How to Rebalance a Portfolio With New Contributions

To rebalance a portfolio with new contributions, direct new cash toward assets that are below their target allocations instead of immediately selling assets that are above target.

For example, assume a $10,000 portfolio has the following allocation:

With a new $1,000 contribution, US stocks are already overweight, so none of the new cash needs to go there. One transparent buy-only method allocates the contribution in proportion to the target-value gaps:

International stocks: $500.00
Bonds: $466.67
Cash: $33.33
US stocks: $0

The portfolio moves closer to its target without selling. It does not reach the target exactly because $1,000 is not enough new cash to dilute the overweight US stock position back to 50%.

Calculate a buy-only rebalance with the BasisPilot Portfolio Rebalancing Calculator →

Portfolio rebalancing means adjusting current holdings toward a selected target allocation.

A target allocation might be expressed as:

US stocks: 50%
International stocks: 25%
Bonds: 20%
Cash: 5%

Market movements can cause the current allocation to move away from those targets. If stocks rise faster than bonds, for example, the stock allocation may become larger than planned.

Investor.gov describes rebalancing as bringing a portfolio back to its desired asset allocation mix. It identifies three broad methods:

1. Sell overweight assets and buy underweight assets.
2. Purchase additional investments in underweight assets.
3. Redirect ongoing contributions toward underweight categories.

This guide focuses on the second and third methods: rebalancing with new cash instead of selling.

Rebalancing does not choose the target allocation

A rebalancing calculator should not determine whether a portfolio should be:

80% stocks / 20% bonds

or:

40% stocks / 60% bonds

The user supplies the target allocation. The calculator only measures the difference between current and target values and calculates the trades or contributions needed to move toward that target.

Asset allocation depends on factors such as goals, time horizon, risk tolerance, account type and liquidity needs. The same target is not appropriate for every person or every account.

Allocation drift is the difference between the current weight and target weight of an asset.

Step 1: Calculate total portfolio value

Total portfolio value
= Sum of all current asset values

Using the example:

$6,000 + $2,000 + $1,500 + $500
= $10,000

Step 2: Calculate current weight

Current weight
= Current asset value ÷ Total portfolio value

For US stocks:

$6,000 ÷ $10,000
= 60%

Step 3: Calculate absolute drift

Absolute drift
= Current weight − Target weight

For US stocks:

60% − 50%
= +10 percentage points

For bonds:

15% − 20%
= -5 percentage points

A positive number means the asset is overweight. A negative number means it is underweight.

Percentage points vs relative drift

These measures are different.

If an asset has:

Current weight: 25%
Target weight: 20%

Absolute drift:

25% − 20%
= +5 percentage points

Relative drift:

(25% − 20%) ÷ 20%
= +25%

The asset is 5 percentage points above target and 25% above its target weight on a relative basis.

A calculator should label these measures clearly instead of displaying both as “5%” or “25% drift.”

Total portfolio drift

BasisPilot can summarize the portfolio’s total allocation deviation as:

Total allocation drift
=
0.5 × Sum of |Current weight − Target weight|

For the original portfolio:

Current:
60%, 20%, 15%, 5%

Target:
50%, 25%, 20%, 5%

Total drift
=
0.5 ×
(|60% − 50%|
+ |20% − 25%|
+ |15% − 20%|
+ |5% − 5%|)

= 0.5 × (10% + 5% + 5% + 0%)
= 10%

In a frictionless portfolio with no new cash, this also represents the minimum share of total portfolio value that would need to move between assets to reach the target exactly.

The name and formula must be visible in the calculator methodology because “portfolio drift” can be defined differently by different tools.

A buy-only rebalance uses new cash to purchase underweight assets while leaving overweight assets unchanged.

A transparent calculation can use the following process.

Step 1: Add the new contribution to total portfolio value

Post-contribution portfolio value
=
Current portfolio value + New contribution

Example:

$10,000 + $1,000
= $11,000

Step 2: Calculate each target value after the contribution

Target asset value
=
Post-contribution portfolio value × Target weight

For a 25% international-stock target:

$11,000 × 25%
= $2,750

Step 3: Calculate each positive target gap

Target gap
=
Maximum of:
- Target asset value − Current asset value
- 0

For international stocks:

$2,750 − $2,000
= $750 gap

For US stocks:

$5,500 − $6,000
= -$500

Because US stocks are already above their target value:

Buy-only target gap
= $0

Step 4: Add all positive target gaps

Total positive gap
= Sum of all positive target gaps

Example:

Total positive gap
= $750 + $700 + $50
= $1,500

The portfolio has only $1,000 of new cash, so it cannot fill every gap completely.

Step 5: Allocate new cash in proportion to the gaps

Contribution to asset
=
New cash × Asset positive gap ÷ Total positive gap

International stocks:

$1,000 × $750 ÷ $1,500
= $500.00

Bonds:

$1,000 × $700 ÷ $1,500
= $466.67

Cash:

$1,000 × $50 ÷ $1,500
= $33.33

The allocations sum to $1,000.

This proportional-gap approach is transparent and moves all underweight assets toward target. Other buy-only optimization methods may produce different allocations, particularly when minimum trade sizes, “do not buy” restrictions or tax lots are involved.

Starting portfolio:

New contribution:

$1,000

Suggested buy-only allocation:

The portfolio becomes closer to target:

Original total allocation drift: 10.00%
New total allocation drift: approximately 4.55%

US stocks remain overweight because the investor did not sell any shares and the contribution was not large enough to dilute the position to 50%.

Why not send the entire contribution to the most underweight asset?

A different buy-only rule could direct all $1,000 to bonds or international stocks first.

That method may be appropriate under a predefined priority rule, but it can leave other underweight assets unchanged. The proportional-gap method distributes cash across all positive target gaps.

The calculator should state which algorithm it uses. There is no universally required buy-only allocation algorithm.

It is possible to calculate the minimum amount of new cash required to reach the target exactly without selling any current holding.

For each asset with a positive target weight:

Required total portfolio value for asset i
=
Current value of asset i ÷ Target weight of asset i

The minimum feasible total portfolio value is the largest of these values:

Minimum required total portfolio value
=
Maximum across all assets of:
Current value ÷ Target weight

Then:

Minimum new cash required
=
Minimum required total portfolio value
− Current portfolio value

Example

Current values and targets:

The largest required total is:

$12,000

Current portfolio value:

$10,000

Minimum new cash:

$12,000 − $10,000
= $2,000

With $2,000 of new cash, the portfolio can reach the targets without selling:

Result:

US stocks: 50%
International stocks: 25%
Bonds: 20%
Cash: 5%

When an exact buy-only rebalance is impossible

An exact buy-only rebalance is impossible when:

• an asset has a target weight of 0% but currently has a positive value;
• the user prohibits purchases of an underweight asset;
• minimum investment rules prevent the required purchases;
• assets cannot be purchased in sufficiently small units;
• the new cash amount is fixed below the minimum required amount.

A calculator should return:

Exact buy-only rebalance is not possible with the selected constraints.

It should then show the closest allocation under the stated algorithm rather than silently changing the target.

Buy-only rebalancing

Uses:

• new deposits;
• payroll contributions;
• dividends;
• interest;
• other cash flows.

Potential advantages:

• avoids selling solely for allocation purposes;
• may reduce realized taxable gains in taxable accounts;
• can reduce the number of transactions;
• is easy to apply during regular contributions.

Limitations:

• may take a long time for a large portfolio;
• may not fully correct a large overweight;
• cannot remove an asset with a 0% target;
• may require more new cash than is available;
• can leave portfolio risk away from target.

Buy-and-sell rebalancing

For a post-cash portfolio:

Target value
=
Post-cash total × Target weight

Trade amount
=
Target value − Current value

Positive result:

Buy

Negative result:

Sell or reduce

Using the original $10,000 portfolio with no new cash:

Potential advantages:

• can restore the target immediately;
• works without waiting for future contributions;
• can remove or sharply reduce an unwanted allocation.

Limitations:

• selling may create taxes;
• trades may incur fees, spread and slippage;
• minimum lots may prevent exact values;
• account restrictions may apply.

Neither method is universally better. The appropriate method depends on account type, drift size, tax treatment, transaction costs, available cash and how urgently the target needs to be restored.

Two common review methods are calendar-based and threshold-based rebalancing.

Calendar-based review

Review the portfolio on a schedule, such as:

Every six months
Every 12 months

The review does not have to produce trades. It determines whether the allocation still needs adjustment.

Threshold-based review

Review or rebalance when an asset moves beyond a predefined drift threshold.

Example:

Target weight: 20%
Allowed absolute band: ±5 percentage points

Allowed range:

15% to 25%

A current weight of 26% is outside the band.

Relative threshold

A relative band scales with the target.

Example:

Target weight: 20%
Relative threshold: 25%

Threshold size:

20% × 25%
= 5 percentage points

Allowed range:

15% to 25%

For a 5% target, the same 25% relative threshold becomes:

5% × 25%
= 1.25 percentage points

Allowed range:

3.75% to 6.25%

No universal schedule or threshold

Investor.gov notes that some experts suggest reviewing at regular intervals, such as every six or 12 months, while others use preset drift thresholds. It also notes that rebalancing tends to work best when done relatively infrequently.

This is educational context, not a recommendation to use a specific schedule or band.

Frequent rebalancing can create:

• more transactions;
• higher costs;
• more taxable events;
• small trades;
• operational complexity.

Infrequent rebalancing can leave the portfolio farther from the selected target for longer.

A mathematically exact rebalance may not be operationally efficient.

Potential transaction costs include:

• commissions;
• bid-ask spread;
• markups or markdowns;
• fund sales loads;
• exchange fees;
• currency conversion;
• account-level charges.

Investor.gov explains that transaction fees can apply each time an investment is bought, sold or exchanged and that both transaction and ongoing fees reduce portfolio value.

Minimum trade amount

Assume the calculation suggests:

Buy $7.50 of Asset A
Buy $492.50 of Asset B

If the selected minimum trade amount is $25:

Asset A: Below minimum trade
Asset B: Buy $492.50
Unallocated cash: $7.50

The calculator should not automatically add the $7.50 to another asset unless the allocation algorithm explicitly defines that behavior.

Whole shares

If fractional shares are unavailable:

Shares to buy
=
Floor(Suggested dollar amount ÷ Current price)

The remaining cash should be displayed as:

Unallocated cash

It should not disappear through rounding.

Cost-aware rebalancing

A useful calculator can display:

Gross suggested trade
Estimated transaction cost
Net invested amount
Allocation after estimated costs

BasisPilot should not estimate costs without user inputs or clearly disclosed assumptions.

Selling assets can create tax consequences depending on jurisdiction, account type, holding period and cost basis.

Investor.gov advises considering transaction fees and tax consequences before choosing a rebalancing method. Its robo-adviser bulletin also notes that rebalancing can have different tax implications depending on the account.

In the United States, the IRS generally treats the difference between adjusted basis and sale proceeds as a capital gain or capital loss when a capital asset such as stocks or bonds is sold.

This does not mean the same rules apply in every jurisdiction.

Why new contributions may reduce taxable sales

A buy-only rebalance can move the allocation toward target without realizing gains from selling overweight assets.

However, it does not automatically produce the lowest tax outcome. Other considerations may include:

• unrealized losses;
• holding period;
• tax lots;
• wash-sale or similar rules;
• account wrappers;
• capital-loss limitations;
• transaction costs;
• local tax law.

BasisPilot’s rebalancing calculator should not:

• select tax lots;
• estimate capital-gains tax;
• recommend which security to sell for tax reasons;
• describe a buy-only plan as “tax-free.”

Tax consequences vary. Users should consult current local rules or a qualified tax professional for decisions that depend on tax treatment.

New cash does not have to come from one lump-sum deposit.

Potential sources include:

• monthly contributions;
• payroll deposits;
• dividends;
• bond interest;
• cash distributions;
• maturing securities;
• proceeds from an unrelated withdrawal or sale.

Redirecting recurring contributions

Suppose monthly contributions are currently allocated:

US stocks: 50%
International stocks: 25%
Bonds: 20%
Cash: 5%

If US stocks become overweight, future contributions can temporarily shift toward underweight assets.

Example:

Next monthly contribution: $1,000
US stocks: $0
International stocks: $500
Bonds: $466.67
Cash: $33.33

The contribution mix can be recalculated after each deposit.

Redirecting dividends

Instead of automatically reinvesting each dividend into the asset that paid it, available cash distributions can be directed toward underweight assets where the account permits.

This can reduce the need to sell, but it may also change the user’s preferred reinvestment process or incur different execution costs.

Do not assume cash arrives continuously

A buy-only plan should show:

Current drift
Expected new contribution
Allocation after contribution
Remaining drift
Minimum cash required for exact rebalance

It should not assume future contributions will occur unless the user includes them.

A household may hold investments across:

• taxable brokerage accounts;
• retirement accounts;
• employer plans;
• savings accounts;
• other investment platforms.

Two calculation scopes are possible.

Account-level rebalancing

Each account is compared with its own target.

Household-level rebalancing

Assets across accounts are combined into one allocation.

These methods can produce different trades.

Example:

Taxable account: Mostly stocks
Retirement account: Mostly bonds

Each account may look unbalanced individually while the combined household allocation is on target.

A simple portfolio calculator should clearly state whether:

• each row is an asset;
• each row is an account;
• all entered values are being treated as one portfolio.

It should not infer tax location or account restrictions.

Allocation drift and concentration are related but different.

A portfolio can match its target and still be concentrated if the target itself is concentrated.

One descriptive concentration measure is the Herfindahl-Hirschman Index:

HHI
= Sum of each portfolio weight squared

For four equal holdings:

HHI
= 4 × 0.25²
= 0.25

Effective number of holdings:

Effective holdings
= 1 ÷ HHI
= 4

For:

70%, 10%, 10%, 10%

HHI
= 0.70² + 3 × 0.10²
= 0.52

Effective holdings
= 1 ÷ 0.52
≈ 1.92

This describes how evenly value is distributed. It does not determine whether the allocation is suitable, diversified or low risk.

FINRA notes that diversification includes spreading investments both among and within asset classes. Holding several narrowly focused funds does not necessarily eliminate overlapping exposure.

1. Letting the calculator choose the target

The calculator should implement the user’s target, not recommend one.

2. Using percentages that do not sum to 100%

The target weights must total 100%, subject to a small rounding tolerance.

3. Treating percentage points and percentages as the same

A move from 20% to 25% is:

+5 percentage points

and:

+25% relative to the target

4. Assuming new cash can always restore the target exactly

A large overweight may require more new cash than is available.

5. Ignoring minimum trade sizes

Tiny suggested trades may be impractical or unavailable.

6. Ignoring transaction costs

Frequent small rebalances can create costs that exceed the benefit of a small allocation adjustment.

7. Ignoring taxes when selling

Selling an appreciated asset may create a taxable gain in some accounts and jurisdictions.

8. Treating buy-only rebalancing as automatically tax-free

Buying does not itself remove all tax, account or transaction considerations.

9. Rebalancing too frequently without a defined rule

Reacting to every small market movement can create unnecessary turnover.

10. Never reviewing the target

A target may need reconsideration when the goal, time horizon, liquidity need or risk tolerance changes.

11. Combining different valuation dates

All asset values should use approximately the same valuation time.

12. Forgetting uninvested cash

Cash is part of the portfolio if it is included in the allocation target.

13. Ignoring overlapping funds

Two different fund names may hold many of the same securities.

14. Assuming the post-trade allocation will be exact

Prices can move between calculation and execution. Whole-share rounding and fees can also change the result.

15. Hiding unallocated cash

Rounding remainders must remain visible.

A transparent process is:

1. Define the portfolio scope.
2. Enter current asset values using one valuation date.
3. Enter target weights that sum to 100%.
4. Calculate current weights and drift.
5. Enter available new cash.
6. Choose buy-only or buy-and-sell mode.
7. Add minimum trade and sale restrictions.
8. Review suggested transactions.
9. Review remaining drift after the transactions.
10. Review fees, taxes and account rules.
11. Execute only through the user’s chosen broker or platform.
12. Recalculate using actual post-trade values.

The calculator provides arithmetic, not execution or personalized allocation advice.

Can I rebalance a portfolio without selling?

Yes, if new contributions, dividends or other cash flows can be directed to underweight assets. The portfolio may move closer to target without reaching it exactly.

How do I know which asset should receive a new contribution?

Calculate each asset’s target value after adding the new cash, then compare it with the current value. Assets below their target values have positive gaps.

What is the formula for portfolio rebalancing?

For buy-and-sell mode:

Suggested trade
=
(Post-cash total × Target weight)
− Current value

For buy-only mode, negative trade amounts are set to zero and available cash is allocated among positive target gaps according to the selected algorithm.

How much new cash do I need to rebalance without selling?

Calculate:

Minimum required total
=
Maximum(Current asset value ÷ Target weight)

Then subtract the current portfolio total.

What if one asset has a target of 0%?

If the asset currently has value, an exact buy-only rebalance is impossible. New contributions can dilute it but cannot reduce it to exactly 0%.

Should I rebalance monthly?

There is no universal schedule. Investor.gov describes calendar and threshold approaches and notes that rebalancing tends to work best when done relatively infrequently.

Is portfolio rebalancing the same as diversification?

No. Diversification describes how investments are spread. Rebalancing adjusts current holdings back toward a selected allocation.

Does rebalancing improve returns?

Rebalancing is primarily a risk- and allocation-maintenance process. It does not guarantee higher returns or prevent losses.

Does rebalancing create taxes?

Selling can create tax consequences depending on jurisdiction and account type. Buying with new cash may reduce the need to sell, but it is not automatically the best tax strategy.

Should dividends be reinvested into the same asset?

That depends on the investment plan and account settings. Directing available dividends toward underweight assets is one possible buy-only rebalancing method.

What is an allocation threshold?

It is a predefined amount of drift that triggers a review or rebalance, such as 5 percentage points from target or 25% of the target weight.

What happens if target weights total 99.99% or 100.01%?

A calculator can permit a small tolerance for decimal rounding, such as ±0.01%. Larger differences should produce a validation error.

Can I rebalance several accounts together?

Yes, if all entered values are intentionally treated as one combined portfolio. Account-specific tax and trading restrictions still need separate review.

Does the calculator account for changing market prices?

It uses the values entered at calculation time. Prices may change before trades are executed, so the final allocation can differ.

• Investor.gov: Asset Allocation and Diversification
• Investor.gov: Beginners’ Guide to Asset Allocation, Diversification, and Rebalancing
• Investor.gov: Year-End Investment Considerations for Individual Investors
• Investor.gov: Is It Time to Rebalance Your Investment Portfolio?
• Investor.gov: How Fees and Expenses Affect Your Investment Portfolio
• FINRA: Asset Allocation and Diversification
• IRS: Topic No. 409, Capital Gains and Losses

The calculations in this guide use current values and user-selected target weights. They do not recommend an asset allocation, security, account type or rebalancing schedule.

The proportional-gap buy-only method is one transparent allocation method. Other optimization methods may produce different suggested purchases.

Actual post-trade weights may differ because of market movement, bid-ask spreads, fees, taxes, minimum lots, fractional-share availability and execution timing.

BasisPilot provides educational calculations and does not provide personalized investment, tax, legal or financial advice.