How to Calculate Average Cost After Buying More Shares

When you buy more shares of a stock you already own, your new average cost is not usually the simple average of the two purchase prices.

It is a weighted average based on how many shares you bought at each price.

The basic formula is:

New average cost
= Total amount paid for all shares ÷ Total number of shares

For example, suppose you bought:

10 shares at $100
10 shares at $70

Total cost:

(10 × $100) + (10 × $70)
= $1,000 + $700
= $1,700

Total shares:

10 + 10
= 20 shares

New average cost:

$1,700 ÷ 20
= $85 per share

Your new average cost is $85, not $70 and not $100.

Because the two purchases contained the same number of shares, $85 also happens to be the simple midpoint. But when the share quantities are different, a simple average gives the wrong result.

If you make two purchases:

Average cost
= [(Shares 1 × Price 1) + (Shares 2 × Price 2)]
  ÷ (Shares 1 + Shares 2)

For multiple purchases:

Average cost
= Total purchase cost ÷ Total shares

Or:

Average cost
= Σ(Shares × Purchase price) ÷ Σ(Shares)

If fees are included in your tracking method:

Average cost
= (Total purchase cost + Included purchase fees)
  ÷ Total shares

Whether a particular fee belongs in your tax basis depends on the asset, account, broker, and local tax rules. The arithmetic in this guide is for investment tracking, not tax-lot accounting.

Suppose you originally own:

20 shares at an average cost of $120

Your current total cost is:

20 × $120
= $2,400

The stock falls to $90 and you buy another 15 shares:

15 × $90
= $1,350

Combined cost:

$2,400 + $1,350
= $3,750

Combined shares:

20 + 15
= 35 shares

New average cost:

$3,750 ÷ 35
= $107.14

Your average cost falls from $120 to approximately $107.14.

This is commonly called averaging down.

Suppose you bought:

10 shares at $100
30 shares at $70

A simple average of the two prices would be:

($100 + $70) ÷ 2
= $85

But $85 is wrong because you bought three times as many shares at $70.

The correct calculation is:

10 × $100 = $1,000
30 × $70  = $2,100

Total:

$3,100 ÷ 40 shares
= $77.50

Your correct average cost is $77.50.

The $70 purchase has more weight because it contains more shares.

You do not need the full purchase history if you already know:

• current number of shares;
• current average cost;
• number of new shares;
• new purchase price.

Use:

Current cost
= Current shares × Current average cost

Then:

New purchase cost
= New shares × New purchase price

Finally:

New average cost
= (Current cost + New purchase cost)
  ÷ (Current shares + New shares)

Example:

Current shares = 50
Current average cost = $42
New shares = 20
New purchase price = $35

Current cost:

50 × $42
= $2,100

New purchase cost:

20 × $35
= $700

Combined cost:

$2,800

Combined shares:

70

New average:

$2,800 ÷ 70
= $40

Buying 20 additional shares at $35 lowers the average cost from $42 to $40.

Sometimes you know how much new cash you want to invest rather than how many shares you will buy.

Suppose:

Current shares = 40
Current average cost = $75
Current stock price = $60
New investment = $1,200

If fractional shares are allowed and fees are ignored:

New shares
= $1,200 ÷ $60
= 20 shares

Current cost:

40 × $75
= $3,000

New purchase cost:

$1,200

New average cost:

($3,000 + $1,200) ÷ (40 + 20)
= $4,200 ÷ 60
= $70

The new average cost is $70.

If only whole shares are allowed, the actual share count and unused cash can change the result.

For example, if you had $1,200 to invest at $67 per share:

$1,200 ÷ $67
≈ 17.91 shares

With whole shares only, you could buy 17 shares:

17 × $67
= $1,139

Unused cash:

$1,200 − $1,139
= $61

The average-cost calculation should use the 17 shares actually purchased, not 17.91.

Averaging down means buying additional shares below your existing average cost.

If:

New purchase price < Current average cost

then, ignoring unusual fee effects:

New average cost < Current average cost

But the new average will remain between the old average cost and the new purchase price.

Example:

Current average = $100
New purchase price = $70

After buying more shares, the new average might become:

$95
$90
$85
$80

depending on how many shares you add.

But it cannot become lower than $70 merely by buying at $70.

The more shares you buy at the lower price, the closer the weighted average moves toward $70.

Average cost is not only relevant when a stock falls.

If you add shares above your existing average price, your average cost rises.

Suppose:

10 shares at $50
5 new shares at $80

Total cost:

10 × $50 = $500
5 × $80 = $400

Combined cost:

$900

Combined shares:

15

New average cost:

$900 ÷ 15
= $60

Your average rises from $50 to $60.

This is sometimes called averaging up.

A higher average cost is not automatically bad. It simply reflects that additional capital was deployed at a higher price.

You can also work backward.

Suppose:

Current shares = 20
Current average cost = $120
New purchase price = $90
Target average cost = $105

Let x be the number of new shares.

The equation is:

(20 × $120 + x × $90)
÷ (20 + x)
= $105

Solving for x:

$2,400 + $90x
= $2,100 + $105x

$300
= $15x

x
= 20 shares

So you would need to buy 20 additional shares at $90 to bring the average cost to $105, ignoring fees.

Check:

Old cost = 20 × $120 = $2,400
New cost = 20 × $90 = $1,800
Total cost = $4,200

Total shares = 40

$4,200 ÷ 40
= $105

The general formula is:

New shares required
= Current shares × (Current average − Target average)
  ÷ (Target average − New purchase price)

This formula only works in a meaningful way when the target average lies between the current average and the new purchase price.

If your current average is $120 and you are buying at $90, you cannot reach an $85 average by buying more at $90. A weighted average of $120 and $90 cannot fall below $90.

Once you know the required new share count:

Required investment
= New shares required × New purchase price

Using the previous example:

20 shares × $90
= $1,800

So lowering the average from $120 to $105 requires another $1,800 at a $90 purchase price, before fees.

This is an important reality check.

Reducing an average cost substantially can require a large amount of new capital.

Suppose you own:

10 shares at $100

Buying another 10 shares at $80 changes the average to:

$90

But suppose you already own:

1,000 shares at $100

Buying only 10 more shares at $80 barely moves the average:

Old cost = 1,000 × $100 = $100,000
New cost = 10 × $80 = $800

New average
= $100,800 ÷ 1,010
≈ $99.80

The original position has much more weight.

As a position becomes larger, moving its average cost meaningfully may require increasingly large additional purchases.

Ignoring selling costs and taxes, your average cost is approximately the price at which the combined position reaches purchase-cost break-even.

Suppose your new average cost is:

$85

If the stock trades at $85:

Market value
= Shares × $85

and your purchase cost is approximately:

Cost
= Shares × $85

So the unrealized gain or loss on the combined position is around zero before other costs.

However, fees, taxes, currency conversion, financing, and other costs can make your true economic break-even different from the displayed average purchase cost.

Suppose:

Average cost = $85
Current price = $70

Dollar increase required:

$85 − $70
= $15

But the percentage recovery should be calculated from the current price:

Required gain
= ($85 − $70) ÷ $70 × 100
≈ 21.43%

So the stock must rise about 21.43% from $70 to return to $85.

This is different from the percentage loss measured from the average cost:

Loss from $85 to $70
= ($70 − $85) ÷ $85 × 100
≈ -17.65%

A 17.65% decline requires a 21.43% gain to recover because percentage losses and gains use different starting values.

Suppose:

10 shares at $100

The stock falls to $70.

Before adding:

Cost = $1,000
Market value = $700
Unrealized loss = -$300

Now buy another 10 shares at $70:

Additional cost = $700
New total cost = $1,700
New total shares = 20
New average = $85

Immediately after the purchase, if the market price is still $70:

Market value
= 20 × $70
= $1,400

Unrealized loss:

$1,400 − $1,700
= -$300

The dollar loss has not disappeared.

What changed is:

• your share count increased;
• your capital invested increased;
• your average cost fell;
• the percentage rise needed to reach break-even became smaller.

Averaging down changes the structure of the position. It does not retroactively remove the loss already created by the price decline.

Lowering your average cost often requires committing more capital.

That means you are also increasing exposure to the same stock.

Example:

Before averaging down:

10 shares × $70 market price
= $700 current exposure

After buying 20 more shares:

30 shares × $70
= $2,100 current exposure

Your average cost may improve, but the portfolio now depends more heavily on the same company.

That introduces another question:

Do you want a lower average cost, or do you want a larger position in this stock?

Those are not always the same decision.

A lower average cost can feel attractive because it reduces the price needed to break even.

But the calculation does not tell you whether buying more is a good investment decision.

Before adding, it can be useful to ask:

• Has the investment thesis changed?
• Did the company’s fundamentals deteriorate?
• Is the decline company-specific or market-wide?
• Would you buy the stock today if you did not already own it?
• How large would the position become after the purchase?
• Does it create excessive concentration?
• Are you adding because the expected return improved, or only because you want to get back to break-even?

Average cost is an accounting and planning number. It is not a valuation signal by itself.

Suppose you buy:

10 shares at $50

Purchase value:

$500

If you also include a $5 purchase fee in your investment-tracking cost:

Total tracked cost
= $505

Average tracked cost:

$505 ÷ 10
= $50.50 per share

If you later buy:

10 shares at $40
Fee = $5

Second tracked cost:

$405

Combined tracked cost:

$505 + $405
= $910

Combined shares:

20

Average tracked cost:

$910 ÷ 20
= $45.50

Fees can therefore affect the average.

However, how fees are treated for tax purposes varies by jurisdiction and transaction type. Do not assume a portfolio-tracking average is the same as your broker's or tax authority's adjusted cost basis.

These terms are sometimes used interchangeably in casual investing discussions, but they should not automatically be treated as identical.

A simple weighted average tells you:

Total tracked purchase cost
÷ Total shares held

Tax cost basis can involve additional rules, including:

• specific tax lots;
• FIFO;
• LIFO where permitted;
• specific identification;
• wash-sale or similar adjustments;
• reinvested distributions;
• return of capital;
• corporate actions;
• transfers between brokers;
• jurisdiction-specific rules.

BasisPilot's Stock Average Cost Calculator is designed to combine purchases and estimate weighted average purchase cost. It is not a tax-lot accounting tool.

Once sales are involved, the calculation becomes more dependent on the accounting method you are using.

Suppose you bought shares at several different prices and later sell only part of the position.

Which shares are treated as sold can matter for realized gains, remaining cost basis, and taxes.

A simple purchases-only weighted average does not model:

• which lot was sold;
• realized gain or loss;
• FIFO or specific identification;
• tax adjustments.

If you are calculating a figure for tax reporting, use your broker's records and the rules applicable to your jurisdiction rather than relying on a simple average-cost calculation.

A stock split normally changes the number of shares and the per-share cost while leaving the total economic cost unchanged.

Suppose you own:

10 shares
Average cost = $100
Total cost = $1,000

After a 2-for-1 stock split:

Shares = 20
Adjusted average cost = $50
Total cost = $1,000

The total tracked cost remains the same.

If you combine pre-split and post-split records, make sure the share quantities and prices are adjusted to the same basis before calculating an average.

The terms sound similar, but they mean different things.

Average Cost

This is the weighted average price of shares you already bought:

Total purchase cost ÷ Total shares

Dollar-Cost Averaging

Dollar-cost averaging, or DCA, is an investment schedule in which you invest a set amount repeatedly over time.

For example:

$500 every month

Each contribution may buy a different number of shares.

A DCA strategy creates an average purchase cost over time, but “average cost” is the result of the purchases, while “dollar-cost averaging” describes the contribution process.

When deciding how an additional purchase would affect your position:

1. Record your current share count.
2. Record your current average purchase cost.
3. Calculate the current total tracked cost.
4. Enter the new purchase price.
5. Determine how many shares you will actually buy.
6. Add any fees you intentionally include in the tracked cost.
7. Calculate the combined cost.
8. Divide by the combined share count.
9. Compare the new average with the current market price.
10. Check how much capital and concentration the additional purchase creates.

The main formulas are:

Current cost
= Current shares × Current average cost

New purchase cost
= New shares × New purchase price

New average cost
= (Current cost + New purchase cost)
  ÷ (Current shares + New shares)

You can also use the BasisPilot Stock Average Cost Calculator to combine purchase history or test a planned additional buy.

How do I calculate my new average price after buying more shares?

Use:

New average
= (Old shares × Old average + New shares × New price)
  ÷ (Old shares + New shares)

If I buy the same number of shares at a lower price, is the new average halfway between?

Yes, if the old position and the new purchase contain exactly the same number of shares and fees are ignored.

For example:

10 shares at $100
10 shares at $70

New average:

($100 + $70) ÷ 2
= $85

But this shortcut does not work when the share quantities are different.

If I buy more below my average cost, will my average always go down?

Generally yes, if the new purchase price is below the current average and fees do not overwhelm the difference.

The new weighted average will fall somewhere between the old average and the new purchase price.

Can averaging down bring my average below the new purchase price?

No.

If your current average is $100 and you buy more at $70, the combined weighted average must remain above $70 and below $100.

How much do I need to buy to lower my average cost?

It depends on:

• current share count;
• current average cost;
• new purchase price;
• target average cost.

The closer the target is to the new purchase price, the more shares and capital are generally required.

Does lowering my average cost reduce my existing dollar loss?

Not immediately.

If the stock price does not change, buying more at the current market price lowers the average cost but does not erase the dollar loss on the shares you already owned.

Is average cost the same as break-even?

It is an approximate purchase-cost break-even before taxes and other selling or financing costs.

Your true economic break-even may differ.

Is average cost the same as tax cost basis?

Not necessarily.

Tax basis can depend on tax lots, sales, fees, corporate actions, and jurisdiction-specific rules.

Should I average down just because a stock is below my purchase price?

The math alone cannot answer that.

A lower price reduces the weighted average if you buy more, but it also increases your capital exposure to the same investment.

The core formula is:

New average cost
= Total tracked purchase cost
  ÷ Total shares held

For an existing position plus one new purchase:

New average cost
= (Current shares × Current average cost
   + New shares × New purchase price)
  ÷ (Current shares + New shares)

The arithmetic is straightforward.

The more important decision is whether adding more shares still makes sense for the portfolio—not simply whether it makes the average price look better.