Stock Cost Basis Calculator

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Walk-through

How to Use This Calculator

4 steps

1

Enter every purchase lot

On the Cost Basis tab, add one row per buy. Enter the number of shares, the price you paid per share, and any commission or fee for that order. Use the + Add Lot button for additional purchases — the calculator updates instantly as you type.

2

Read your average cost and break-even

The result card shows your weighted-average cost per share, total shares, total invested (fees included), and the break-even price. Because fees are baked into the basis, the average cost and break-even price are the same number — the price your position must reach to get back to even.

3

Add a current price for profit/loss

Type today's market price into the optional field to see your unrealized gain or loss in dollars and percent. Switch to the P/L vs Price tab to model any holding directly.

4

Model averaging down

On the Average Down tab, enter your existing shares and current average, then a new buy. You'll see how the purchase blends your average and moves your break-even — useful before deciding whether to add to a losing (or winning) position.

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Reference

Formula & Methodology

4 formulas

Weighted-average cost basis

avg cost = ( Σ (sharesᵢ × priceᵢ) + Σ fees ) ÷ Σ sharesᵢ

Sum each purchase’s shares × price plus purchase fees, then divide by total shares. This is a portfolio acquisition average. Its use as tax basis depends on asset eligibility and jurisdiction; it is not an automatic FIFO/specific-lot replacement.

Break-even price

break-even = avg cost per share

Average acquisition cost equals break-even before selling costs and taxes. Zero basis has no defined percentage return. Uniform splits preserve total acquisition cash while changing share units and unit prices. Purchase previews leave the ledger unchanged until you explicitly apply them; Undo can restore a prior revision.

Unrealized profit / loss

P/L = (current price − avg cost) × shares ; P/L % = (current price − avg cost) ÷ avg cost × 100

The paper gain or loss versus what you paid. It is 'unrealized' until you sell — selling realizes the gain or loss and may create a taxable event.

New average after averaging down

new avg = ( old shares × old avg + new shares × new price + fee ) ÷ ( old shares + new shares )

A purchase lowers the average only when its fee-inclusive unit cost, price + fee ÷ new shares, is below the current average. Additional cash remains at risk; a lower average does not guarantee recovery.

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YMYL

Trust, Methodology & Sources

Written by Calculover Editorial Team · Updated 2026-06-13 · 2 sources

Editorial accountability

Author: Calculover Editorial Team - Personal finance content

Owner: Calculover Editorial Team - Review and fact-checking

Last reviewed: 2026-06-13

Last verified: 2026-10-18

Methodology

This is a portfolio acquisition average, not a tax-lot election. Tax rules determine whether average basis is permitted; it is not generally interchangeable with FIFO or specific identification for every security or jurisdiction. Purchase fees are included once. Sales, wash sales, return of capital, gifts, inheritance and cash-in-lieu are not automatically reconciled.

Assumption: All purchases are of the same security, and fees entered apply to that buy.

Assumption: This is a portfolio acquisition average, not a tax-lot election. Tax rules determine whether average basis is permitted; it is not generally interchangeable with FIFO or specific identification for every security or jurisdiction. Purchase fees are included once. Sales, wash sales, return of capital, gifts, inheritance and cash-in-lieu are not automatically reconciled.

Assumption: Average acquisition cost equals break-even before selling costs and taxes. Zero basis has no defined percentage return. Uniform splits preserve total acquisition cash while changing share units and unit prices. Purchase previews leave the ledger unchanged until you explicitly apply them; Undo can restore a prior revision.

Limitations & guidance

Average acquisition cost equals break-even before selling costs and taxes. Zero basis has no defined percentage return. Uniform splits preserve total acquisition cash while changing share units and unit prices. Purchase previews leave the ledger unchanged until you explicitly apply them; Undo can restore a prior revision.

Results are estimates for planning and education; reconcile against your broker's 1099-B and consolidated statements before filing taxes.

Tax treatment varies by jurisdiction and account type and can change; capital-gains rules for crypto in particular differ from those for stocks.

Professional guidance: This calculator is for educational purposes only and is not financial, investment, or tax advice. Consult a qualified tax professional (CPA, EA, or tax attorney) about your specific situation before making investment or filing decisions.

Primary sources

Publication 550 — Investment Income and Expenses - Internal Revenue Service

Topic No. 409 — Capital Gains and Losses - Internal Revenue Service

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Glossary

Key Terms Explained

8 terms

Cost basis The total amount you paid to acquire your shares, including commissions and fees, divided across the shares you own. It is the starting point for calculating capital gains or losses when you sell.

Average cost method This is a portfolio acquisition average, not a tax-lot election. Tax rules determine whether average basis is permitted; it is not generally interchangeable with FIFO or specific identification for every security or jurisdiction. Purchase fees are included once. Sales, wash sales, return of capital, gifts, inheritance and cash-in-lieu are not automatically reconciled.

Average down Buying more shares of a holding at a price below your current average, which lowers your overall average cost per share and your break-even price. It increases your exposure to a position that has already fallen.

Break-even price Fee-inclusive average acquisition cost is break-even before selling costs and tax. Zero basis has no defined percentage return; a uniform split preserves total acquisition cash.

Unrealized P/L The paper gain or loss on a holding you still own, measured against your cost basis. It becomes a realized gain or loss — and a potential taxable event — only when you sell.

FIFO (first in, first out) A cost-basis method that assumes the oldest shares are sold first. FIFO can produce a different taxable gain than the average-cost method, especially after multiple purchases at different prices.

Wash sale A U.S. tax rule that disallows a loss if you buy a substantially identical security within 30 days before or after selling at a loss. The disallowed loss is added to the basis of the replacement shares.

Commission / fee Transaction costs charged by a broker or exchange. Fees raise your cost basis on a buy and reduce proceeds on a sell, so they directly affect your break-even and reported gain.

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Scenarios

Real-World Examples

3 worked examples

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Three equal buys

Dollar-cost averaging into a stock

Lot 1 50 sh @ $20 Lot 2 50 sh @ $22 Lot 3 50 sh @ $24

Total invested is $3,300 for 150 shares, so the average cost is $22.00 per share — exactly the middle price because the lots are equal size. Break-even is $22.00. At a current price of $23 the position is up $150 (+4.55%).

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Averaging down a loser

Adding shares after a drop

Existing 100 sh @ $50 avg New buy 150 sh @ $30 + $5 fee

The existing 100 × $50 costs $5,000. The proposed 150 × $30 plus $5 fee costs $4,505. Total acquisition cash is $9,505 for 250 shares, or $38.02 each. This is a separate preview; committing more cash does not guarantee recovery.

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Crypto DCA with fees

Building a Bitcoin position over time

Buy 1 0.25 @ $60,000 + $15 Buy 2 0.50 @ $42,000 + $12 Buy 3 0.75 @ $30,000 + $10

The three purchases cost $15,015 + $21,012 + $22,510 = $58,537 for 1.5 BTC. The weighted average is $39,024.67 per BTC, including $37 of fees once. This is acquisition arithmetic, not a crypto tax-lot method.

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Reference

Cite This Calculator

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Use either format to cite this calculator in a paper, report, or resource list.

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Deep Dive

Understanding Stock Cost Basis and Averaging Down

5 chapters · 4 min read

This is a portfolio acquisition average, not a tax-lot election. Tax rules determine whether average basis is permitted; it is not generally interchangeable with FIFO or specific identification for every security or jurisdiction. Purchase fees are included once. Sales, wash sales, return of capital, gifts, inheritance and cash-in-lieu are not automatically reconciled.

How the cost basis calculation works

Average acquisition cost equals break-even before selling costs and taxes. Zero basis is allowed, but its percentage return is undefined. A split changes share units and unit prices together while preserving total acquisition cash; the preview purchase does not alter the lot ledger.

Average cost vs FIFO and specific-lot

This is a portfolio acquisition average, not a tax-lot election. Tax rules determine whether average basis is permitted; it is not generally interchangeable with FIFO or specific identification for every security or jurisdiction. Purchase fees are included once. Sales, wash sales, return of capital, gifts, inheritance and cash-in-lieu are not automatically reconciled.

Why fees matter more than they look

A few dollars of commission seems trivial, but fees are added to your basis on every buy and subtracted from proceeds on every sell. On small or frequent trades — and on crypto exchanges that charge a percentage — they quietly raise your break-even. The calculator surfaces total fees separately so you can see exactly how much of your basis is transaction cost rather than the asset itself.

The risk of averaging down

A purchase lowers the average only when its fee-inclusive unit cost, price + fee ÷ new shares, is below the current average. Additional cash remains at risk; a lower average does not guarantee recovery.

Limits and when to get advice

The ledger records acquisitions and user-entered uniform splits. It does not calculate tax-lot sales, wash-sale eligibility, return of capital, gifts, inheritance or cash-in-lieu. Keep those adjustments separately; optional dates do not infer tax holding periods.

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Questions

Frequently Asked Questions

7 questions

What's the difference between average cost and FIFO?+

This is a portfolio acquisition average, not a tax-lot election. Tax rules determine whether average basis is permitted; it is not generally interchangeable with FIFO or specific identification for every security or jurisdiction. Purchase fees are included once. Sales, wash sales, return of capital, gifts, inheritance and cash-in-lieu are not automatically reconciled.

Do commissions and fees count toward my cost basis?+

Yes. Commissions and transaction fees paid on a purchase are added to your cost basis, which raises your break-even price. Fees on a sale reduce your proceeds. This calculator includes a fee field on each lot and adds it into the average automatically, and it shows your total fees separately so you can see their impact.

Does averaging down always lower my break-even?+

A purchase lowers the average only when its fee-inclusive unit cost, price + fee ÷ new shares, is below the current average. Additional cash remains at risk; a lower average does not guarantee recovery.

Can I use this for crypto and fractional shares?+

Yes. Enter fractional quantities (for example 0.25 BTC) and per-trade exchange fees just like whole shares. The average-cost math is identical. Note that crypto tax rules and broker reporting differ by jurisdiction, so confirm how your exchange or tax software tracks basis.

How do sells affect my cost basis?+

Under the average-cost method, a sale removes shares at the current average cost, so your average per share is unchanged but your total shares and total invested both drop. The realized gain or loss is the sale price (minus fees) versus that average cost. Under FIFO or specific-lot methods, a sale removes particular lots, which changes the basis of your remaining shares. This calculator focuses on building your basis from purchases; reconcile actual sells against your broker's records for tax filing.

What is a wash sale and why does it matter?+

U.S. wash-sale rules can disallow a loss and affect replacement-share basis, with exceptions. This calculator does not determine wash sales or apply tax adjustments. Keep jurisdiction-specific tax records separately.

Is my average cost the same as my break-even price?+

Average acquisition cost equals break-even before selling costs and taxes. Zero basis has no defined percentage return. Uniform splits preserve total acquisition cash while changing share units and unit prices. Purchase previews leave the ledger unchanged until you explicitly apply them; Undo can restore a prior revision.

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Related calculators

Guides & articles

Glossary terms

Calculator topicStock Cost Basis

Your purchase lots

Average cost basis

Fee-inclusive unit costs by lot

Add a buy to your position

New average cost

Average before vs after

Profit / loss vs current price

Unrealized P/L

Cost vs market value

How to Use This Calculator

Enter every purchase lot

Read your average cost and break-even

Add a current price for profit/loss

Model averaging down

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Three equal buys

Averaging down a loser

Crypto DCA with fees

Cite This Calculator

Understanding Stock Cost Basis and Averaging Down

How the cost basis calculation works

Average cost vs FIFO and specific-lot

Why fees matter more than they look

The risk of averaging down

Limits and when to get advice

Frequently Asked Questions

Get a branded PDF of your results

Stock Cost Basis Calculator

Your purchase lots

Average cost basis

Fee-inclusive unit costs by lot

Add a buy to your position

New average cost

Average before vs after

Profit / loss vs current price

Unrealized P/L

Cost vs market value

How to Use This Calculator

Enter every purchase lot

Read your average cost and break-even

Add a current price for profit/loss

Model averaging down

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Cite This Calculator

Understanding Stock Cost Basis and Averaging Down

How the cost basis calculation works

Average cost vs FIFO and specific-lot

Why fees matter more than they look

The risk of averaging down

Limits and when to get advice

Frequently Asked Questions

Get a branded PDF of your results

Keep Exploring

Related calculators

Guides & articles

Glossary terms