Cost Basis Calculator
See the capital gain, the long-term tax, and the net cash from selling part of a position, plus the basis you still hold.
How it's calculated
When you sell only part of a position, the tax turns on your cost basis, which is what you paid for the shares. Sell some shares and your gain is the sale proceeds minus the basis on just those shares. Using your average cost per share keeps it simple. Multiply the shares sold by your basis to get the basis sold, subtract that from the proceeds, and the difference is the taxable gain.
Take the default. You own 1,000 shares bought at $40 each, and you sell 300 at $90. The proceeds are $27,000, and the basis on those 300 shares is $12,000, so the capital gain is $15,000. With $60,000 of other taxable income, that long-term gain falls in the 15 percent bracket, so the tax is $2,250 and you keep $24,750 after tax. You still hold 700 shares with $28,000 of cost basis, which sets the gain on any future sale.
Two things drive the tax. Whether the gain is long-term, meaning held more than a year, which earns the lower 0, 15, or 20 percent rates instead of your ordinary rate, and how much other income you have, since the gain stacks on top of it. A lower income can push part of the gain into the 0 percent bracket, while a high income can add the 3.8 percent NIIT. Track the basis on what you keep, because it decides your next tax bill.
Assumptions
- Uses your average cost basis per share. The gain is the proceeds minus the basis on the shares sold, taxed at the long-term capital gains rate for your income.
- Assumes the shares are long-term (held over a year) and taxed at the 2026 federal 0/15/20 percent rates plus the 3.8 percent NIIT where it applies. State tax is not included.
Last updated: 2026-08-08 · Tax year 2026
These assumptions follow our general methodology.
Frequently asked questions
How do I calculate cost basis on a partial sale?
With the average-cost method, multiply the number of shares you sell by your average cost per share. That is the basis on the shares sold. Subtract it from the sale proceeds to get your capital gain. The shares you keep retain the same per-share basis for next time.
What tax rate applies to my gain?
If you held the shares more than a year, the gain is long-term and taxed at 0, 15, or 20 percent depending on your total income, often far below your ordinary rate. Held a year or less, it is short-term and taxed as ordinary income. This calculator uses the long-term rates.
Can I choose which shares to sell?
Yes, if your broker supports specific-lot identification. Selling the highest-cost lots first shrinks the gain and the tax, a tactic called tax-loss or tax-lot harvesting. This calculator uses your average cost, which is simpler and is the default at many brokers for mutual funds.
What is the basis of the shares I keep?
With average cost, each remaining share keeps the same per-share basis, so 700 shares bought at $40 still carry $28,000 of basis. That figure sets the gain when you eventually sell them, so it is worth tracking as you trim a position over time.