Capital Gains Tax Calculator

Your gain does not start at zero

The most common misunderstanding about capital gains tax is that your gain gets its own set of brackets starting from zero. It does not. Your other income fills the lower brackets first, and the gain stacks on top of whatever is left.

On the default figures here — $75,000 of other income, single filer — the standard deduction leaves $58,900 of taxable income. The 0% long-term bracket ends at $49,450, so your other income has already used it up entirely. That means all $30,000 of gain lands in the 15% band, and the bill is $4,500.

Change the other income to zero and the same $50,000 gain produces a tax bill of $82.50. Almost all of it falls inside the 0% band, and only $550 spills into 15%. Same gain, same rates — a hundred times less tax, purely because of what sits underneath it.

The one-year line is worth more than any other decision

Hold an asset for one year or less and the gain is taxed as ordinary income. Hold it for a year and a day and it gets the preferential rates. Nothing else you do moves the bill as much as crossing that line.

These are the numbers at $150,000 of other income, single filer:

• $10,000 gain — $1,500 long-term against $2,400 short-term.
• $50,000 gain — $7,500 against $12,000.
• $100,000 gain — $16,900 against $28,470.
• $250,000 gain — $45,100 against $86,000.

At the largest size the difference is $40,900, which is more than most people save in a year. And it hinges on one day: the holding period runs from the day after you buy through the day you sell, so buying on 1 March 2025 means the earliest long-term sale is 2 March 2026.

This does not mean you should hold a losing position for tax reasons — a bad investment does not become good because the tax rate improves. But when you are already close to the line, waiting is usually the cheapest decision available.

The 0% bracket is the biggest planning opportunity

If your taxable income stays inside the 0% band, long-term gains are taxed at nothing at all. Because the band is generous — up to $49,450 for a single filer and $98,900 for a married couple — a low-income year turns into a nearly free window for selling appreciated assets.

Here is a $50,000 long-term gain at different levels of other income, single filer:

• No other income — $82.50 of tax, an effective rate of 0.17%.
• $20,000 of other income — $667.50, about 1.33%.
• $40,000 — $3,667.50, about 7.33%.
• $60,000 — $6,667.50, about 13.33%.
• $80,000 — $7,500, the full 15%.

The rate climbs gradually rather than jumping, because each extra dollar of other income pushes a little more of the gain out of the 0% band. That gradual slope is the whole mechanism, and it is why the effective rate on a gain is usually well below the headline rate people quote.

The practical version of this: if you have a year with unusually low income — early retirement, a sabbatical, a gap between jobs, a year you took losses — that is the year to realise gains. The tax rate on the same sale can differ by fifteen percentage points depending on which year you do it in.

The 3.8% that catches high earners

Above the thresholds, an additional 3.8% net investment income tax applies on top of the regular capital gains rate. It is calculated on the lesser of your net investment income or the amount by which your income exceeds the threshold — so it is not automatically 3.8% of everything.

The thresholds are $200,000 for single filers and $250,000 for joint filers, and they have not moved since the tax took effect in 2013. They are not adjusted for inflation, which means more taxpayers cross them every year even without a real increase in income.

At the top end the combination reaches 23.8%: the 20% long-term rate plus 3.8%. You can see it in the marginal rate output — at $500,000 of other income with a $200,000 gain, the last dollar of gain is taxed at 23.80%.

One consequence worth knowing: because the tax applies to the amount you exceed the threshold by, spreading a large sale across two tax years can keep more of it below the line. This calculator models a single year — run both years separately to see the difference.

Cost basis is the part you actually control

Your gain is the sale price minus your basis, and basis is where most of the avoidable tax lives. For assets bought in pieces — reinvested dividends, dividend reinvestment plans, stock acquired over years — the real basis is often higher than people assume, and every dollar of basis you cannot document is a dollar you pay 15% or 20% on unnecessarily.

It works the other way too. A $10,000 reduction in basis is not a $10,000 loss — it is roughly $1,500 to $2,380 of extra tax, depending on your bracket. That asymmetry is why keeping records matters more for tax than for any other purpose.

What this calculator leaves out

This is federal tax only, and several categories follow different rules entirely.

• State and local tax, which ranges from zero to well over 10% depending on where you live.
• Capital losses, which offset gains dollar for dollar. Up to $3,000 of net loss beyond that can offset ordinary income each year, with the rest carried forward.
• The wash-sale rule, which disallows a loss if you buy a substantially identical asset within 30 days before or after the sale.
• Collectibles, which are taxed at 28%, and qualified small business stock, which can be entirely exempt.
• Depreciation recapture on real estate, which is taxed at up to 25% and applies even to gain that would otherwise be excluded.
• The alternative minimum tax, credits, and any itemised deductions beyond the standard one.