Every capital gains estimator on the internet asks for the same five things, and the reason they disagree with each other is never the table of rates, which is published and identical everywhere. It is what they do with the inputs. This page sets out what those five inputs are, which of them you are most likely to get wrong, and what the IRS's own published figures do with them once they are right.
The five inputs, and the two that go wrong
An estimate needs the purchase amount, the sale amount, the two dates, your filing status and your other taxable income for the year. The two that go wrong are the first and the last. The purchase amount is not what you paid: it is adjusted basis, which rises with capital improvements and, on a rental, falls by the depreciation allowed or allowable. Other taxable income is not your salary: it is taxable income after deductions, because that is the figure the published bands are measured against, and being a few thousand dollars out moves you between bands.
What the published table then does
The IRS publishes the long-term scale directly: the tax rate on most net capital gain is no higher than 15% for most individuals, with a 0% rate where taxable income is at or below $48,350 for single filers and married filing separately, $96,700 for married filing jointly and qualifying surviving spouse, and $64,750 for head of household, and a 20% rate above the 15% band. A short-term gain is not on this table at all: held one year or less, it is taxed with your ordinary income. That is the whole of the federal rate logic, and any estimator producing a different shape of answer has made an assumption it has not told you about.
The two adjustments an estimate usually omits
The first is loss netting. Losses come off gains before the rate, and an excess loss is deductible against ordinary income up to the lesser of $3,000, or $1,500 if married filing separately, or the total net loss, with the rest carried forward. The second is the Net Investment Income Tax: a further 3.8 percent on the lesser of net investment income, which includes capital gains, or the amount by which modified adjusted gross income exceeds $200,000 single or head of household, $250,000 married filing jointly, or $125,000 married filing separately. An estimator that stops at the rate table is understating a large gain by that 3.8 percent.
Then add the state, which no federal estimator knows
A national estimate is a federal estimate. California publishes that it does not have a lower rate for capital gains and that all are taxed as ordinary income. Washington charges 7% on the sale or exchange of long-term capital assets and nothing by way of income tax. New Jersey states that a resident's capital gains are all subject to tax except gains from the sale of exempt obligations. The record on this site holds each of those as a quotation with the page and the date it was read.
Questions people ask about tax on capital gains calculator
Why do two calculators give me different numbers?
Almost never because of the rates, which are published and the same everywhere. It is usually adjusted basis, or the taxable income figure the bands are measured against, or that one of them omits the 3.8 percent Net Investment Income Tax.
What income figure should I enter?
Taxable income, the figure after deductions, because that is what the IRS's published thresholds for the 0%, 15% and 20% bands are measured against. Gross salary will put you in the wrong band.
Does a short-term gain have its own rate?
No. The IRS draws the line at more than one year of holding for long-term treatment. A short-term gain is taxed alongside your ordinary income, with no separate scale.
Should I trust an estimate for a filing?
Use it to know the order of magnitude. The figures here are the authorities' own published ones, but applying them to your basis, your dates and your state is what a CPA is paid to do, and that is what the enquiry form is for.