An estimate is worth having and it is worth knowing what it is. A capital gains estimator takes a handful of figures, applies a published table and returns a number that is right to within whatever its assumptions are worth. Three of those assumptions are almost always made silently, and each of them can move the answer by more than the difference between the rate bands.
Assumption one: that your basis is what you paid
Every estimator asks for a purchase price and treats it as basis. On shares that is usually close enough. On anything improved, inherited, gifted or rented it is not: improvements raise basis, depreciation allowed or allowable on a rental lowers it, and property acquired by gift or from a decedent follows its own rules, which the IRS flags in the same breath as the holding period. The gain is measured from adjusted basis, so an estimator that has not asked about any of this is answering a simpler question than the one you asked.
Assumption two: that your income is your salary
The published bands are thresholds of taxable income, not of gross pay. The IRS puts the 0% band at taxable income 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, with a rate of no higher than 15% for most individuals above it and 20% above the 15% band. Taxable income is the figure after deductions, so entering a gross salary systematically places you in a higher band than the one you are actually in.
Assumption three: that the rate table is the whole charge
Two things sit outside it. Losses net against gains before any rate applies, and where losses exceed gains the excess deductible against ordinary income is the lesser of $3,000, or $1,500 if married filing separately, or your total net loss, with the rest carried forward. And a 3.8 percent Net Investment Income Tax applies on the lesser of net investment income or the amount by which modified adjusted gross income exceeds $200,000, $250,000 or $125,000 by filing status. An estimator that stops at the table understates a large gain by that 3.8 percent.
And the state, which no national estimate includes
State treatment is not a modifier on the federal number, it is a second computation with its own rules. California publishes that it does not have a lower rate for capital gains and that all are taxed as ordinary income. Washington charges a 7% tax on the sale or exchange of long-term capital assets and no income tax at all. Texas's constitution forbids a tax on the realized or unrealized capital gains of an individual, family, estate or trust. Each of those is quoted on the jurisdiction record here with the page and the date it was read.
Questions people ask about capital gains tax estimator
How accurate is an online estimate?
As accurate as its three silent assumptions: that your basis is the purchase price, that your income figure is taxable income, and that the rate table is the whole charge. Where any of those is wrong for you, so is the number.
What is the one input worth getting right?
Adjusted basis. It is the figure the gain is measured from, and on anything improved, inherited or rented it is not what you paid.
Should an estimate include state tax?
It should, but a national estimator cannot, because states differ completely: some tax gains as ordinary income, one charges a separate excise, and two bar the tax in their constitutions.
When is an estimate not good enough?
When the disposal is large enough that the 3.8 percent surtax or a band boundary is in play, or when basis is uncertain. That is the point at which the enquiry form puts your own facts to an adviser.