The standard deduction is a fixed amount subtracted from your income before the federal tax brackets are applied. You can take it or itemise your actual deductible expenses, but not both, according to IRS Topic 551. Most people take the standard figure because their itemised total would be smaller.
The amount depends on your filing status. For 2026 it is $16,100 for a single filer or someone married filing separately, $32,200 for a married couple filing jointly and $24,150 for a head of household, as the IRS 2026 adjustments set out. It is higher if you are 65 or over or blind, and smaller if someone else can claim you as a dependant. A married person filing separately cannot take it at all if their spouse itemises.
The deduction lowers taxable income, not tax. A single filer on $75,000 is taxed on $58,900, and because the last dollars of that sit in the 22% bracket, the $16,100 saves far less than $16,100 of tax. Its value is the tax those dollars would otherwise have paid, which is why a higher earner gains more from the same deduction.
The UK personal allowance plays the same role, but it is set per person, offers no itemising alternative and is withdrawn above £100,000 of income. Many US states have a standard deduction of their own, with different amounts from the federal one.
The take-home pay calculator applies the federal standard deduction for the filing status you choose, and the state figure when you pick a state. It does not model itemising, so if your own deductions are larger your taxable income, and your tax, will be lower than the result.