Social Security tax is the larger half of FICA. You pay 6.2% of your wages and your employer pays a matching 6.2%, according to the IRS. The money funds the federal old age, survivors and disability insurance programme, which is why payroll systems often label the line OASDI.
Unlike Medicare, it has a ceiling. Only wages up to the wage base are taxed, and for 2026 that base is $184,500, so the most an employee can pay in the year is $11,439. Once your earnings for the year reach the base, the deduction stops and your pay rises for the rest of the year. On a salary of $240,000 paid evenly, that happens around the start of October.
If you have two jobs, each employer withholds up to the base without knowing about the other, so you can pay more than $11,439 in total. The excess is not lost: you claim it back as a credit on your income tax return, as IRS Topic 608 explains. On a joint return each spouse works out any excess separately.
The tax is not the same as your future benefit. What you pay goes to the programme as a whole, and the retirement benefit is worked out later from your earnings record. The retirement calculator treats Social Security income as something you subtract from the income your savings must provide.
The take-home pay calculator charges 6.2% of gross pay up to the base for the tax year you choose and lists the employer's match in the total cost to employer, so above the base you can see the effective rate fall.