Pre-Tax vs Post-Tax Deduction Calculator
See how pre-tax deductions like 401k and medical premiums lower your taxable income and change your net take home pay.
This calculator runs entirely in your browser. Nothing you type is uploaded or saved anywhere.
How to use this tool
Enter your gross pay for a single pay period, then list your pre-tax deductions: 401k or other retirement contributions, your share of medical premiums, and anything else taken out before taxes such as HSA, FSA, or dental. Add your combined tax rate (federal, state, and FICA together as a rough percentage), then any post-tax deductions like a Roth 401k contribution or life insurance. Click Calculate to see your taxable income, the estimated tax, and your net take home pay, along with a clear breakdown of where each dollar goes.
How pre-tax deductions lower your taxes
Estimated tax = Taxable income x Tax rate
Net take home = Taxable income - Estimated tax - Post-tax deductions
The key idea is that pre-tax deductions are subtracted from your gross pay before tax is calculated, so they shrink the amount you are taxed on. A post-tax deduction comes out after tax has already been applied, so it does not reduce your tax at all. That is why putting a dollar into a pre-tax 401k or paying a medical premium pre-tax costs you less than a dollar of take home pay. The higher your tax rate, the bigger the savings on each pre-tax dollar.
A real example
Say your gross pay is $3,000 per period. You put $200 into a pre-tax 401k and pay a $150 medical premium pre-tax, for $350 of pre-tax deductions. Your taxable income drops to $2,650. At a combined 22% rate, the estimated tax is $583. After a $50 post-tax Roth contribution, your net take home is $2,650 - $583 - $50 = $2,017. Without those pre-tax deductions, tax would have been $3,000 x 22% = $660, so the $350 in pre-tax deductions saved you $77 in tax. In effect, $350 of benefits only reduced your paycheck by $273.
Common questions
What counts as a pre-tax deduction?
Common pre-tax deductions include traditional 401k and 403b contributions, your share of employer medical, dental, and vision premiums, HSA and FSA contributions, and some commuter benefits. These come out of your pay before income tax is calculated, which is why they lower your taxable income.
How is the 401k pre-tax reduction figured?
A traditional 401k contribution is subtracted from gross pay before tax. So a $200 contribution at a 22% tax rate only reduces your take home by about $156, because you would have paid roughly $44 in tax on that money anyway. The tool shows this effect in the breakdown.
Does a medical premium really change my paycheck impact?
Yes. When your medical premium is paid pre-tax, it lowers your taxable income, so part of the premium cost is offset by lower taxes. The net hit to your take home pay is the premium minus the tax you no longer owe on that amount.
Why are post-tax deductions treated differently?
Post-tax deductions such as Roth 401k contributions, wage garnishments, or after-tax life insurance are taken out after tax is calculated. They reduce your net pay dollar for dollar but give you no tax reduction in the current period.
Is this calculator exact?
No. It uses one combined tax rate to estimate the benefit impact on your net take home, so results are an educational estimate, not exact payroll math. Real paychecks use separate federal, state, Social Security, and Medicare calculations with brackets and caps. Treat this as a planning guide, not tax or financial advice.