The order deductions come out
Payroll processes in a specific sequence, and it matters. Pre-tax deductions — traditional 401(k), health premiums, HSA and FSA — come out first and reduce the income that income tax is calculated on. FICA taxes (Social Security and Medicare) are then applied, though health and HSA amounts also escape FICA while 401(k) contributions do not. Income tax is calculated on what remains, and post-tax deductions such as Roth contributions come out last.
Why pre-tax benefits are worth more than they look
A $1,500 HSA contribution in a 24% federal and 5% state bracket avoids roughly $435 of income tax plus about $115 of FICA — an effective discount of over a third. HSAs are unique in being triple tax advantaged: deductible going in, tax-free growth, and tax-free withdrawals for qualified medical costs.
Withholding is an estimate, not the tax
Your employer withholds based on the W-4 you filed. It is a projection, and it is frequently wrong — particularly for two-earner households, people with bonus income, or anyone whose situation changed mid-year. A large refund means you lent the government money interest-free; a large bill means you underpaid. Adjusting the W-4 mid-year fixes both.
Bonuses and the supplemental rate
Bonuses are commonly withheld at a flat supplemental rate, which is often higher than your actual effective rate. The bonus is not taxed at a special rate — the excess simply comes back at filing. It is a withholding artefact, not a penalty.
Getting the estimate closer
Look at last year's tax return: divide total tax by total income to get your true effective federal rate, and do the same for state. Entering those figures here will produce a much more accurate take-home number than guessing from bracket tables. This calculator is an estimate and does not model credits, itemised deductions or local payroll taxes.