Methodology & Sources

Every Calcyet calculator uses a published formula. When a law or a rate is involved, the figure comes straight from the official source. This page lists those sources and formulas, section by section. It also explains how we review pages and how we fix errors.

Principles

• Primary sources. Tax and payroll figures come from the IRS, the Social Security Administration and state revenue departments. They don't come from other websites.
• Show the work. Each page shows the formula and the steps with that page's numbers. So you can check the result by hand.
• State assumptions. Some calculators simplify. For example, one might assume a single filer with the standard deduction and no other income. When a calculator simplifies, the page says so.
• Round at the end. We keep full precision through each calculation. We round to cents only for display. Other tools often round in the middle. That's where most small differences come from.

Paycheck & taxes

Federal income tax uses the 2026 tax brackets and standard deductions from IRS Revenue Procedure 2025-32. For 2026 the standard deduction is $16,100 for single filers. It is $32,200 for married couples filing jointly. Stop on that number for a second: a single filer pays no federal income tax on the first $16,100. Tax is progressive. Each slice of taxable income is taxed at its own bracket rate. Then the slices are added together. In other words, a higher bracket only touches the dollars inside that bracket. Taxable income = gross income − pre-tax deductions − standard deduction.

FICA is two taxes. Social Security tax is 6.2% on wages up to the SSA wage base ($184,500 for 2026). Medicare tax is 1.45% on all wages. There is also an additional 0.9% Medicare tax on wages above $200,000 (single). Self-employment tax is 15.3% on 92.35% of net self-employment earnings. The Social Security part stops at the wage base. Half of self-employment tax is deductible for income tax. Bonus withholding uses the IRS flat supplemental rate of 22%. Supplemental wages over $1 million in a year are withheld at 37%.

State income tax uses each state's brackets, rates and standard deductions or exemptions. Those come from the state's department of revenue. Some states tax wages. Others don't. States with no wage income tax show $0. Local and city income taxes are not included unless a page says so.

Paycheck results estimate the tax owed on your wages. Your actual withholding is different. It depends on your W-4 and your employer's payroll system (IRS Publication 15-T). So the two can differ a little.

Salary & wages

Hourly-to-salary conversions use a full-time year: 40 hours × 52 weeks = 2,080 hours. You can enter different hours or weeks. Monthly pay = annual ÷ 12. Biweekly pay = annual ÷ 26. Weekly pay = annual ÷ 52.

Overtime uses the Fair Labor Standards Act rule. Hours over 40 in a workweek pay 1.5× the regular rate. Some employers and some states (such as California) require or offer double time. Where they do, the page shows double time too.

Mortgages and loans: the amortization formula

The monthly principal-and-interest payment on a fixed-rate loan is:

M = P × r(1 + r)^n ÷ ((1 + r)^n − 1)
where P = loan amount, r = annual rate ÷ 12, n = number of monthly payments. If the rate is 0%, M = P ÷ n.

Each month, interest = remaining balance × r. The rest of the payment reduces the principal. The amortization schedule repeats this until the balance reaches zero. That's all an amortization schedule is. Mortgage pages add property tax, homeowners insurance, PMI and HOA dues as separate monthly amounts. Example mortgage rates are based on the Freddie Mac Primary Mortgage Market Survey. Example auto loan and credit card rates are based on the Federal Reserve G.19 release. You can always enter your own rate. Affordability estimates use the debt-to-income ratios explained by the CFPB.

Credit card payoff runs the balance month by month. Interest = balance × APR ÷ 12. Then your payment is subtracted. This repeats until the balance is paid.

Saving & investing: compound interest

FV = P(1 + r)^n + C × ((1 + r)^n − 1) ÷ r
where P = starting balance, C = contribution per period, r = rate per period, n = number of periods.

Returns are hypothetical and constant. Real investments are not. They go up and down from year to year, and they can lose value. 401(k) pages use the IRS contribution limits for the year and the employer match you enter. Inflation-adjusted ("today's dollars") figures divide by (1 + inflation)^years. The inflation context comes from the BLS Consumer Price Index.

Percentages

X% of Y = Y × X ÷ 100. Percent change = (new − old) ÷ old × 100. Sale price = price × (1 − discount ÷ 100). Tips are calculated on the amount you enter. That amount can be before tax or after tax. It's your choice.

Dates & time

Date math uses the Gregorian calendar with leap years. A leap year is divisible by 4, except century years not divisible by 400. Business-day counts skip Saturdays and Sundays. Where a page says so, they also skip federal holidays from the OPM holiday schedule. Tax Day moves when April 15 falls on a weekend. It also moves when April 15 falls on Emancipation Day in Washington, D.C. Then Tax Day is the next business day (26 U.S.C. 7503). Generation ranges follow the Pew Research Center definitions.

Source list

Review and corrections process

1. Automated checks. Each section's math lives in one module. That module has automated checks against examples worked out by hand. The checks run whenever the code changes.
2. Annual updates. The IRS, the SSA and the states publish new figures for each tax year. When they do, we update the values and re-run the checks. Then we change the "Updated" date on the pages they touch.
3. Reader reports. Spot an error? Contact us. We check the report against the primary source. When the error is real, we fix the calculator and update the page.

Updated: October 2, 2026 · Sources and methodology