Calculator Methodology
What formulas our calculators use, and where the underlying figures come from.
Every calculator on The Tools Kit produces an estimate for planning purposes, not a substitute for a tax professional, accountant, or lender. This page explains the formula and data source behind each calculator, so you can judge how much to rely on the result. See our Editorial Policy for how we keep this content accurate over time.
Paycheck Calculator
Federal withholding uses IRS Publication 15-T withholding tables. Federal tax brackets and FICA (Social Security and Medicare) limits are the 2026 figures published in IRS Revenue Procedure 2025-32. State-level figures are approximate; confirm your state's exact withholding with your state's tax authority (for example, California's FTB or New York's DTF) before relying on the result for a real paycheck decision.
Sales Tax Calculator
State and local combined sales tax rates are sourced from the Tax Foundation's 2026 midyear update, using population-weighted average local rates. City-level rates are shown where available. These figures are approximate; sales tax rates can change and vary by exact location within a state, so verify the current rate with your state's revenue department before filing or invoicing based on this result.
Mortgage, personal loan, auto loan, and credit card payoff calculators
These use the standard fixed-rate amortization formula (monthly payment = principal times monthly interest rate times (1 + rate)number of payments, divided by ((1 + rate)number of payments minus 1)), the same formula used by most lenders to calculate a fixed-rate installment payment. They do not include lender-specific fees, points, PMI, or insurance unless you enter them yourself, and they assume a fixed rate for the full term. Your actual loan terms depend on your lender, credit profile, and the specific loan product.
Other calculators (BMI, calorie/TDEE, compound interest, tip, discount, percentage, age, time card, net worth, and similar)
These use widely published, standard formulas for their category, for example the Mifflin-St Jeor equation for calorie/TDEE estimates, or the standard compound interest formula. None of these calculators access a private or proprietary data source; each performs the calculation entirely in your browser from the numbers you enter. Where a calculator has a stated assumption or limitation (for example, a health calculator not accounting for individual medical conditions), that limitation is stated in the calculator's own FAQ section.
Something look wrong?
Tax brackets, FICA limits, and sales tax rates change over time. If you find a figure that is out of date or a formula that looks incorrect, tell us at thetoolskit.co@gmail.com and we will review and correct it.