How Our Calculators Work
ProCalculatorLab explains what each result means, how it is calculated, and when it may differ from a real-world figure. Use the assumptions below to choose the right tool for your question.
Methods and Sources
- APR Calculator
Estimates nominal annual APR from regular monthly installment payments and user-entered qualifying finance charges. It models withheld or financed charges but does not classify fees or produce a certified disclosure.
References on the calculator page: Consumer Financial Protection Bureau.
- 401(k) Retirement Calculator
Projects annual balance growth, modeled fees, employee contributions, and employer matching. It uses 2026 IRS limits as a baseline and treats future limits and returns as assumptions. Contributions are added at year-end.
References on the calculator page: Internal Revenue Service.
- Loan Calculator
Uses the stated annual interest rate divided by 12 to amortize a fixed-rate loan. A withheld origination fee reduces proceeds, not the amount on which scheduled payments are calculated. The stated rate is not APR.
References on the calculator page: OpenStax, Rice University; Consumer Financial Protection Bureau.
- Credit Card Payoff Calculator
Estimates interest and payoff time from fixed monthly payments, including optional extra payments. It uses APR divided by 12 as a monthly approximation and excludes new purchases and fees.
References on the calculator page: Consumer Financial Protection Bureau; Financial Consumer Agency of Canada.
- Bra Size Calculator
Uses underbust and bust measurements to estimate a starting band and cup size in the selected system and show regional equivalents. Sizing and fit vary by brand; this is not a medical assessment.
The calculator page documents the sizing method used.
- Mortgage Calculator
Estimates fixed-rate payments and amortization. The US model uses a monthly rate; the Canadian model converts a nominal rate compounded semiannually to the payment period. Housing costs and eligible mortgage insurance are modeled separately as described on the calculator.
References on the calculator page: Financial Consumer Agency of Canada; Canada Mortgage and Housing Corporation.
- Salary / Take-Home Pay Calculator
Estimates 2026 US federal withholding from pay and W-4 settings, with Social Security and Medicare where applicable. It does not calculate state or local taxes or final tax liability.
References on the calculator page: Internal Revenue Service; Social Security Administration.
- Auto Loan / Car Payment Calculator
Amortizes a fixed-rate balance over monthly payments. Down payment, trade-in equity, fees, and sales tax affect the amount financed according to the selected options and the entered rate.
References on the calculator page: Consumer Financial Protection Bureau; Federal Trade Commission; Financial Consumer Agency of Canada.
- Compound Interest Calculator
Projects compound growth at the selected frequency. Optional fixed monthly contributions are supported only with monthly compounding. The entered rate is an assumption, not a forecast.
References on the calculator page: SEC Investor.gov; Consumer Financial Protection Bureau; Board of Governors of the Federal Reserve System.
- Debt-to-Income Ratio Calculator
Divides monthly housing costs or total included monthly debt by gross monthly income to estimate front-end or back-end ratios. It does not determine eligibility or approval.
References on the calculator page: Consumer Financial Protection Bureau; Fannie Mae; Freddie Mac; U.S. Department of Housing and Urban Development.
- Age Calculator
Counts completed calendar years and months, then remaining days between two dates. It uses date-only Gregorian calendar rules, not birth time, time zones, or legal age rules.
References on the calculator page: U.S. Naval Observatory; International Organization for Standardization; NASA Planetary Data System.
- Sales Tax / GST-HST Calculator
Adds tax to a price or divides a tax-inclusive total by one plus the entered rate. US and Canadian labels do not look up rates, determine exemptions, or convert currencies.
References on the calculator page: Canada Revenue Agency; Revenu Québec; Province of British Columbia; California Department of Tax and Fee Administration; Washington Department of Revenue.
- Income Tax Calculator
Uses 2026 US federal brackets, filing status, and supported deductions and credits to estimate federal income tax. It does not prepare a return or cover state or local tax.
References on the calculator page: Internal Revenue Service.
Testing and Limits
Automated tests check worked examples, boundary cases, and rounding where relevant. Individual calculator pages describe their applicable rules, known limits, and review dates. A review date is not a guarantee that every later rule change is included.
Calculators run in your browser and provide estimates for education and planning. See the Disclaimer and Privacy Policy.
Reporting an Issue
See Publisher Information and editorial standards, or send the page link and a reproducible example to support@procalculatorlab.com. Use sample values rather than personal information.