Calculator Center

Calculation methodology & sources

Every calculator on this site was audited against primary publications. Each entry below lists the formulas used, the assumptions that bound the estimate, and the authoritative source we verified against.

How this audit works. Financial formulas follow disclosures published by the Consumer Financial Protection Bureau under Regulation Z. Health calculators follow the CDC and peer-reviewed clinical equations. Tax brackets come from the annual IRS Revenue Procedure that sets inflation-adjusted amounts. Each citation includes the date we most recently verified it.

Mortgage Calculator

Formulas

  • Loan = Home price − Down payment
  • M = P × [r(1+r)^n] ÷ [(1+r)^n − 1] where r = APR/12, n = years × 12
  • Total monthly = P&I + Tax/12 + Insurance/12 + HOA + PMI + Extra principal

Assumptions

  • Fixed interest rate for the full term.
  • Property tax, homeowners insurance, HOA dues, and PMI stay constant.
  • Extra principal is applied every month until payoff.
  • PMI is not automatically removed at 78% LTV — enter 0 once it should stop.

Sources

  1. How is my monthly mortgage payment calculated? — Consumer Financial Protection Bureau (CFPB). Verified 2026-07-22.
  2. Homeowners Protection Act (PMI cancellation at 78% LTV) — 12 U.S.C. § 4901 et seq.. Verified 2026-07-22.

Car Payment Calculator

Formulas

  • Financed = Price + Sales tax + Fees − Down − Trade-in + Trade-in owed − Rebate
  • M = P × [r(1+r)^n] ÷ [(1+r)^n − 1] where r = APR/12, n = term in months

Assumptions

  • Sales tax is applied to the vehicle price only (state rules vary).
  • Fixed APR; no prepayment penalty, balloon, or dealer add-ons after signing.
  • Trade-in reduces the financed amount; negative equity increases it.

Sources

  1. Auto loans: Understanding your loan and payments — Consumer Financial Protection Bureau (CFPB). Verified 2026-07-22.
  2. Buying a New Car — sales tax and fees — Federal Trade Commission. Verified 2026-07-22.

Loan Calculator

Formulas

  • M = P × [r(1+r)^n] ÷ [(1+r)^n − 1] where r = APR/12, n = months
  • Total interest = M × n − P

Assumptions

  • Fixed APR for the entire term.
  • Equal monthly installments; no origination fees or prepayment penalty.
  • APR is treated as the nominal annual rate compounded monthly.

Sources

  1. Truth in Lending Act (Regulation Z) — APR and payment disclosure — Consumer Financial Protection Bureau (CFPB). Verified 2026-07-22.

BMI Calculator

Formulas

  • Metric: BMI = weight(kg) ÷ height(m)²
  • US: BMI = 703 × weight(lb) ÷ height(in)²
  • Categories: <18.5 underweight, 18.5–24.9 healthy, 25.0–29.9 overweight, ≥30 obesity

Assumptions

  • Applies to adults age 20 and older.
  • Not appropriate for athletes with high muscle mass, pregnant people, or children (use CDC growth charts).
  • BMI is a screening tool, not a diagnosis of body fatness or health.

Sources

  1. About Adult BMI — U.S. Centers for Disease Control and Prevention (CDC). Verified 2026-07-22.
  2. Obesity: preventing and managing the global epidemic (WHO Technical Report 894) — World Health Organization. Verified 2026-07-22.

Calorie Calculator

Formulas

  • Mifflin–St Jeor BMR (kcal/day):
  • Male: 10 × kg + 6.25 × cm − 5 × age + 5
  • Female: 10 × kg + 6.25 × cm − 5 × age − 161
  • TDEE = BMR × activity factor (1.2 sedentary → 1.9 extremely active)
  • Weight change target: ≈ 3,500 kcal deficit/surplus per pound

Assumptions

  • Estimates only; individual metabolism varies by ~±10%.
  • Activity multipliers follow the Harris–Benedict-style categories widely used in dietetics.
  • 3,500 kcal ≈ 1 lb rule is a linear approximation; actual loss slows over time.

Sources

  1. Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241–247. — American Journal of Clinical Nutrition. Verified 2026-07-22.
  2. Dietary Guidelines for Americans, 2020–2025 — Appendix 1 (Estimated Calorie Needs) — U.S. Departments of Agriculture and Health and Human Services. Verified 2026-07-22.

Tax Calculator

Formulas

  • Taxable income = max(Gross income − Pre-tax deductions − Deduction, 0)
  • Tax = Σ (amount in bracket × bracket rate) over ordinary-income brackets
  • Net tax = max(Tax − Credits − Dependent credit, 0)
  • Effective rate = Net tax ÷ Gross income

Assumptions

  • Federal ordinary income tax only — no state income tax, FICA (Social Security/Medicare), AMT, NIIT, or self-employment tax.
  • No capital gains, qualified dividends, or Schedule-specific treatment.
  • Dependent credit is a flat illustrative $2,000 per dependent (real Child Tax Credit has phase-outs and refundability limits).
  • Standard deduction assumes no additional amounts for age 65+ or blindness.

Sources

  1. Rev. Proc. 2023-34 — 2024 inflation-adjusted brackets and standard deduction — Internal Revenue Service. Verified 2026-07-22.
  2. Rev. Proc. 2024-40 — 2025 inflation-adjusted brackets and standard deduction — Internal Revenue Service. Verified 2026-07-22.
  3. Publication 17 — Your Federal Income Tax — Internal Revenue Service. Verified 2026-07-22.

Retirement Calculator

Formulas

  • Monthly compounding: B_{t+1} = B_t × (1 + r/12) + contribution
  • Employer match added to monthly contribution as (income × match%) ÷ 12
  • Real balance = Nominal balance ÷ (1 + inflation)^years
  • Retirement income = Balance × safe withdrawal rate (default 4%)

Assumptions

  • Constant return, contribution, and inflation across all years (real markets vary).
  • Contributions are made at the end of each month.
  • The 4% safe withdrawal rate follows Bengen's 1994 historical study of 30-year retirements.
  • Tax treatment of withdrawals is not modeled.

Sources

  1. Compound Interest Calculator — methodology — U.S. SEC Office of Investor Education (Investor.gov). Verified 2026-07-22.
  2. Bengen WP. Determining Withdrawal Rates Using Historical Data. Journal of Financial Planning, October 1994. — Journal of Financial Planning. Verified 2026-07-22.

Age Calculator

Formulas

  • Age = full years, then remaining months, then remaining days between two dates.
  • Total days = ⌊(now − birth) ÷ 86,400,000 ms⌋

Assumptions

  • Uses the proleptic Gregorian calendar; leap years handled automatically.
  • Dates are interpreted in the browser's local timezone.

Sources

  1. ISO 8601-1:2019 — Date and time representations — International Organization for Standardization. Verified 2026-07-22.

Percentage Calculator

Formulas

  • X% of N = (X ÷ 100) × N
  • A is what % of B = (A ÷ B) × 100
  • % change from A to B = ((B − A) ÷ A) × 100
  • % difference = |A − B| ÷ ((A + B) ÷ 2) × 100

Assumptions

  • Undefined when the reference value is zero (division by zero).
  • Percent change is directional; percent difference is symmetric.

Sources

  1. NIST Guide to the SI, Chapter 7 — Rules for expressing values of quantities — U.S. National Institute of Standards and Technology. Verified 2026-07-22.

GPA Calculator

Formulas

  • Semester GPA = Σ (grade_points × credits) ÷ Σ credits
  • Cumulative GPA = (prior_GPA × prior_credits + new_points) ÷ total_credits

Assumptions

  • Uses the standard 4.0 unweighted scale (A/A+ = 4.0, A− = 3.7, … F = 0.0).
  • Weighted (Honors/AP/IB) and +/− policies vary by institution — check your school's registrar.

Sources

  1. How to Convert Your GPA to a 4.0 Scale — College Board. Verified 2026-07-22.
  2. State of College Admission — GPA reporting practices — National Association for College Admission Counseling (NACAC). Verified 2026-07-22.

Review schedule

Tax brackets and standard deductions are refreshed each year against the IRS Revenue Procedure that publishes inflation-adjusted amounts (typically released in November for the following tax year). BMI categories, the Mifflin–St Jeor equation, and amortization formulas are stable — we re-verify links and publisher pages at least annually.