How our calculators work, which formulas they use, and where the data comes from. Last reviewed: January 2026.
All loan payment calculations use the standard amortization formula: M = P[r(1+r)ⁿ]/[(1+r)ⁿ−1] where P = principal, r = monthly interest rate, n = number of payments. This is the same formula used by banks and mortgage servicers. CFPB
Uses 2024 federal income tax brackets from the IRS. Does not include state taxes, the standard deduction, itemized deductions, or tax credits. Results are estimates only. IRS.gov Updated Jan 2026
Uses future value of annuity formula with monthly compounding. The 4% safe withdrawal rate is based on the Trinity Study (Cooley, Hubbard, Walz, 1998) and subsequent research. Trinity Study
Uses compound inflation formula with a user-specified rate. The US historical average of ~3% is based on CPI data from the Bureau of Labor Statistics going back to 1913. BLS.gov
Compares after-tax retirement value assuming tax-free Roth growth vs pre-tax Traditional growth taxed at withdrawal. Does not account for state taxes, required minimum distributions, or contribution limit phase-outs. IRS Publication 590
Uses the standard WHO formula: BMI = weight(kg) / height(m)². Imperial conversion: BMI = 703 × weight(lbs) / height(inches)². BMI categories follow WHO classifications (underweight <18.5, normal 18.5–24.9, overweight 25–29.9, obese ≥30). WHO
The Mifflin-St Jeor equation (1990) is used by default: Men: BMR = 10W + 6.25H − 5A + 5; Women: BMR = 10W + 6.25H − 5A − 161, where W = weight in kg, H = height in cm, A = age. Activity multipliers follow standard exercise physiology guidelines. Mifflin et al., 1990
Uses the U.S. Navy circumference method, validated for use in large populations. Not as accurate as DEXA scanning or hydrostatic weighing but widely accepted for general fitness assessment. U.S. Navy / Hodgdon & Beckett, 1984
Uses Naegele's Rule (LMP + 280 days) as the standard clinical calculation. Adjusted for cycle length variation from the 28-day baseline. Ultrasound dating is more accurate after 8–10 weeks. ACOG
Uses the Widmark formula: BAC = (A × 5.14 / W × r) − 0.015 × H, where A = alcohol in oz, W = body weight in lbs, r = gender constant (0.73 male, 0.66 female), H = hours drinking. This is an estimate — individual metabolism varies significantly. Widmark, 1932
Cap rate = Net Operating Income / Purchase Price. Cash-on-cash return = Annual Pre-Tax Cash Flow / Total Cash Invested. NOI excludes mortgage payments (it's a property metric, not a financing metric). CCIM Institute
Calculates minimum viable rate by grossing up desired net income for self-employment tax (15.3% SE tax + income tax rate), then dividing by actual billable hours. Industry benchmark: freelance rate should be 2–3× equivalent employee hourly rate. IRS SE Tax Guidelines
Driving emissions use EPA's 8.887 kg CO₂/gallon for gasoline. Flight emissions use ICAO average factors (short-haul: ~255g CO₂e/km per passenger; long-haul: higher radiative forcing multiplier applied). Dietary data from University of Oxford meta-analysis (Poore & Nemecek, 2018). EPA · ICAO · Oxford
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We review and update our calculators annually or when underlying regulations, formulas, or data sources change. If you believe a formula is incorrect or outdated, please contact us.