How the math gets done

Loan / Payment Calculator

Standard amortization formula as used by every US bank, credit union, and dealer F&I office.

M = P · r·(1+r)^n / ((1+r)^n − 1)

Where P is the financed amount (price + tax + fees − down − trade-in), r is APR/12/100, and n is the term in months. We special-case r = 0 to avoid a divide-by-zero on promo financing. Source: standard textbook formula (e.g. Fundamentals of Corporate Finance, Ross/Westerfield/Jordan, ch. 6).

Wind Chill Calculator

National Weather Service 2001 wind-chill model, the standard adopted by the NWS and Environment Canada after the older Siple-Passel formula was retired.

Imperial (°F, mph):
WC = 35.74 + 0.6215·T − 35.75·V^0.16 + 0.4275·T·V^0.16

Calibrated for T ≤ 50 °F and V ≥ 3 mph. Outside that range we still display a number but flag it as a reference value only. Source: NWS public documentation (weather.gov/safety/cold-wind-chill-chart).

Tire Size Calculator

Pure geometry from the metric tire-size spec WIDTH/ASPECT-RIM:

Overall diameter (mm) = Rim × 25.4 + 2 × (Width × Aspect/100)
Speedometer error %   = (D_new − D_old) / D_old × 100

Real tires deviate ±1% from this mathematical aspect in practice (tire manufacturers publish their own measured section heights). Source: ETRTO / Tire and Rim Association standards.

Chain Length Calculator

Standard ANSI roller-chain length approximation, identical to the formula used by every major chain manufacturer’s sizing chart:

L = 2·(C/p) + (F+R)/2 + ((R−F)/(2π))² · (p/C)

Result is rounded up to the nearest even number of links so the master link fits. Source: ANSI B29.1 (roller chain), DID / RK manufacturer sizing tables.

Gear Ratio Calculator

Drivetrain multiplication chain:

Overall = Primary × Gearbox(gear) × (Rear / Front)
Speed   = (Engine RPM / Overall) × tire circumference

Static-tire assumption — real tires grow 1–2% of their diameter at high RPM, so the calculator under-reports theoretical top speed by the same percent. Acceptable for street use; racers should add a 1–2% margin. Source: standard powertrain mechanics; cross-checked against published OEM gearing tables.

Unit tests

Every formula above is covered by automated unit tests that run on every commit. Test cases include published reference points (e.g. NWS wind-chill chart, textbook amortization examples). Test suite is open-source in the project repository.

Last reviewed: June 4, 2026. Methodology page is reviewed quarterly or whenever a formula changes. Spot an error or have a source to cite? Email us — corrections ship the same week.