Methodology
Last updated: May 28, 2026
Every formula on UnitPlanet is derived from a named authoritative source. This page describes the standard we apply to each category and links to the source documents. Archive URLs are included so you can verify the source even if the original page changes.
Unit conversions
Standard unit conversions (length, mass, volume, area, temperature, speed) use exact SI and US customary definitions as published by NIST SP 811 (Guide for the Use of the International System of Units). Conversions are performed at full floating-point precision; rounding occurs only at display.
Cooking conversions
Volume conversions use the US customary system as defined by NIST (National Institute of Standards and Technology). 1 US cup = 236.588 ml; 1 US tablespoon = 14.787 ml; 1 US fluid ounce = 29.574 ml. Oven temperature conversions use the exact Fahrenheit/Celsius formula (°C = (°F − 32) × 5/9) with no rounding at the formula level; rounding occurs only at display.
Weight conversions for cooking ingredients (oz ↔ grams) use the international avoirdupois ounce: 1 oz = 28.3495 g.
Parenting
Clothing and shoe size conversions are drawn from published ISO and manufacturer sizing tables.
Home measurements
Length and area conversions use exact SI / US customary definitions: 1 inch = 2.54 cm (exact); 1 foot = 30.48 cm; 1 meter = 3.28084 ft. Paint coverage estimates use an industry-standard coverage rate of 350–400 sq ft per gallon, presented as a range with explanation. This figure reflects the typical spread rate published by major US paint manufacturers for interior flat and eggshell finishes on previously painted, prepared surfaces; actual coverage varies with surface texture, porosity, and application method.
Money math
Compound interest uses A = P(1 + r/n)^(nt) extended for periodic contributions, per Investor.gov (U.S. SEC). Loan and mortgage monthly payments use the standard amortization formula M = P × [r(1+r)^n] / [(1+r)^n − 1], cited from the Consumer Financial Protection Bureau and Investopedia. APR/APY conversions use APY = (1 + APR/n)^n − 1 and its inverse, per the CFPB. Calculations are performed at floating-point precision; rounding occurs only at display. All tools in this category render a MoneyMathDisclaimer — they present math, not financial advice.
Fitness tools
The weight converter (kg, lbs, stone) uses NIST SP 811 exact factors: 1 lb = 0.45359237 kg; 1 stone = 14 lb. The running pace calculator applies standard arithmetic (pace = time ÷ distance) with no external conversion constant. BMI uses WHO = weight(kg) / height(m)², cited from WHO Technical Report Series 894 (2000); the imperial variant uses the published 703 scaling factor. 1-rep max estimates use the Epley (1985) formula weight × (1 + reps/30) and the Brzycki (1993) formula weight × (36 / (37 − reps)), cited from the original published papers. Plate math uses a greedy algorithm (heaviest plate first) applied to standard Olympic plate denominations; bar weight defaults follow IWF specifications (20 kg / 45 lb). Body fat estimation uses the US Navy circumference method published by Hodgdon and Beckett (1984, Naval Health Research Center Report 84-29): metric log₁₀ regression on height, waist, neck, and hip (women only) circumferences. All fitness tools render a FitnessDisclaimer — they present reference measurements, not health advice.
Pets tools
Dog age equivalences use the size-adjusted human-year table from the AAHA Canine Life Stage Guidelines (2019), grouped into five size classes (toy, small, medium, large, giant) by adult body weight. The Wang et al. (2020) epigenetic formula (human_age = 16 × ln(dog_age) + 31, Cell Systems) is described in the tool content as a research finding for a single breed. Cat age equivalences use the AAFP/AAHA Feline Life Stage step-function: Year 1 = 15 human years, Year 2 = +9, each subsequent year = +4. Pet weight uses NIST SP 811 exact factors: 1 lb = 0.45359237 kg; 1 oz = 0.028349523125 kg. Aquarium volume uses the exact US gallon definition of 231 cubic inches (NIST HB 44) for rectangular tanks and the standard cylinder formula (π × r² × h) for cylindrical tanks. Aquarium substrate weight uses empirical dry bulk density values from Aquarium Co-Op reference guidelines (~0.058 lb/in³ for gravel, ~0.052 lb/in³ for fine sand, ~0.035 lb/in³ for planted substrate). All pets tools render a PetsDisclaimer — they present reference calculations, not veterinary advice.
Source archival
Every source URL in our tool configs ships with a Wayback Machine archive URL and a retrieval date. If a source moves or goes offline, the archived version documents what we relied on at the time of writing.