Methodology & Sources

Length, Weight, Area, Volume, Speed, Time, Pressure, Energy

Every conversion factor for these categories is derived from the International System of Units (SI) as defined by the Bureau International des Poids et Mesures (BIPM), with U.S. customary and imperial equivalents cross-referenced against NIST Special Publication 811 and NIST Handbook 44. The base unit per category (meter, kilogram, second, square meter, cubic meter, pascal, joule) is the SI base or derived unit, and all other units in the category convert to and from that base using exact, deterministic factors.

Where the U.S. survey foot and international foot differ (since the U.S. survey foot was retired on January 1, 2023), Calcflux uses the international foot exactly (1 ft = 0.3048 m).

• BIPM — The International System of Units (SI Brochure), 9th edition — Defines the seven SI base units, derived units, and prefixes. Updated through 2025.
• NIST Special Publication 811 — Guide for the Use of the International System of Units — Conversion factors and style rules for SI usage in the United States.
• NIST Handbook 44 — Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices — U.S. legal metrology reference adopted by all 50 states for weighing and measuring devices.
• BIPM — Measurement Units — Authoritative entry point for the SI as maintained by the BIPM after the 2019 redefinition.

Temperature

Temperature is non-linear across scales, so Calcflux uses explicit conversion formulas rather than multiplicative factors. Celsius is the internal base; Fahrenheit, Kelvin, Rankine, Réaumur, Newton, and Delisle all convert through Celsius. The defining relationships (0 °C = 273.15 K, 0 °C = 32 °F, −40 °C = −40 °F) come from the SI specification.

• BIPM — SI Brochure (kelvin definition) — Defines the kelvin via the Boltzmann constant and the Celsius scale offset of 273.15 K.
• NIST Special Publication 811 — Lists the official conversion relationships between Celsius, Fahrenheit, kelvin, and Rankine.

Cooking — Ingredient Densities

Cooking conversions route volume through ingredient density before converting to weight, because 1 cup of flour and 1 cup of honey weigh very different amounts. Densities for the 60+ ingredients Calcflux supports are derived from USDA FoodData Central reference values, cross-checked against published professional baking references where the USDA entry is ambiguous.

U.S. cup, tablespoon, and teaspoon definitions use the exact U.S. customary cup of 236.5882365 mL (1/16 U.S. gallon). The FDA-style 240 mL “legal cup” used on U.S. nutrition labels is approximately 1.4% larger; consumers cross-checking gram totals against nutrition labels should account for this difference.

• USDA FoodData Central — USDA Agricultural Research Service food composition database used as the primary source for ingredient mass-per-volume reference data.

Digital Storage and Data Transfer

Calcflux distinguishes decimal prefixes (kB, MB, GB, TB — powers of ten) from binary prefixes (KiB, MiB, GiB, TiB — powers of two) per IEC 80000-13 and IEEE 1541-2021 (superseding IEEE 1541-2002). Hard drive manufacturers and network bandwidth specs use decimal; operating systems and memory modules typically use binary. Both are supported as distinct units.

• NIST — Prefixes for binary multiples — Official NIST reference listing kibi, mebi, gibi, tebi, pebi, exbi prefixes and their exact decimal equivalents.
• IEEE 1541-2021 — Prefixes for Binary Multiples (Wikipedia overview, superseding IEEE 1541-2002) — IEEE standard establishing the binary prefix nomenclature (kibi, mebi, gibi, tebi) in digital electronics and computing. The IEEE catalog page itself is paywalled; the Wikipedia article summarizes the standard and its supersession.

Fuel Economy

MPG (US), MPG (imperial UK), L/100 km, and km/L are non-linear with respect to each other — they are reciprocal relationships, not proportional ones. Calcflux uses exact formula conversions rather than interpolated tables. The US gallon (3.785411784 L) and the imperial gallon (4.54609 L) differ, which is why UK MPG ratings are higher than US MPG ratings for the same vehicle.

• fueleconomy.gov — US DOE/EPA — Official U.S. government source for vehicle MPG ratings, jointly administered by the Department of Energy and the EPA.

Mortgage and Loan Amortization

Mortgage and loan calculators use the standard amortization formula: monthly payment = P × (r(1+r)ⁿ) / ((1+r)ⁿ − 1), where P is principal, r is the monthly interest rate, and n is the term in months. The amortization schedule is computed iteratively per period so extra-payment effects on payoff date and total interest are reflected accurately. The 20% LTV PMI cutoff under U.S. mortgage convention is noted in the calculator UI so users see when PMI may apply; PMI is not itself added to the computed monthly payment.

Results are estimates for educational purposes. Closing costs, escrow, property taxes, and insurance vary widely by jurisdiction and lender; consult your Loan Estimate disclosure for binding numbers.

• CFPB — Understanding Your Loan Estimate — Consumer Financial Protection Bureau explainer for the federally required mortgage Loan Estimate disclosure form.

Body Mass Index (BMI)

BMI is computed as mass (kg) ÷ height (m)². Calcflux uses the WHO/CDC adult category cutoffs: underweight below 18.5, healthy weight 18.5–24.9, overweight 25.0–29.9, and obesity 30.0 or greater. These cutoffs were standardized for population health use by the WHO and adopted by the CDC for U.S. adult screening.

BMI is a screening tool, not a diagnosis. It does not distinguish muscle from fat and is not appropriate for athletes, pregnant individuals, or children (who use age- and sex-specific percentiles).

• CDC — Adult BMI Calculator — CDC's official adult BMI categories: underweight (<18.5), healthy (18.5–24.9), overweight (25.0–29.9), obesity (≥30.0).
• WHO — Obesity and overweight fact sheet — WHO definitions: overweight BMI ≥ 25, obesity BMI ≥ 30 for adults.

BMR and TDEE (Calorie Calculator)

Calcflux uses the Mifflin-St Jeor equation, which is the standard reference for resting energy expenditure in healthy individuals and consistently outperforms older equations (Harris-Benedict, Schofield) when validated against indirect calorimetry. TDEE applies activity-level multipliers to BMR using the conventional sedentary (1.2) through extra-active (1.9) scale.

• Mifflin MD, St Jeor ST, et al. (1990) — A new predictive equation for resting energy expenditure in healthy individuals — American Journal of Clinical Nutrition 51(2):241–7. Original derivation of the Mifflin-St Jeor equation from 498 healthy subjects.

Compound Interest

The compound interest engine uses the standard actuarial formula A = P(1 + r/n)^nt, with separate handling for periodic contributions (annuity). Compounding frequency options match the SEC Investor.gov calculator (annually, semiannually, quarterly, monthly, daily). Results are pre-tax; actual returns vary with fees, taxes, and market conditions.

• SEC Investor.gov — Compound Interest Calculator — U.S. Securities and Exchange Commission consumer-facing calculator; defines the input set and compounding frequencies replicated here.
• SEC Investor.gov — Compound Interest glossary entry — Formal SEC definition: interest paid on principal and on accumulated interest.

Tip Calculator

There is no government standard for gratuity. Calcflux's defaults (15–20% pre-tax for sit-down restaurants) match the most widely cited U.S. etiquette guidance. The calculator simply computes the indicated percentage and any bill-splitting requested; users should adjust to local norms.

• The Emily Post Institute — General Tipping Guide — Widely referenced U.S. etiquette guidance: 15–20% pre-tax for sit-down wait service, additional categories for bar, delivery, and valet.

Date Math and Age Calculation

Date inputs follow the ISO 8601 representation (YYYY-MM-DD). Age and date-difference calculations operate on the proleptic Gregorian calendar and count full elapsed periods: a person born on 1990-03-15 turns 35 on 2025-03-15, not on the eve. Leap years are handled by the underlying date-fns library, which is itself ISO 8601 compliant.

• Wikipedia — ISO 8601 (overview of the standard) — Practical overview of ISO 8601-1:2019 and ISO 8601-2:2019, which define the international date and time representation Calcflux follows. The ISO catalog page itself is paywalled.