Methodology
How Calclumo calculates
Every formula behind the calculators, the published source it comes from, how often the underlying figures are revised, and where the estimates stop being reliable.
Last updated
Every number on this site comes from a published formula, not from a model we invented. This page lists which formula each calculator uses, where that formula was published, how often the figures behind it are revised, and — the part most calculator sites leave out — where the estimate stops being trustworthy.
If you find a result you believe is wrong, tell us with the inputs you used. Corrections are made against the source, and the page's last updated date moves when they are.
Principles
- A published formula, always. No calculator uses a coefficient we tuned ourselves. If a field needs a value with no standard source, the calculator asks you for it instead of guessing.
- Arithmetic runs in your browser. Nothing you type is sent to a server, so results do not depend on our uptime and your figures stay on your device.
- Full precision until display. Intermediate steps are not rounded; rounding happens once, at the point a number is shown. This is why a total may differ by a cent from adding up the rounded rows.
- Estimates are labelled as estimates. Health and money calculators produce ranges that depend on individual circumstances. None of them is advice, and each page says so.
Health and fitness
These carry the widest error bars on the site. Every equation here was fitted to a study population, and you are not that population — the formulas describe averages, and individual metabolism varies well beyond them.
| Calculator | Formula | Source |
|---|---|---|
| BMI | BMI = kg ÷ m², with the standard 18.5 / 25 / 30 category thresholds |
Quetelet index; thresholds per the World Health Organization |
| BMR | Mifflin-St Jeor by default, Harris-Benedict shown alongside for comparison | Mifflin et al., Am J Clin Nutr (1990); Harris & Benedict (1919), Roza-Shizgal revision (1984) |
| Calories / TDEE | TDEE = BMR × activity factor, using Mifflin-St Jeor for the BMR term |
Standard activity multipliers, 1.2 (sedentary) to 1.9 (very active) |
| Body fat | US Navy circumference method, logarithmic form | Hodgdon & Beckett, Naval Health Research Center (1984) |
| One rep max | Epley w × (1 + r/30), Brzycki w × 36/(37 − r), Lombardi w × r^0.10 |
Epley (1985); Brzycki (1993); Lombardi (1989) |
| Pregnancy due date | Naegele's rule — 280 days from the last menstrual period, adjusted for cycle length or transfer day | Naegele's rule, as used in routine obstetric dating |
Where these stop being reliable
- BMI does not measure body composition. It cannot tell muscle from fat, so it misclassifies athletes as overweight and can miss low muscle mass in older adults.
- BMR equations carry roughly ±10% error for an individual even when the inputs are exact, and the activity multiplier adds more error than the equation itself.
- The Navy body-fat method is sensitive to tape placement. A centimetre at the waist moves the result by about a percentage point.
- Due dates are estimates. Only about 4% of births occur on the calculated date; a clinical dating scan is more accurate than any formula.
Finance
The arithmetic here is exact — it is contract mathematics, not statistics. The uncertainty is entirely in the assumptions you supply: a rate, a term, a projected return.
| Calculator | Formula | Note |
|---|---|---|
| Loan payment, mortgage, car payment | Standard amortization, P × i ÷ (1 − (1 + i)^−n) |
Monthly compounding; extra payments applied to principal |
| Compound interest | A = P(1 + r/n)^(nt), plus contributions where entered |
Results are nominal — not adjusted for inflation |
| Simple interest | I = P × r × t |
No compounding, by definition |
| Credit card payoff | Monthly interest on the outstanding balance, payments applied after interest accrues | Assumes no new spending on the card |
| Take-home pay | US federal brackets applied marginally, Social Security at 6.2% to the wage cap, Medicare at 1.45% | Uses 2026 federal figures, including the $184,500 Social Security wage cap |
| Retirement | Compound growth on contributions at the return rate you enter | A projection under fixed assumptions, not a forecast |
Where these stop being reliable
- Take-home pay is an approximation of federal withholding. It does not model every credit, filing situation, or local tax, and it does not replace a payroll calculation or a tax professional.
- Lenders round differently. Your actual payment can differ by a small amount depending on day-count convention and fee treatment.
- Projections assume a constant rate of return. Real markets do not deliver one, and sequence of returns matters as much as the average.
Conversions, dates and geometry
These are exact by definition. Where a conversion factor is defined by international agreement, that exact factor is used rather than a rounded one.
| Area | Basis |
|---|---|
| Length, weight, general units | SI definitions and exact international factors — 1 inch = 25.4 mm, 1 lb = 0.45359237 kg, 1 US gallon = 3.785411784 L |
| Temperature | °F = °C × 9/5 + 32; K = °C + 273.15 |
| Area, volume | Standard plane and solid geometry; π to full double precision |
| Date difference, age | Proleptic Gregorian calendar with leap years; whole calendar days |
| Shoe sizes | Published manufacturer conversion tables — these are conventions, not standards, and differ by brand |
How often figures are revised
Most of this site is built on mathematics that does not change. A small part is built on figures that are revised on a schedule, and those are the ones worth dating.
| What | Revised | Currently reflects |
|---|---|---|
| US federal tax brackets, standard deduction, FICA caps | Annually, by the IRS | Tax year 2026 — IRS Rev. Proc. 2025-32 and the SSA wage base for 2026 |
| Health reference ranges (BMI bands, body-fat ranges) | Rarely; on guideline change | Current WHO categories |
| Unit conversion factors | Effectively never | Current SI definitions |
| Metabolic equations | Effectively never | As originally published |
Each calculator carries its own last updated date, which reflects the last change to that page's formula, copy, or figures — not a sitewide redeploy.
Corrections
If a result looks wrong, it may well be. Send the calculator, the exact inputs, the result you got and the result you expected to our contact page. Anything traced to a formula or a stale figure is fixed and the page is redated. Anything that turns out to be a documented limitation gets written into this page, so the next person reading it knows before they rely on the number.