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    How we build and test our calculators

    A calculator is only useful if you can trust the number it gives you. This page explains where our formulas come from, how we check them, and what we do when we get something wrong.

    Formulas & sources

    Every calculator implements a published, standard method for its field rather than a rule of thumb we made up — for example the Mifflin-St Jeor equation for resting energy needs, the U.S. Navy circumference method for body fat, Naegele's rule (last period + 280 days) for due dates, and plain geometry plus manufacturer coverage figures for construction estimates. Each page explains its formula in plain language, including the constants it uses, so you can check our work.

    Where a result could affect your health or money, the page lists its sources, with links to the original papers or official documents, and shows the date it was last reviewed. These calculators currently cite their sources:

    How we test

    The math behind every calculator lives in a small, separate module with its own automated test suite, which runs before every release. Tests cover worked examples from the original sources, boundary values (a pregnancy at exactly 14 weeks, a roof of exactly one square, a leap-day birthday), zero, blank and negative inputs, very large numbers, and unit conversions in both directions. When a formula has an independent alternative form — like the inch and centimetre versions of the Navy body-fat equations — we test one against the other.

    Results that involve counting things you buy — bags, boxes, bundles, gallons — are rounded up, because you can't buy part of a bag. Everything else is rounded only for display, never in the middle of a calculation.

    Showing the math

    Most calculators have a Show the math panel under the result. It plugs the numbers you entered into the formula, step by step, so you can follow exactly how the answer was reached — or spot an input you mistyped.

    Updates & dates

    The "Updated" date on a calculator changes only when its formula, inputs, or guidance change — not when we tweak the page design. We review cited pages when the underlying guidance is revised, and any change that alters results is recorded in the corrections log below.

    Your data

    Calculations run entirely in your browser. The numbers you type aren't sent to our servers; they only appear in the page address if you choose to copy a shareable link. Your recently used calculators and your metric/imperial preference are remembered in your own browser's storage, and you can clear them at any time. See the privacy policy for analytics and cookies.

    What calculators can't do

    Formulas describe averages. A BMR equation can be off by 10% for a particular person, a tape-measure body-fat estimate isn't a DEXA scan, and a concrete estimate doesn't know your site is out of level. Health results aren't medical advice, and financial results aren't financial advice — confirm anything important with a qualified professional. Our disclaimer covers this in full.

    Corrections log

    When a calculator gave a wrong result, we fix it, add a test so it can't come back, and record it here. If you think something is off, please tell us with the inputs you used.

    1. Pregnancy & Due Date calculators

      Trimester boundaries were one week early: week 13 was labelled second trimester and week 27 third. They now follow ACOG — first trimester through 13 weeks 6 days, second from 14w0d to 27w6d, third from 28 weeks.

    2. Calorie Calculator

      The weight-loss target had no lower limit and could suggest as little as about 550 kcal a day. It now never goes below the commonly used minimums of 1,200 kcal (women) or 1,500 kcal (men), or below maintenance, and shows a warning when that limit applies.

    3. Body Fat Calculator

      For women, a blank hip measurement was treated as 0 and produced a result of 0%. The hip measurement is now required, and switching to metric converts the tape measurements instead of relabelling them.

    4. Protein Calculator

      The athletic target of 2.2 g/kg was above the 2.0 g/kg upper limit of the sources the page cites; targets now stay within the cited ranges. Switching to metric now converts your weight.

    5. Paint, Roofing, Flooring, Tile, Gravel & Square Footage calculators

      Switching to metric changed the unit labels but kept the US numbers, so a coverage of 33.3 sq ft per bundle was read as 33.3 m² (about 10× too much). The toggle now converts every value and uses metric coverage figures, so both unit systems give the same answer for the same space.

    6. BMR & Calorie calculators

      Switching to metric kept the US numbers, so a default of 154 lb was read as 154 kg. Values are now converted when you change units.

    7. Roofing Calculator

      The default of 33.3 sq ft per bundle rounded up to 4 bundles for exactly one roofing square (100 sq ft); the standard is 3. Coverage is now entered as bundles per square (default 3), and the result also shows squares.

    8. Probability Calculator

      An impossible P(A and B) could produce P(A or B) above 100%, and large binomial inputs (n above about 1,000) returned blank results. Joint probabilities are now validated, and the binomial is computed in a way that stays accurate up to 10,000 trials.

    9. Fraction Calculator

      Dividing by a fraction equal to zero (such as 0/3) returned a number instead of an error. It now reports that you can't divide by zero.

    10. Scientific Calculator

      Very large and very small results were displayed wrongly (2^100 lost digits from its exponent, and 6.626e-34 showed as 0). Results now use scientific notation with 12 significant digits, and tan(90°) reports undefined.

    11. GPA Calculator

      IB courses were weighted as regular classes even though the page said they get the same +1.0 as AP, and a failing grade in a weighted course still received the bonus. Both now follow common school practice.

    12. ROI Calculator

      A blank or zero holding period was treated as one year, showing an annualized return that wasn't real; it now shows a dash. The page also said a shorter holding period needs a higher total ROI to match an annual rate — it's a longer one.

    13. Body Fat Calculator

      Tape measurements in inches were passed to the centimetre form of the U.S. Navy equations, so results read about 6 points low for men and much lower for women. Inputs are now converted to centimetres first, and a test checks the result against the independent inch-form equations.