How we calculate

Where the formulas come from

Every calculator uses the original published equation or a recognised clinical reference, never a shortcut of our own. For example:

• Kidney function: the CKD-EPI 2021 creatinine equation (the race-free version) for eGFR, and Cockcroft-Gault for creatinine clearance.
• Body surface area: Mosteller, Du Bois, Haycock and Gehan-George, so you can use the method your setting prefers.
• Body weight: Devine for ideal body weight, and the standard adjusted body weight formula built on it.
• Paediatric maintenance fluids: the Holliday-Segar method (the 4-2-1 and 100-50-20 rules).
• Electrolytes: albumin-corrected calcium in the style of Payne, and sodium corrected for hyperglycaemia with either the Katz (1.6) or the Hillier (2.4) factor.
• Doses and infusions: the standard arithmetic of medication calculations: dose ÷ concentration, mg/kg × weight, volume ÷ time, drop factors, C1V1 = C2V2 and unit conversions.

The references behind each calculator are listed at the bottom of its page. When two formulas compete, the guides and comparisons explain what each one measures and when to use it.

How is each formula tested?

Each formula is covered by automated tests. The tests feed in values whose correct results are already known and check that the code returns the same answer, to the expected number of decimal places. Because they run automatically, the tests are repeated after every change, so a mistake introduced by an edit is likely to be caught before it reaches the site.

No early rounding

Numbers keep their full precision throughout the calculation and are rounded only in the result you see. Intermediate values in the working may be shown rounded so they are easier to read, but the calculation never uses those rounded figures. Rounding halfway through is a common source of small errors that add up, especially in infusion calculations with several steps.

Every result shows its working

Under each result you will find the steps that produced it: the formula, the numbers you entered and each intermediate value. If your hand calculation disagrees, you can see exactly where the two part ways.

Input checks

Before calculating, every value is checked:

• anything that is not a number is rejected with a clear message;
• negative values are rejected, and so is zero wherever it makes no sense (a weight, a concentration, a duration);
• a decimal comma is accepted as well as a decimal point, so 12,5 and 12.5 mean the same thing.

Warnings about implausible results

A result can be arithmetically correct and still be a sign that something was entered wrong. Where it makes sense, the calculators add a warning, for example when:

• the volume to draw up is very small and may be hard to measure accurately;
• the number of tablets is not a whole or half tablet, or is unusually high;
• a weight-based dose goes over the maximum you entered, in which case the maximum is used and the step is shown.

A warning is a prompt to stop and check, not a clinical judgement. No warning does not mean a dose is appropriate.

Review and corrections

The clinical content has not yet been reviewed by a named healthcare professional. We plan to add an independent clinical reviewer, and this page will name them once they have agreed. Until then, the formulas rest on their published sources and the automated tests described above. Formulas and wording are updated when references change. DoseStep is not certified and is not a medical device: see the disclaimer.

If you spot an error, an unclear step or an outdated reference, write to [email protected]. Every report is read, and corrections are made as quickly as possible.