How every calculator on Water Calculator is researched, built, and checked before it reaches you.
Last updated
Where our formulas come from
We build each calculator on the recognised standard for that quantity, taken from a primary source — the original research, the governing engineering equation, or the authoritative dataset — never a competitor's page. The standards behind our tools include:
- Water properties (density, thermal expansion, boiling point): the Kell equation and NIST thermophysical data.
- Vapour pressure, humidity and dew point: the Magnus–Tetens formulation.
- Viscosity and surface tension: IAPWS formulations.
- Pipe and channel flow: Hazen–Williams, Darcy–Weisbach (with Colebrook–White / Swamee–Jain friction factors), and Manning's equation.
- Pressure and head: hydrostatic and Bernoulli energy relationships.
- Pumps: the affinity laws and Hydraulic Institute methods.
- Water chemistry and treatment: the Langelier and Ryznar indices, and standard grains-per-gallon and TDS relationships.
- Usage, cost and conservation: reference data from the EPA (WaterSense), the USGS, and the Water Research Foundation.
- Units and constants: internationally defined values from the BIPM (SI) and NIST.
Each calculator lists the specific sources it relies on in its own “Sources & references” section, so you can trace any figure back to its origin.
Constants and conversions
We use exact, internationally defined conversion factors and standard reference values — for example, one US gallon as exactly 3.785411784 litres, and the density of water from the Kell equation rather than a rounded “1 kg/L.” Calculations run at full numerical precision internally; only the displayed result is rounded.
How we verify every calculator
Before a tool is published, every number we print — in the reference tables, the worked examples, and the FAQ — is computed independently and checked against what the live calculator returns for the same inputs. If they disagree, the page does not ship.
We also test the extremes: empty and partial inputs, values at the edges of a formula's valid range, and unusually small or large numbers, so the tool shows a calm, correct state rather than a misleading result or an error.
Our single hardest rule: a reference table or example that disagrees with the live calculator is a defect, not a rounding quirk. The two must always agree.
Assumptions and scope
Most of our science tools model pure, air-free water at atmospheric pressure unless you tell them otherwise, and each page states the conditions it assumes — temperature, pressure, salinity. Where a result depends on something we cannot know (your local water rate, your soil, a specific product's rating), we ask you to supply it rather than bury a guess in the answer.
Our calculators are planning aids: accurate within their stated assumptions, but not a replacement for site-specific testing or a licensed professional on a live project.
Units and rounding
Where a unit is ambiguous we make the choice explicit — US versus imperial gallons, gauge versus absolute pressure, °C versus °F — and offer the common alternatives directly in the tool. Intermediate steps keep full precision; results are rounded to a sensible number of significant figures for the quantity, so what you read is both accurate and easy to use.
Review, dates and corrections
Every calculator is reviewed by a credentialed specialist in its field before it is published, and each page shows when it was published and last updated. We revisit pages as standards, reference data, and utility rates change.
If you find an error, please tell us — corrections are a priority, and we date every update. You can reach us through our contact page. For the principles behind this process, see our editorial policy.