FreeHVACCalculators

Methodology and accuracy policy

How each calculator is built, checked and kept up to date — and what that does and does not guarantee.

Last updated 22 October 2026

Principles

  • Every result comes from a documented method. Constants are traceable to a public source, which is linked on the calculator page.
  • The working is shown. Under each result, “Show the working” lists every step with your numbers substituted.
  • Estimates are labelled as estimates. A sizing tool on this site is not an ACCA Manual J calculation, and the page says so.
  • Assumptions and limitations are listed on every page, not hidden in a footnote.
  • Advertising and sponsorship never change a calculation or a recommendation.

How a calculator is built

All calculators share one calculation engine. Each calculator is defined once: its inputs and their units and limits, the formula, known-answer test cases, references, assumptions and a version number. Calculations run in your browser; the values you enter are not sent to our server.

The engine works internally in US customary units. When you choose metric, your inputs are converted on the way in and results on the way out using exact or NIST factors. Temperature differences are converted without the 32° offset, and psychrometric enthalpy is calculated separately in each system because the two scales use different zero points.

How results are checked

  1. Known-answer tests: hand-calculated or published values (for example the ENERGY STAR sizing chart or NIST unit definitions) that the engine must reproduce.
  2. Independent implementations: psychrometric results are compared with PsychroLib, an open-source implementation of the ASHRAE Handbook equations, and the load and duct methods are re-implemented separately in Python. The two must agree within stated tolerances.
  3. Reference data: saturation pressures are checked against ASHRAE table values.
  4. Validation tests: every input is tested with missing, non-numeric, below-minimum, above-maximum and boundary values. No combination may produce a hidden “NaN” or infinite result.
  5. Engineering sanity checks: for example, that sizing grows with floor area and that a well-built house lands in the range published Manual J studies report.

A release cannot ship unless all of these pass. Passing tests show the maths does what the method says; they do not prove the method fits every building.

Rounding and precision

Results are calculated at full precision and rounded only for display, to a number of digits that reflects the precision of the inputs. Equipment suggestions round up to the next standard size and say so. Treat the last displayed digit of an estimate as indicative, not exact.

Default prices

Energy-price defaults were last verified on 2026-10-22:

Sources

The calculators currently cite 26 public sources. Each was retrieved and the supporting fact checked before it was cited. Where a primary standard is paywalled (for example ASHRAE Handbook—Fundamentals or ACCA Manual J), we cite a public source that reports the same method and say so.

Review status

16 of 16 calculators are marked validated. Each page shows the date its method was last reviewed and its version number; the version changes whenever a formula, default or assumption changes.

Corrections

If you find an error, please report it. Confirmed errors are fixed, a regression test is added so the error cannot return, and the change is logged. See the corrections policy.