Notchline
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Notchline

How the numbers are checked

Every formula in the app has test cases with a source. Here are the published values the free calculators on this site reproduce, and how the site is held to the app.

The engine

The stair and roof maths runs in one engine, tested with 353 cases, 91 of them against published worked examples: carpentry references, code commentary, trade schools and stair-maker guides. A failing case stops a release.

This site against the app

The calculators on this site are a port of the same engine to JavaScript. Every change runs them against 2079 stairs across 11 rule sets and 318 roof and birdsmouth cases computed by the app's own engine: every number must match to a billionth and every rule must reach the same verdict.

Published values

Values from published examples that the calculators on this page reproduce:

Published values and this site's calculator
SourceWhatPublishedThis siteResult
swansontoolco.comPitch in degrees9.46239.4623✓ within tolerance
swansontoolco.comPitch in degrees14.036214.0362✓ within tolerance
swansontoolco.comPitch in degrees18.434918.4349✓ within tolerance
swansontoolco.comPitch in degrees22.619922.6199✓ within tolerance
swansontoolco.comPitch in degrees26.565126.5651✓ within tolerance
en.wikipedia.orgRafter per 12 of run14.422214.4222✓ within tolerance
en.wikipedia.orgRafter per 12 of run16.970616.9706✓ within tolerance
en.wikipedia.orgRafter per 12 of run19.209419.2094✓ within tolerance
en.wikipedia.orgRafter per 12 of run21.633321.6333✓ within tolerance
swansontoolco.comRafter per 12 of run12.649112.6491✓ within tolerance
swansontoolco.comSlope factor1.05411.0541✓ within tolerance
swansontoolco.comCommon rafter8.43278.4327✓ within tolerance
swansontoolco.comCommon rafter10.540910.5409✓ within tolerance
swansontoolco.comCommon rafter12.649112.6491✓ within tolerance
swansontoolco.comCommon rafter14.757314.7573✓ within tolerance
swansontoolco.comCommon rafter16.865516.8655✓ within tolerance
swansontoolco.comCommon rafter18.973718.9737✓ within tolerance
swansontoolco.comCommon rafter21.081921.0819✓ within tolerance
www.maine.govCommon rafter7878✓ within tolerance
inspectapedia.comCommon rafter197.7197.7✓ within tolerance
inspectapedia.comCommon rafter16.22516.225✓ within tolerance
inspectapedia.comCommon rafter10.419310.4193✓ within tolerance
www.jlconline.comFewest risers1414✓ within tolerance
www.jlconline.comRiser height7.88467.8846✓ within tolerance
www.jlconline.comRiser height7.32147.3214✓ within tolerance
www.jlconline.comRiser height6.83336.8333✓ within tolerance
www.jlconline.comRiser height6.40636.4063✓ within tolerance
www.jlconline.comStair angle33.647133.6471✓ within tolerance
www.jlconline.comStep along the stringer13.213813.2138✓ within tolerance
www.jlconline.comTotal run143143✓ within tolerance
www.jlconline.comNosing line171.8171.8✓ within tolerance
www.firgelliauto.comStringer length120.4120.4✓ within tolerance
www.isb.bayern.deRiser height17.817.8✓ within tolerance
www.isb.bayern.deGoing from the step rule27.427.4✓ within tolerance
www.isb.bayern.deTotal run383.6383.6✓ within tolerance
www.isb.bayern.deRiser height19.071419.0714✓ within tolerance
www.isb.bayern.deTotal run325325✓ within tolerance

The rule sets

The stair calculator offers each country's home rule set. Numbers from trade summaries rather than the official text are labelled.

Rule sets on the stair calculator
CountryRule setRulesNumbers checkedNote
United StatesIRC 2021 · one- and two-family dwellings112026-11-01
CanadaNBC 2020 · private stairs112026-11-01
EnglandApproved Document K 2013 · private stair112026-11-01
EnglandApproved Document K 2013 · loft conversion112026-11-01
GermanyDIN 18065:2020-08 · home, main stair92026-11-01Secondary source
FranceArrêté du 24 décembre 2015 · inside a home62026-11-01
NetherlandsBbl · new home72026-11-01
SpainCTE DB SUA 2022 · restricted use (inside a home)62026-11-01
ItalyDM 236/1989 · private stair42026-11-01Secondary source
AustraliaNCC 2022 · housing132026-11-01
TürkiyePAİY 2017 · inside a home, no lift72026-11-01

Sources

  1. swansontoolco.com
  2. swansontoolco.com
  3. swansontoolco.com
  4. swansontoolco.com
  5. swansontoolco.com
  6. en.wikipedia.org
  7. www.maine.gov
  8. inspectapedia.com
  9. www.jlconline.com
  10. www.firgelliauto.com
  11. www.isb.bayern.de

Results are design aids. Check the current rules with your local building authority before you build.

Checked against the sources on November 2, 2026.