About Rafter Length Calculator
A small, single-purpose site: free rafter calculators built from framing geometry and the IRC span tables, checked by hand, with the working shown.
What this site is
Rafter Length Calculator does one job: the arithmetic of cutting roof rafters. The homepage calculator works out common rafter lengths with the ridge deduction, overhang tail and birdsmouth included; the rest of the family — hip, valley, jack, shed and gambrel rafters, cut angles, pitch conversions — each gets a dedicated tool, and the span and size checkers read the IRC 2021 span tables directly.
Everything runs in your browser, free, with no account and nothing to install. The site exists because most rafter calculators on the web are either bare-bones (a length and nothing else) or buried inside general construction calculators where the framing details — HAP, ridge deductions, the IRC birdsmouth notch limit — get lost.
Why I built it
Rafter work punishes small mistakes in a way most measuring jobs do not. A board cut a fraction short cannot be stretched, a birdsmouth taken too deep removes wood the rafter needs, and a span chosen by guesswork is not just a wasted board — it can become a safety problem once the roof is loaded.
The geometry itself is only Pythagoras and a framing square, but the safe span for a board depends on its species, grade, size, spacing and loads. Those limits sit in the span tables inside the IRC code books — in this site’s case, the published 2021 IRC Tables R802.4.1(3) and R802.4.1(4) — which most DIY builders never open. I built this site to put that geometry and those published tables into plain calculators, with the inputs, formulas, worked examples and limits shown before anyone picks up a saw.
About the author
I’m Ian Welkins, an independent web developer based in Houston, Texas. I build small, single-purpose calculator websites — tools that do one job, show their working, and ask for nothing: no sign-up, no accounts, nothing to install.
Rafter Length Calculator is researched, built and maintained by me personally. I derive the framing geometry, encode the calculators, test the outputs and answer the error reports. The span data is taken from the published IRC tables named above and cross-checked before it is encoded; where a table value could not be verified, it was left out rather than guessed.
You can find me, and check that for yourself, on these profiles:
Spotted a wrong length, angle or span? Tell me through the report-an-error page — I read every report myself and fix what is wrong. For anything else, the contact page is the way to reach me.
A note on AI
I use AI tools to help draft page text and code, the way many independent developers do now. They do not choose the span data: every span figure on this site comes from the published IRC tables named on the pages, and each value is verified against those tables before it is published. If a result looks wrong, the report-an-error form reaches a person — me — and I check it against the method and the source.
How the calculators are checked
A calculator that cannot show its working is asking for trust it has not earned. Each tool here is verified three ways before it ships:
- Every length formula is derived from first principles (run, rise and the Pythagorean theorem) and tested against worked examples computed independently by hand
- Span and size data are transcribed from the published IRC 2021 tables — never interpolated, extended or 'corrected'
- Fraction outputs are rounded once, to the nearest 1/16 in, at the final step only
- Each calculator page carries fully worked examples whose inputs and outputs are stated in the text, so anyone can reproduce them
What this site is not
These are planning and layout tools, not engineering. They do not replace the building code in your jurisdiction, a span check by your building department, or a structural engineer for anything outside the prescriptive tables — long spans, heavy snow loads, raised ties, trusses or engineered lumber. Verify every measurement before you cut; lumber is expensive and roofs are unforgiving.