A small, single-purpose site: free rafter calculators built from framing geometry and the IRC span tables, checked by hand, with the working shown.
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.
The site is built and maintained by Ian Welkins, who writes the calculators, checks the worked examples and answers the error reports. The byline is a person, not a team: if a number on this site is wrong, there is exactly one desk it lands on.
Ian can be reached through the contact page, and calculation problems are best sent through the report-an-error form, which captures the exact inputs that produced the result.
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:
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.