About This Gambrel Roof Calculator

This site exists for one reason: there is no good free tool for calculating gambrel roof geometry. The calculators that exist are either trapped behind sign-up walls, hidden inside CAD software, or limited to a single building width. Carpenters, owner-builders, barn restorers, and architecture students all need the same numbers: rafter lengths, pitch angles, roof area, attic volume. They should not have to pay for them or wait for a load to complete.

Every calculator on this site runs in your browser. Nothing is sent to a server. There is no account to create, no email to enter, and no pop-up to dismiss. Type a building width and the answer appears on the same keystroke. The math comes from standard trigonometric formulas for gambrel geometry, the half-circle method documented in barn-building references going back to the 19th century, and ASCE 7 and IRC code references for snow, wind, and lumber sizing.

The tool is for estimation. A gambrel roof on a real building needs an engineer's stamp anywhere the span exceeds standard IRC rafter tables, anywhere the wind speed exceeds 130 mph design, and anywhere the snow load exceeds the warm-roof slope-factor reduction allowance. Use the numbers here to plan, to order lumber, to estimate costs, and to understand what you are building. Use a licensed professional for final structural design and code review.

If you find a bug, a typo, or a formula error, the contact page accepts notes. Corrections are applied within a day or two.