A rafter calculator figures out how long your roof rafters need to be based on the building’s span and the roof pitch. It also shows the rise, the roof angle and often the cut angles for the plumb and seat cuts. It’s a must-have tool for building sheds, garages, additions and pergolas.
Roof pitch is expressed as rise over 12 inches of run, like 6/12. The run is half the building span (minus half the ridge board thickness for precision). The rafter length is found with the Pythagorean theorem: rafter length = √(run² + rise²). For example, a 24-foot-wide building has a 12-foot run. At a 6/12 pitch, the rise is 6 feet. Rafter length = √(12² + 6²) = √180 ≈ 13.42 feet, before adding the overhang.
The roof angle equals the arctangent of rise ÷ run; a 6/12 pitch is about 26.6 degrees. Add the overhang length (measured along the slope) to get the total rafter length, and buy lumber long enough for the cuts. A 14-foot board would work for the example above with a short overhang.
Tips: account for the ridge board thickness, use a speed square to mark plumb and seat cuts, and cut a test rafter before cutting the rest. Rafter size and spacing depend on span, load and local building codes, including snow loads, so check your local requirements or consult a professional.
Why use an online Rafter Calculator: it saves time, avoids simple arithmetic mistakes and shows each step, so you can check your homework, plan a project or make a decision with confidence. Bookmark the page so it’s ready the next time you need it.
Tips for getting accurate results from the Rafter Calculator: double-check that every input uses the right units, enter numbers without commas or symbols, and run the calculation a second time with a quick estimate in your head to confirm the answer looks reasonable.
Learn more on Wikipedia’s rafter article. Find stair stringer, board feet and stud calculators on Minedit.