Rafter Calculator | Known Run

Calculate the length of rafters based on the run and pitch using feet and inches. One sixteenth of an inch accuracy. Use the forms below to get a fast result for the length and the rise. The first row has three fields – feet, inch, and fraction. This is for the run. The run is the horizontal distance the rafter will travel. For a standard gable you start with the span and subtract the thickness of the ridge. For example: 2 x 10 ridge – subtract 1.5 inches from the span, then divide the result by two. This yields the run. For gable roofs, click here for the Common Rafter Calculator that expects the span and the ridge thickness along with the roof pitch. This model is best suited for lean to rafters, or any situation where the run is known. This