Rafter Length Calculator

How to use the rafter length calculator

What this calculator does

This calculator treats one roof side as a right triangle. The horizontal run is one leg, the vertical rise is the other leg, and the sloped rafter is the hypotenuse. It estimates the geometric line length from the wall plate to the ridge centerline, together with pitch ratio, pitch angle, pitch percentage, and four proportional layout checkpoints. It does not size lumber, verify structural capacity, account for snow or wind loads, design connections, or add practical allowances for a ridge board, birdsmouth, eave overhang, fascia, seat cut, plumb cut, or waste. Those details require plans, local code requirements, and often professional engineering.

When to use it

Use it while sketching a simple gable roof, checking a measured run-and-rise pair, comparing roof slopes, estimating the minimum stock length before cut allowances, or teaching the geometry behind roof pitch. For work at height, review OSHA's fall-protection guidance before construction activity.

How to calculate

1. The calculator opens with a ready-to-use example: a 12 ft horizontal run and a 6 ft vertical rise. Its example workbook is immediately available.
2. Replace Horizontal run and Vertical rise with positive decimal values. Choose ft, in, m, or cm beside each field. The two inputs may use different units; the model converts both to meters internally.
3. Choose the Result unit. Read Rafter length, then use Pitch ratio, Pitch angle, and Pitch percent to describe the same slope in common formats.
4. Review Common layout checkpoints for quarter points along the ideal straight slope.
5. Select Download Excel to create a current-state OOXML workbook. Select Reset to clear the demonstration data. Reset may disable the download until both required dimensions are complete and valid again.

Input guide

Horizontal run is required and accepts a positive ordinary decimal number, such as 12 ft or 360 cm. It is the level distance from the outside wall line to the ridge centerline, not the full building span. A larger run increases rafter length and, for a fixed rise, reduces the pitch. Do not enter a full gable span unless you first divide it by two. Vertical rise is required and accepts the same positive decimal format, such as 6 ft or 1.8 m. A larger rise increases both rafter length and steepness. Do not confuse rise with ceiling height or total ridge elevation. Result unit is a required display choice. It converts the results and table without changing the underlying geometry; switching units is not a new design assumption.

Output guide

Rafter length is the ideal straight-line hypotenuse in the selected unit. It is an estimate of geometric length, not a cut-list recommendation. Pitch ratio reports vertical rise per 12 units of horizontal run; 6.00:12 means six units up for every twelve units across. Pitch angle is the slope above horizontal in degrees. Pitch percent is rise divided by run times 100. A 0% slope would be flat, but this calculator requires a positive rise. The summary pills repeat the current run, rise, and ratio. The checkpoint table shows the fraction of run, cumulative horizontal distance, proportional vertical height, and cumulative distance along the same straight rafter.

Worked example

For the startup example, run = 12 ft and rise = 6 ft. The Pythagorean calculation is √(12² + 6²) = √180 = 13.4164 ft, displayed as 13.42 ft. The pitch ratio is (6 ÷ 12) × 12 = 6.00:12; the angle is arctan(6 ÷ 12) = 26.57°; and the grade is 6 ÷ 12 × 100 = 50.00%. The halfway checkpoint is therefore 6.00 ft horizontally, 3.00 ft vertically, and 6.71 ft along the sloped line. The underlying geometry is the Pythagorean theorem for right triangles.

Learn more

For consistent unit interpretation, consult NIST's SI guidance for length. For structural design and permitted construction, use the adopted building code and approved plans for the project location; the International Code Council provides an overview of its model building codes.