Rafter Calculator
Calculate the sloped length of a common rafter from horizontal run and roof pitch. Add an optional horizontal overhang without mixing structural sizing or cut details into the geometry.
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Estimate shingle quantity for this roof →Common rafter length formula
The pitch multiplier converts horizontal run into sloped rafter length. For an X:12 pitch, the rise/run ratio is X ÷ 12.
rafter length = horizontal run × √(1 + (pitch ÷ 12)²)
If an overhang is entered, its horizontal projection is converted by the same multiplier and added to the base rafter length.
What this rafter calculator does not size
Rafter length is geometry, not structural design. Member size, spacing, species, grade, snow and wind loads, ridge details, birdsmouth cuts and allowable spans require project-specific structural and code information.
Measure horizontal run consistently
The horizontal run is the plan-view distance from the outside wall line toward the ridge, not the already-sloped roof surface. Enter the run directly, or switch to Building Span for a simple centered ridge. In span mode, the calculator uses half the span and subtracts half of the ridge-board thickness you enter.
US measurements can be entered as decimal feet or feet, inches and fractions such as 12' 6 1/2". Results show both decimal feet and a feet-and-inches value rounded to the nearest 1/16 inch.
If an overhang is included, enter its horizontal projection rather than the sloped fascia-to-wall distance. For roofs with hips, valleys, multiple pitches or nonstandard framing, calculate simple common-rafter geometry only where the assumptions match the actual roof plan. Use drawings or qualified structural guidance for member sizing and cut details.
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Rafter length is geometry, not structural sizing
The calculator can convert horizontal run and roof pitch into common-rafter length and add an entered overhang. That geometric result does not determine rafter depth, spacing, species, grade, bearing, connection or allowable span.
Use measured run and the intended pitch for layout planning, and keep ridge details, birdsmouth geometry and fascia conditions in the actual framing plan. Structural member sizing and code compliance require project-specific design information beyond a length calculation.
Example: same run, different roof pitch
For a 10 ft horizontal run, a 4/12 pitch gives a geometric common-rafter line of 10 × √(1 + (4 ÷ 12)²) ≈ 10.54 ft. With an 8/12 pitch and the same horizontal run, it becomes 10 × √(1 + (8 ÷ 12)²) ≈ 12.02 ft. The second roof has a longer slope because the vertical rise is greater even though the plan-view run has not changed.
If overhang is included, enter its horizontal projection and calculate the additional sloped line consistently. Neither result is a final board cutting dimension: ridge deductions, bearings, fascia details and birdsmouth cuts must follow the actual plan. Structural adequacy cannot be inferred from either slope length.