Roof Truss Calculator — Rafter Length, Truss Count & Cost
Use this Roof Truss Calculator to quickly estimate rafter length, the number of trusses you’ll need, and a rough cost. Enter span, pitch and overhang — results update instantly.
Calculation Results
How It’s Calculated
Rafter length is calculated using the Pythagorean theorem:
Rafter Length = √(Run² + Rise²)
Pitch is calculated as Rise / Run.
How Roof Truss Calculations Work
Rafter Length Formula
The rafter length is calculated using the Pythagorean theorem, where the rafter is the hypotenuse of a right triangle formed by the run and rise:
Rafter Length = √(Run² + Rise²)
When including overhang, the formula becomes:
Rafter Length = √((Run + Overhang)² + Rise²)
Pitch Conversion
Pitch is typically expressed as a ratio of vertical rise to 12 units of horizontal run. To convert pitch to degrees:
Angle = arctan(Rise / Run)
Truss Count Formula
The number of trusses needed is calculated as:
Truss Count = ceil(Roof Length / (Truss Spacing / 12)) + 1
The “+1” accounts for the additional truss needed at the end of the roof.
Frequently Asked Questions
Roof pitch is the steepness of a roof, expressed as a ratio of vertical rise to horizontal run. For example, a 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run.
Rafter length is calculated using the Pythagorean theorem. If you know the run (horizontal distance) and rise (vertical distance), the rafter length is the square root of (run² + rise²).
For a 30 ft roof with standard 24-inch spacing: 30 ft × 12 = 360 inches. 360 ÷ 24 = 15 spaces. You need one truss at each end plus one for each space, so 15 + 1 = 16 trusses total.
This calculator provides estimates for planning and budgeting purposes only. It is not a substitute for professional engineering analysis. Always consult a licensed structural engineer for final design and construction.
To convert pitch to degrees, use the formula: angle = arctan(rise/run). For example, a 6/12 pitch: arctan(6/12) = arctan(0.5) ≈ 26.57 degrees.
The “+1” accounts for the additional truss needed at the end of the roof. If you have N spaces between trusses, you need N+1 trusses total to cover both ends.
Span is the total horizontal distance between the outside faces of exterior walls. Run is half the span for a symmetrical gable roof, representing the horizontal distance from the outside wall to the ridge center.
Standard truss spacing is typically 24 inches on center (OC) for residential construction, though 16 inches OC is also common. Commercial buildings may use wider spacing. Always follow local building codes.