24 ft span, 6/12 pitch, 1 ft overhang
- Rafter ≈ 13.42 ft
- Total ≈ 14.42 ft
construction calculator
Rafter Length Calculator
Calculate rafter length and angle from span, pitch, and overhang.
Results
- Rafter length (ft)
- 13.42
- Length incl. overhang (ft)
- 14.42
- Roof angle (degrees)
- 26.57
How to use this calculator
- Enter total roof span (wall to wall).
- Enter pitch (rise per 12 inches of run) and desired overhang.
- Review rafter length to the wall plate, total length with overhang, and bevel angle.
Inputs explained
- Roof span
- Distance from outside wall to outside wall across the building.
- Pitch
- Rise per foot of run (e.g., 6 = 6-in rise per 12-in run).
- Overhang
- Additional length beyond the wall plate for eaves.
How it works
Half the span is the run. Rise = run × (pitch/12).
Rafter length uses the Pythagorean theorem. Overhang is added separately.
Formula
Run = span ÷ 2 Rise = Run × (pitch/12) Rafter = √(Run² + Rise²)
When to use it
- Estimating board lengths to purchase before cutting rafters.
- Confirming saw bevel angles for plumb cuts and birdsmouth layout.
- Checking how pitch changes total lumber length for remodels.
Tips & cautions
- Include extra length for ridge thickness and birdsmouth depth when marking cuts.
- Keep units consistent—inputs are in feet, but pitch is inches of rise per foot of run.
- Round up lumber length to the next available stock size to allow trimming.
- Does not size structural members—follow span tables and local code for allowable spans.
- Ridge thickness, heel height, and seat cuts can change final cut length; add those manually.
- Assumes a simple gable run; hips/valleys need separate layout math.
Worked examples
20 ft span, 4/12 pitch, 1.5 ft overhang
- Rafter ≈ 11.18 ft
- Total ≈ 12.68 ft
Deep dive
This rafter length calculator converts span, roof pitch, and overhang into rafter length and bevel angle. Enter your dimensions to see how much lumber you need before heading to the yard.
Use it to plan cuts for gable roofs, set saw angles, and budget materials. Add allowances for ridge thickness and birdsmouth cuts to match your framing details.
FAQs
- Does this include ridge thickness?
- No. Add half the ridge thickness to each side if you’re cutting rafters to meet at a ridge board.
- How do I convert pitch to angle for my saw?
- The calculator outputs roof angle in degrees—use that as your bevel angle for plumb cuts.
- Can I enter metric values?
- Inputs are in feet/inches. Convert to feet before entering for correct results.
- Does this account for birdsmouth depth?
- It doesn’t subtract seat/plumb cut depth. Add extra length if you need more material for those cuts.
- Is this enough for structural sizing?
- No. Use span tables or an engineer to verify rafter size and spacing for your loads and code requirements.
Related calculators
Engineering requirements vary by snow load and code. Verify sizing with a professional.