Inputs to result
Horizontal run = (span − ridge thickness in feet) ÷ 2 + horizontal overhang; slope length = horizontal run × √(12² + rise²) ÷ 12; stock estimate = slope length + cut allowance.
Carpentry & Framing
Rafter Length Calculator
Estimate common rafter slope length and framing stock length from building span, ridge thickness, roof pitch, and horizontal overhang.
Enter full outside span, ridge-board thickness, pitch and horizontal overhang. The result calculates one common rafter's geometric slope length.
Calculation and worked example reviewed 2026-11-12. See our calculation methodology.
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Carpentry & Framing
US UNITSEnter your measurements
YOUR ESTIMATE
18.32 ft stock estimate17.82 ft slope geometry15.94 ft horizontal run including overhang.
Not a finished rafter cut length or structural sizing result.
Formula and example
How this calculation works
This estimator converts your entered jobsite inputs (outside building span, ridge board thickness, roof rise per 12, horizontal overhang, stock/cut allowance) into consistent US trade units, computes the exact geometric baseline first, and then applies any selected waste or order allowance so you can verify both numbers before purchasing materials.
How does the rafter length calculator formula work?
Horizontal run = (span − ridge thickness in feet) ÷ 2 + horizontal overhang; slope length = horizontal run × √(12² + rise²) ÷ 12; stock estimate = slope length + cut allowance.
(Rafter Length Calculator)where:
- Outside building span – Entered in ft (default reference: 30 ft);
- Ridge board thickness – Entered in in (default reference: 1.5 in);
- Roof rise per 12 – Entered in in (default reference: 6 in);
- Horizontal overhang – Entered in ft (default reference: 1 ft);
- Stock/cut allowance – Entered in in (default reference: 6 in);
Check the math
For a 30 ft span, 1.5 in ridge, 6:12 pitch and 1 ft horizontal overhang, geometric slope length is about 17.82 ft before cut allowance.
Quick lookup chart
Common Rafter Slope Length by Span & Roof Pitch (1.5 in Ridge, 1 ft Overhang)
Geometric line length from ridge face to fascia plumb line including a 12 in horizontal overhang.
| Outside Span | 4:12 Pitch | 6:12 Pitch | 8:12 Pitch |
|---|---|---|---|
| 20 ft span | 11.53 ft (12 ft stock) | 12.23 ft (14 ft stock) | 13.15 ft (14 ft stock) |
| 24 ft span | 13.64 ft (14 ft stock) | 14.46 ft (16 ft stock) | 15.55 ft (16 ft stock) |
| 30 ft span | 16.80 ft (18 ft stock) | 17.82 ft (18 ft stock) | 19.15 ft (20 ft stock) |
| 36 ft span | 19.96 ft (20 ft stock) | 21.17 ft (22 ft stock) | 22.76 ft (24 ft stock) |
Before you use this result
This is a geometric planning length, not a cut schedule. Birdsmouth, plumb cut, fascia, tail, structural size, span and ridge details require field layout and design checks.
- Enter full outside-of-wall building span; the calculator automatically deducts half the ridge board thickness before dividing by two.
- Enter overhang as horizontal projection (level measurement from outer wall to sub-fascia), not sloped tail length.
- Verify birdsmouth seat cut depth (heel height), sub-fascia thickness, and maximum allowable rafter span for your lumber species, grade, and snow load.
Questions answered
Calculator FAQ
Should I enter the full span or half span?+
Enter full outside span. The calculator subtracts ridge thickness and divides the remainder in half.
Is overhang measured along the slope?+
No. Enter horizontal overhang. The pitch factor converts that projection to slope length.