How do I calculate rafter length?

Use the Pythagorean theorem: rafter length = √(run² + rise²), then subtract half the ridge-board thickness (projected along the slope) for the plumb cut at the ridge. For a 12′ run at a 6/12 pitch, run = 12, rise = 6 (= 12 × 6 ÷ 12), rafter length ≈ 13.35′ ≈ 13′ 4-3/16″. Metric: Use the Pythagorean theorem: rafter length = √(run² + rise²), then subtract half the ridge-board thickness projected along the slope. For a 4 m run at 30°, rise ≈ 2.31 m and the finished rafter length is about 4.59 m.

What does "6/12 pitch" mean?

6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run - a 26.57° slope angle. The denominator is always 12 by convention, so a "6 pitch" and "6/12 pitch" mean the same thing. Metric: Metric mode uses degrees. For example, a 30° pitch means the roof rises by tan(30°) for each meter of horizontal run, so a 4 m run has about 2.31 m of rise.

What pitch should I use for my roof?

3/12 to 4/12 = low-slope (will not shed snow well, needs membrane roofing). 5/12 to 8/12 = standard residential, works with shingles. 8/12 to 12/12 = steep, sheds snow and water aggressively, costs more in materials. 12/12 is exactly 45°. Metric: 14° to 18° = low-slope (will not shed snow well, needs membrane roofing). 23° to 34° = standard residential, works with shingles. 34° to 45° = steep, sheds snow and water aggressively, costs more in materials. 45° is a very steep roof.

Why does the calculator deduct half the ridge board thickness?

Common rafters meet at the ridge board, which is typically 1.5″ thick (or 45 mm metric). Each rafter's plumb cut sits against half of that thickness - so the rafter's actual length is shorter than the bare hypotenuse by that amount, projected along the rafter's slope. Metric: Common rafters meet at the ridge board, which is typically about 45 mm thick in metric mode. Each rafter's plumb cut sits against half of that thickness, so the actual rafter is shorter than the bare hypotenuse by that amount projected along the slope.

Does this include the rafter tail / overhang?

No. This calculates from the wall plate to the ridge only. Add your overhang length separately (typically 12–24″ for eaves). The full board length you order = ridge length + overhang + cut waste. Metric: No. This calculates from the wall plate to the ridge only. Add your overhang length separately, often around 300-600 mm for eaves. The full board length you order = ridge length + overhang + cut waste.

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Rafter Length Calculator

Common rafter length from run and pitch, with ridge-board deduction. Imperial (X/12 pitch) or metric (degrees), auto-detected by region. Mobile-first.

Units

Rafter Dimensions

Run, wall to ridge centre (ft)

Pitch (X / 12)

Rafter Length -

Ridge height (rise) - Plumb cut -° Seat cut -° Rafter × -

For roof overhang sloped length, multiply your horizontal eave by the rafter factor. For example, a 12″ eave at this pitch becomes -.

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Roof framing guides

For adjacent roof layout jobs, use the ridge height calculator, the roof overhang calculator, the valley rafter calculator, the rafter cut calculator, or the lean-to rafter calculator before adding rafter tail length to your order.

Need pitch-based lengths?

Use the rafter length by pitch guide for common roof pitch, run, rise, and rafter length examples.

How to use this calculator

Measure the horizontal run from the wall plate to the centre of the ridge. Pick your roof pitch - imperial is "X/12" (X inches of rise per 12 inches of run), metric is degrees. The calculator returns the rafter length, the rise of the roof, and the slope angle. Ridge-board deduction is included (1.5″ imperial, 45 mm metric). For overhang, add your eave length separately.

The formula

The rafter is the hypotenuse of a right triangle whose legs are the rise and the run: Rafter = √(run² + rise²) − (ridge ÷ 2) ÷ cos(angle)

Worked example. 12 ft run, 6/12 pitch:

Rise: 12 × (6 ÷ 12) = 6 ft
Bare hypotenuse: √(12² + 6²) = √180 ≈ 13.42 ft
Angle: atan(6 ÷ 12) ≈ 26.57°
Ridge deduction: (1.5″ ÷ 2) ÷ cos(26.57°) ÷ 12 ≈ 0.070 ft
Rafter length: 13.42 − 0.070 ≈ 13.35 ft (13′ 4-3/16″)

Common rafter lengths at a glance

Run × pitch → rafter length (no overhang, no ridge correction). Run × roof angle → rafter length in meters (no overhang, no ridge correction).

Run	4/1222.5°	6/1230°	8/1235°	12/1245°
8 ft2.5 m	8.432.71	8.942.89	9.613.05	11.313.54
10 ft3 m	10.543.25	11.183.46	12.023.66	14.144.24
12 ft4 m	12.654.33	13.424.62	14.424.88	16.975.66
14 ft5 m	14.765.41	15.655.77	16.826.10	19.807.07
16 ft6 m	16.876.49	17.896.93	19.237.32	22.638.49

Lengths in feet, no overhang, no ridge correction.Lengths in meters, no overhang, no ridge correction.

What pitch should I use?

3/12 to 5/1214° to 23° - low slope. Common for ranch homes and some modern designs. Won't shed snow well; needs membrane roofing.
6/12 to 8/1227° to 34° - medium slope, the most common residential range. Drains water well, takes any roofing material.
9/12 to 12/1237° to 45° - steep. Colonial / cape / victorian. Sheds snow well but harder to walk on.
Above 12/12Above 45° - very steep. Decorative or specific architectural choices.

Common Questions

How do I calculate rafter length?How do I calculate rafter length in meters?: Use the Pythagorean theorem: rafter length = √(run² + rise²), then subtract half the ridge-board thickness (projected along the slope) for the plumb cut at the ridge. For a 12′ run at a 6/12 pitch, run = 12, rise = 6 (= 12 × 6 ÷ 12), rafter length ≈ 13.35′ ≈ 13′ 4-3/16″.Use the Pythagorean theorem: rafter length = √(run² + rise²), then subtract half the ridge-board thickness projected along the slope. For a 4 m run at 30°, rise ≈ 2.31 m and the finished rafter length is about 4.59 m.
What does "6/12 pitch" mean?What pitch format should I use in metric?: 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run - a 26.57° slope angle. The denominator is always 12 by convention, so a "6 pitch" and "6/12 pitch" mean the same thing.Metric mode uses degrees. For example, a 30° pitch means the roof rises by tan(30°) for each meter of horizontal run, so a 4 m run has about 2.31 m of rise.
What pitch should I use for my roof?What roof angle should I use?: 3/12 to 4/12 = low-slope (will not shed snow well, needs membrane roofing). 5/12 to 8/12 = standard residential, works with shingles. 8/12 to 12/12 = steep, sheds snow and water aggressively, costs more in materials. 12/12 is exactly 45°.14° to 18° = low-slope (will not shed snow well, needs membrane roofing). 23° to 34° = standard residential, works with shingles. 34° to 45° = steep, sheds snow and water aggressively, costs more in materials. 45° is a very steep roof.

Why does the calculator deduct half the ridge board thickness?Why does the calculator deduct half the ridge board thickness?: Common rafters meet at the ridge board, which is typically 1.5″ thick (or 45 mm metric). Each rafter's plumb cut sits against half of that thickness - so the rafter's actual length is shorter than the bare hypotenuse by that amount, projected along the rafter's slope.Common rafters meet at the ridge board, which is typically about 45 mm thick in metric mode. Each rafter's plumb cut sits against half of that thickness, so the actual rafter is shorter than the bare hypotenuse by that amount projected along the slope.
Does this include the rafter tail / overhang?Does this include the rafter tail / overhang?: No. This calculates from the wall plate to the ridge only. Add your overhang length separately (typically 12–24″ for eaves). The full board length you order = ridge length + overhang + cut waste.No. This calculates from the wall plate to the ridge only. Add your overhang length separately, often around 300-600 mm for eaves. The full board length you order = ridge length + overhang + cut waste.

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