What is the rafter run?

The run is the horizontal distance from the outside wall plate to the centerline of the ridge board — roughly half the building width for a symmetrical gable roof. Do not include the overhang in the run; enter overhang separately.

What is roof pitch and how does it relate to rise?

Pitch is expressed as X/12, meaning X inches of vertical rise for every 12 inches of horizontal run. A 6/12 pitch on a 10-foot run gives a rise of (6 ÷ 12) × 10 ft = 5 ft. Enter the pitch and the calculator derives the rise automatically.

Does the result include the overhang?

Yes. The overhang is measured horizontally (in feet or inches). Its contribution to rafter length is calculated the same way — √(overhang² + rafter_rise_over_overhang²) — and added to the main span length.

Why add a waste factor?

Lumber is sold in standard lengths (8, 10, 12, 14, 16 ft, etc.). A 5–10% waste factor accounts for end cuts, defects, and rounding up to the nearest standard length. The default is 10%.

What board dimensions should I enter?

Use nominal dimensions such as 2×6 or 2×8. Board feet are calculated from nominal size: (Thickness in × Width in × Length ft) ÷ 12. Most residential rafters are 2×6 or 2×8 lumber.

Is the ridge board included in the calculation?

No. This tool calculates common rafter lumber only. The ridge board, ceiling joists, and hurricane ties are separate line items in your material list.

Rafter Length Calculator

Name: Rafter Length Calculator
Availability: InStock
Author: Nham Vu

Calculate rafter length from run and rise (or roof pitch), then estimate total board feet needed for the full roof span.

Roof Dimensions

Run (horizontal, ft) — wall to ridge centerline

Roof Pitch (in/12)

Overhang (horizontal, ft)

Lumber & Quantity

Thickness (in, nominal)

Width (in, nominal)

No. of Rafters

Waste Factor (%)

Results

Rafter Length

—

feet (incl. overhang)

Pitch Angle

—

degrees

BF / Rafter

—

board feet

Total Board Feet

—

all rafters + waste

Parameter	Value
Horizontal Run	—
Vertical Rise	—
Horizontal Overhang	—
Rafter Span (no overhang)	—
Overhang Extension	—
Total Rafter Length	—
Lumber Size (nominal)	—
Number of Rafters	—
Waste Factor	—
Board Feet per Rafter (before waste)	—
Total Board Feet (incl. waste)	—

How the Formula Works

1
Rise from pitch: If using pitch X/12: Rise = Run × (X ÷ 12)
2
Rafter span: Span = √(Run² + Rise²) — the Pythagorean theorem applied to the roof triangle.
3
Overhang extension: The rafter extends past the wall by the overhang. Its slope-length is √(Overhang² + Overhang_rise²) where Overhang_rise = Overhang × (Rise ÷ Run).
4
Total rafter length: Total = Span + Overhang Extension
5
Board feet: BF = (Thickness × Width × Length_ft) ÷ 12 — using nominal lumber dimensions.
6
Project total: Total BF = BF_per_rafter × Count × (1 + Waste%)

Summary

Calculate rafter length from run and rise (or roof pitch), then estimate total board feet needed for the full roof span.

How it works

Choose whether to enter Rise directly or use a Pitch (in/12) value.
Enter the horizontal Run — the distance from the wall plate to the ridge centerline.
Enter the Rise or select a standard pitch (e.g. 6/12 means 6 inches of rise per 12 inches of run).
Enter the rafter width and depth (nominal lumber dimensions) and the number of rafters.
The calculator applies the Pythagorean theorem: Rafter = √(Run² + Rise²) and adds a user-defined overhang.
Board feet are computed for a single rafter and for the full set, with an optional waste factor.

Use cases

Framing a gable, hip, or shed roof and estimating rafter lumber.
Checking rafter length before ordering pre-cut lumber from a yard.
Verifying roof geometry during a remodel or addition.
Teaching students the relationship between pitch, rise, run, and hypotenuse.
Preparing a material takeoff for a roofing permit application.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu