# Rafter Calculator

Plan a rafter takeoff. Calculates rafter count, total lumber length, board feet, cut angles, and 10% waste. Covers both sides of a gable roof for framing orders.

## What this calculates

This roof rafter calculator plans a complete rafter order for a gable roof. Enter building length, span, pitch, and spacing; the calculator returns rafter count per side, total rafters, lumber length to order, total linear feet with 10% waste, board feet, and the plumb and seat cut angles. For single-rafter length only, use the separate Rafter Length Calculator.

## Inputs

- **Building Length** (ft) — min 0 — Length of the building along the ridge
- **Building Span** (ft) — min 0 — Total span across the building (wall to wall, perpendicular to ridge)
- **Roof Pitch** (:12) — min 0, max 24 — Rise per 12 inches of run (6 = 6:12 pitch)
- **Rafter Spacing** — options: 12" OC, 16" OC (standard), 24" OC
- **Eave Overhang** (in) — min 0 — Horizontal overhang past the wall (typical: 12-24 in)
- **Lumber Length** — options: Auto (nearest standard size), 10 ft, 12 ft, 14 ft, 16 ft, 20 ft
- **Rafter Lumber Size** — options: 2x6, 2x8, 2x10, 2x12 — Used for the board feet estimate

## Outputs

- **Rafters per Side**
- **Total Rafters (Both Sides)**
- **Rafter Length** (ft) — Total rafter length including overhang
- **Lumber Length to Order** (ft) — Nearest standard lumber length that accommodates the rafter
- **Total Linear Feet (with 10% waste)** (lin ft)
- **Total Board Feet** (BF)
- **Plumb Cut Angle** (°)
- **Seat Cut Angle** (°)

## Details

How Rafter Counts Are Calculated

Rafters per side = ceil(building length / rafter spacing in feet) + 1 (for the starting rafter).

For a 40-foot building at 16-inch spacing: 40 / 1.333 = 30, plus 1 = 31 rafters per side. A gable roof needs these on both sides, so total rafters = 62.

Rafter Length Formula

Each rafter is the hypotenuse of a right triangle:

Rafter length = sqrt(run² + rise²) + overhang rafter

Where:

- Run = half the building span

- Rise = run x (pitch / 12)

- Overhang rafter is the extra length past the wall, calculated the same way

For a 28-foot span with a 6:12 pitch and 12-inch overhang:

- Run = 14 ft, Rise = 14 x (6/12) = 7 ft

- Main rafter = sqrt(14² + 7²) = sqrt(245) = 15.65 ft

- Overhang rafter = sqrt(1² + 0.5²) = 1.12 ft

- Total rafter length = 16.77 ft, order 18 ft lumber

Rafter Length Table (common pitches, per foot of run)

Pitch
Length per foot of run

3:12
1.031 ft

4:12
1.054 ft

5:12
1.083 ft

6:12
1.118 ft

8:12
1.202 ft

10:12
1.302 ft

12:12
1.414 ft

Multiply run (in feet) by this factor to get rafter length (main rafter only, add overhang separately).

Lumber Quantity and Board Feet

The rafter calculator rounds to standard lumber lengths (8, 10, 12, 14, 16, 20 ft) and applies a 10% waste factor to cover miscuts, knots, and breakage.

Board feet per linear foot of common rafter sizes:

- 2x6 = 1.0 BF/ft

- 2x8 = 1.33 BF/ft

- 2x10 = 1.67 BF/ft

- 2x12 = 2.0 BF/ft

Cut Angles

Plumb cut angle (vertical cuts at the ridge and rafter tail) equals the roof angle: arctan(pitch / 12). For a 6:12 pitch, that's 26.57°.

Seat cut angle (horizontal cut of the birdsmouth) is the complement: 90° - plumb angle. For a 6:12 pitch, the seat cut is 63.43°.

When to Use This Calculator vs Rafter Length Calculator

Use this rafter calculator when you need a complete lumber takeoff for a roofing order (count, total linear feet, board feet). Use the rafter length calculator when you need the precise length of a single rafter including ridge board adjustment for actual cutting.

## Frequently Asked Questions

**Q: How do I calculate the number of rafters I need?**

A: Divide the building length in feet by the rafter spacing in feet (16" = 1.33 ft), round up, and add 1 for the starting rafter. Then double for both sides of a gable roof. A 40-foot building at 16" OC needs 31 rafters per side, or 62 total.

**Q: What length rafter do I need for a 24-foot span?**

A: With a 6:12 pitch and 12" overhang, a 24-foot span needs about 14.5-foot rafters (12 ft run + 6 ft rise + 1 ft overhang rafter). Order 16-foot lumber and trim. With an 8:12 pitch, rafter length grows to about 15.4 feet so you would still order 16-foot stock.

**Q: What is the rafter length for a 12:12 pitch?**

A: A 12:12 pitch has rafter length of 1.414 feet per foot of run. For a 28-foot span (14-foot run): 14 x 1.414 = 19.8 feet. Add overhang (about 1.4 ft for a 12" horizontal overhang on a 12:12) to get 21.2 feet total. Order 24-foot lumber.

**Q: How much waste should I add for rafters?**

A: 10% is standard for straight-cut rafters on a simple gable. For complex roofs with hips, valleys, or dormers, use 15-20%. For very small jobs with expensive lumber, measuring each rafter individually and using 5% waste can save money.

**Q: What is the plumb cut angle for a 6:12 roof?**

A: The plumb cut angle for a 6:12 pitch is 26.57°, measured from horizontal. The seat cut is the complement: 63.43°. For framing, set your speed square to 6 on the common scale (or 26.5 degrees) for the plumb cut at the ridge and tail.

**Q: How many board feet are in a typical rafter?**

A: A 16-foot 2x8 rafter has 1.33 BF per linear foot x 16 ft = 21.3 board feet. A 16-foot 2x10 has 26.7 BF. A 16-foot 2x12 has 32 BF. Multiply by total rafter count and add 10% waste to get the total board feet for your order.

---

Source: https://vastcalc.com/calculators/construction/rafter
Category: Construction
Last updated: 2026-04-21