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Cut rafters. Get it right the first time.

The free calculator framers trust. Rafter length, birdsmouth cuts, HAP, ridge height, lumber count, all in under 60 seconds, backed by IRC R802.

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6/12 Pitch

Rafter · 13.34 ft

HAP · 3.67 in

48 Rafters

Measurements

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Setup time

end to end

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Rafter Calculator

IRC R802 · American Wood Council Standards

Free

Step 1 of 3

Measure your building

Outside-wall to outside-wall, span is the hardest part, everything else is fast.

Building Span (Width)

feet

Building Length

feet

Step 2 of 3

Set your pitch & eave

Watch the preview morph as you adjust, steeper pitch = longer rafters.

Live Preview

Roof Pitch

Eave Overhang

inches

Ridge Board Thickness

Step 3 of 3

Pick your lumber

Choose rafter size and spacing, the calculator will enforce IRC HAP requirements.

Rafter Lumber Size

Rafter Spacing (On-Center)

Instant Results

All 10 measurements calculated in real time. Rafter length, birdsmouth, HAP, and more.

Code-Compliant

Built on IRC R802 and the American Wood Council's framing formulas.

Learn As You Build

Pro tips and methodology at every step so you understand the math.

About This Tool

What Is a Rafter Calculator?

A rafter calculator figures out every dimension you need to cut and install roof rafters: the sloped structural members that give a roof its shape and carry the load down to the walls. Feed it your building width, roof pitch, and a few other inputs, and it gives you rafter length, birdsmouth dimensions, ridge height, and how many boards to order. No trigonometry tables, no ruler-on-a-drawing guesswork.

This tool is built for residential construction: gable roofs, shed roofs, and any simple ridge-board assembly covered under IRC Section R802. The math follows the same formulas a licensed framing carpenter uses, the Pythagorean theorem for rafter length, trigonometry for roof angle, and the IRC's HAP requirements for birdsmouth depth.

Contractors use it to generate a cut list before the lumber truck arrives. Homeowners use it to check a contractor's quote or plan a DIY project. Carpentry students use it to understand the geometry of roof framing before touching a saw. Learn more about how we build our calculators.

Getting accurate results takes less than two minutes if you have your building plans or a tape measure handy. Here's how each step works.

Step 1: Building Dimensions

Enter the outside-to-outside wall width (span) and the building length. For a standard two-car garage that's typically 24 × 30 feet. Common mistake: using the interior dimension. Always use the exterior measurement.

Step 2: Pitch & Eave

Pick your pitch from the visual selector. The live preview updates instantly so you can see how steeper pitches affect your roof. Also set your eave overhang (12 inches is typical) and ridge board thickness.

Step 3: Lumber

Choose rafter size (2×6 through 2×12) and on-center spacing (16″ OC is standard residential). Hit Calculate to get all 10 measurements instantly.

Every number comes from the same geometry framers have used for over a century: the Pythagorean theorem applied to the right triangle formed by a roof rafter.

The Core Formula

For a 24-foot building with a 2× ridge (1.5″ actual) at 6/12 pitch:

Run = (24 ÷ 2) − (1.5 ÷ 2 ÷ 12) = 11.9375 ft
Rise = 11.9375 × (6 ÷ 12) = 5.969 ft
Rafter = √(11.9375² + 5.969²) ≈ 13.34 ft

Birdsmouth & HAP

The birdsmouth seat cut = rafter depth × cos(angle). The plumb cut = rafter depth × sin(angle). Per IRC R802.7.1, HAP must be ≥ 2/3 of the rafter depth to maintain structural integrity at the wall bearing point.

Roof Framing Guide

Everything You Need to Know

Cutting rafters is one of the most technically demanding parts of residential framing. The geometry is straightforward once you understand it, but a mistake in layout gets repeated across every rafter in the building.

Common Rafter vs. Hip Rafter

A common rafter runs perpendicular from the ridge board to the wall plate. A hip rafter runs diagonally from a building corner to the ridge. Hip roofs are more wind-resistant but require three rafter types and more layout work. This calculator handles common rafters. Read our hip roof framing guide for the full breakdown.

Choosing the Right Lumber Size

As a general rule: 2×6 for spans up to ~13 ft at 16″ OC; 2×8 to ~17 ft; 2×10 to ~22 ft; beyond that, 2×12 or engineered lumber. Check our rafter lumber sizing guide for species-specific tables.

Building Codes for Roof Framing in 2026

Most US jurisdictions have adopted the 2021 IRC. Key requirements: 1-1/2″ minimum bearing; birdsmouth ≤ 1/3 rafter depth; notches prohibited in the middle third of span; collar ties or ridge beams required. See our building code guide.

DIY Rafter Cutting Tips

Always cut a test rafter and dry-fit before cutting the full run. Mark your birdsmouth, fit it to the ridge and plate, verify HAP, then use it as a pattern. A 1/4-inch error in the birdsmouth gets amplified across every rafter. See our step-by-step cutting guide.

Who It's For

Built for Everyone Who Frames Roofs

Contractors & Framers

Generate a cut list before lumber delivery. Verify numbers on-site in 30 seconds.

DIY Homeowners

Building a garage or shed? The geometry is the hard part. This removes that barrier.

Carpentry Students

See how rafter length, HAP, and birdsmouth change as you adjust pitch and span.

Inspectors & Estimators

Verify rafter dimensions against plans, or estimate materials without a full drawing set.

FAQ

Common Questions

Rafter length is calculated using the Pythagorean theorem: length = √(run² + rise²). The run is half your building span minus half the ridge board thickness. The rise equals the run multiplied by the pitch ratio (pitch/12). For example, a 24-foot span with 6/12 pitch: run = 12 feet (minus ridge deduction), rise = ~6 feet, rafter line length = √(12² + 6²) ≈ 13.42 feet. Add the overhang tail length for the total rafter length. Always round up to the next standard lumber length when purchasing.

A birdsmouth (or bird's mouth) is a notch cut into a rafter where it sits on the wall plate (top plate of the wall). It consists of two cuts: a level seat cut that rests flat on the wall plate and a vertical plumb cut that fits against the outside of the wall. The birdsmouth allows the rafter to bear evenly on the wall while maintaining the correct roof angle. Per building code, the birdsmouth should not exceed one-third of the rafter depth to maintain structural strength, leaving at least two-thirds as the Height Above Plate (HAP).

HAP stands for Height Above Plate, which is the depth of rafter material remaining above the birdsmouth cut. The IRC requires that HAP be at least two-thirds of the total rafter depth. For a 2×6 rafter (5.5" actual depth), the minimum HAP is approximately 3.67 inches, meaning the birdsmouth can be no deeper than about 1.83 inches. HAP is critical because it determines the structural capacity of the rafter at its weakest point, the wall bearing location. Insufficient HAP can lead to rafter failure under load.

Rafter size depends on the span (run), spacing, species, grade, and load requirements. As general IRC guidelines: 2×6 rafters at 16" OC can span approximately 9-12 feet; 2×8 rafters can span 12-16 feet; 2×10 rafters can span 16-20 feet; and 2×12 rafters can span 20-24 feet. These spans assume #2 grade Southern Pine or Douglas Fir with standard residential loads (20 psf live load, 10 psf dead load). Heavy snow loads, wider spacing, or lower-grade lumber reduce allowable spans. Always consult IRC rafter span tables or a structural engineer for your specific conditions.

Count rafters by dividing the building length (in inches) by the on-center spacing and adding one for the starting rafter. Then double the count for both sides of a gable roof. For a 30-foot building at 16" OC: (360 ÷ 16) + 1 = 23.5, round up to 24 rafters per side, times 2 = 48 total rafters. At 24" OC the same building needs (360 ÷ 24) + 1 = 16 per side, or 32 total. Add extra rafters for hip or valley framing if your roof is not a simple gable design.

Rafters are individual lumber pieces cut and assembled on-site by framers, running from the ridge board to the wall plate. Trusses are pre-engineered triangular assemblies manufactured in a factory and installed as complete units. Rafters provide open attic space (no web members) and allow for cathedral ceilings without special designs. Trusses are faster to install, can span greater distances without interior bearing walls, and are typically more cost-effective for new construction. Rafters are preferred for additions, renovations where crane access is limited, and custom roof designs with dormers or skylights.

Roof pitch is expressed as the number of inches the roof rises vertically for every 12 inches of horizontal run. A 6/12 pitch means the roof rises 6 inches for every 12 inches of run. To measure an existing roof pitch, place a level horizontally against the roof surface, measure 12 inches along the level, then measure the vertical distance from the level down to the roof at that 12-inch mark. Common residential pitches range from 4/12 to 8/12, with 6/12 being the most popular for standard homes.

A ridge board is a non-structural board (typically a 1× or 2× board) placed at the peak where opposing rafters meet. It serves as a nailing surface and alignment guide but does not bear load, the rafter loads are transferred to the walls through the ceiling joists (which act as ties). A ridge beam is a structural member (typically engineered lumber like LVL or glulam) that actually supports the weight of the rafters. Ridge beams are required when there are no ceiling joists or collar ties to prevent the rafters from spreading, such as in cathedral ceiling designs. Ridge beams require support posts at each end.

This calculator provides common rafter measurements for standard gable roofs. Hip roofs use common rafters plus specialized hip rafters (which run diagonally from corners to the ridge) and jack rafters (shorter rafters that connect to hip rafters). Hip rafter length can be estimated by multiplying the common rafter run by 1.414 (the square root of 2) and then calculating the hypotenuse using the appropriate pitch. However, hip roof framing involves complex compound angles that typically require specialized hip/valley rafter tables or framing square calculations beyond what a basic rafter calculator provides.

Written & Reviewed By

Rafter Calculator Team

We build free, accurate roof framing calculators based on IRC R802 and American Wood Council standards.

IRC R802 VerifiedAbout Us

Cut rafters. Get it right the first time.

What Is a Rafter Calculator?

Everything You Need to Know

Common Rafter vs. Hip Rafter

Choosing the Right Lumber Size

Building Codes for Roof Framing in 2026

DIY Rafter Cutting Tips

Built for Everyone Who Frames Roofs

Common Questions

01How do I calculate rafter length?

02What is a birdsmouth cut?

03What is HAP in rafter framing?

04What size rafters do I need?

05How many rafters do I need?

06What is the difference between a rafter and a truss?

07How do I determine roof pitch?

08What is a ridge board vs a ridge beam?

09Can I use a rafter calculator for hip roofs?

Rafter Calculator Team