A rafter length calculator determines the exact cut length for common rafters, the ridge board height above the floor, birdsmouth seat and plumb cut dimensions, and the total number of rafters needed for a gable roof. Getting these numbers right before you touch a saw prevents wasted lumber and ensures your roof frame sits plumb and square.
Rafter Framing Calculator
Measure from outside of wall framing to outside of opposite wall framing.
Select a preset or enter the rise value. Pitch = rise per 12 inches of horizontal run.
Horizontal eave projection past the wall plate. Typical: 12–18 inches.
How to Use the Rafter Length Calculator
Cutting rafters accurately is one of the most math-intensive parts of residential framing. Our free rafter length calculator handles the trigonometry so you can focus on cutting and assembling — not number-crunching. Enter your roof span, pitch, and overhang, and the calculator instantly returns every measurement you need to cut a perfect common rafter.
Step 1: Enter the Building Span
The span is the horizontal distance from the outside face of one wall plate to the outside face of the opposite wall plate. Measure this carefully — a 1-inch error in span creates a half-inch error in run and compounds through every rafter dimension. The run (used in all rafter calculations) is exactly half the span.
Step 2: Select the Roof Pitch
Roof pitch expresses how many inches the roof rises for every 12 inches of horizontal run. A 6/12 pitch rises 6 inches per foot of run. Use the dropdown to choose a common pitch or type the rise value directly. The pitch determines the rafter's angle and controls every cut angle on the rafter — ridge plumb cut, birdsmouth, and tail cut all share this same angle.
Step 3: Set the Eave Overhang
The overhang is the horizontal distance the rafter extends past the wall plate to form the soffit. Typical residential overhangs range from 12 to 18 inches. The calculator adds the angled overhang length to the structural rafter length to give you the total cut length — the measurement from the ridge plumb cut to the tail plumb cut.
Step 4: Enter Building Length and Wall Height
Building length is used to calculate the ridge board length and the total number of rafters needed. Wall plate height (floor to top of plate) is used to calculate the ridge height above the floor — critical for planning ceiling heights and ordering ridge posts. Standard 8-foot walls measure 97.125 inches to the top of the double top plate.
Step 5: Review Birdsmouth Dimensions
The birdsmouth is the notch cut into the bottom of each rafter so it sits flat on the wall plate. The seat cut is horizontal and matches your wall plate width (3.5 inches for a 2×4 wall, 5.5 inches for a 2×6 wall). The plumb cut is vertical and equals the rafter thickness (1.5 inches for standard 2× lumber). Set your framing square to the pitch and mark both cuts at the point where the rafter crosses the wall plate.
Understanding the Ridge Height Result
Ridge height is the vertical distance from the finished floor to the top of the ridge board. It equals the run multiplied by the tangent of the pitch angle, plus the wall plate height. This number helps you plan attic access, ceiling treatments, and the length of any ridge posts. Remember to subtract the ridge board depth when calculating where the rafter top ends.
Ordering Lumber
Standard framing lumber comes in 2-foot increments: 8, 10, 12, 14, 16 feet. Order rafter stock at the next standard length above your total calculated rafter length. For example, if your total rafter length is 15.2 feet, order 16-foot boards. The material estimate in the results section shows exactly how many rafter boards to purchase, plus the ridge board length.
Frequently Asked Questions
Is this rafter length calculator free to use?
Yes, the rafter length calculator is completely free with no limits or signup required. You can calculate rafter dimensions for as many roof framing projects as you need. All calculations run locally in your browser — your measurements are never sent to a server.
Is my data private when using this tool?
Absolutely. All calculations happen entirely in your browser using client-side JavaScript. Your roof dimensions and project details are never transmitted to a server or stored anywhere. Your data stays on your device.
How is rafter length calculated?
Rafter length equals the run divided by the cosine of the pitch angle. The run is half the building span (from the ridge center to the wall plate edge). For example, a 24-foot span with a 6/12 pitch has a 12-foot run and a pitch angle of 26.57°, giving a rafter length of 12 ÷ cos(26.57°) = 13.42 feet.
What is a birdsmouth cut on a rafter?
A birdsmouth is a notch cut into the bottom of a rafter where it sits on the wall plate. It has two parts: the seat cut (horizontal, equal to the wall plate width — typically 3.5 inches for a 2×4 or 5.5 inches for a 2×6) and the plumb cut (vertical, equal to the rafter thickness — typically 1.5 inches). The birdsmouth keeps the rafter from sliding off the plate.
How do I find the ridge height from roof pitch?
Ridge height equals the run multiplied by the tangent of the pitch angle, added to the wall plate height. For a 6/12 pitch with a 12-foot run on 8-foot walls, the ridge is 12 × tan(26.57°) + 96 = 6 feet + 8 feet = 14 feet above the floor. Measure wall height from the subfloor to the top of the wall plate.
What is rafter spacing and how does it affect the count?
Rafter spacing is the on-center distance between rafters, typically 16 or 24 inches for residential roofs. The number of rafters is calculated as (ridge length ÷ spacing) + 1, multiplied by 2 for both roof sides. Always add one extra rafter to account for the final end rafter. Check local building codes for required spacing based on rafter size and snow loads.
What is the ridge board length?
The ridge board runs the full length of the roof peak. For a simple gable roof, the ridge board length equals the building length. If the roof has gable overhangs, add the overhang amount to each end. Ridge boards are typically one size larger than the rafters — use a 2×8 ridge board for 2×6 rafters.
How does overhang length affect total rafter length?
The overhang (eave) extends the rafter past the wall plate to form the soffit. Total rafter length equals the structural rafter length plus the overhang length. For example, a 13.42-foot rafter with a 12-inch overhang has a total cut length of 14.42 feet. Always buy lumber at least 6 inches longer than the total calculated length to allow for the plumb cut at the ridge.