Does this calculator include ridge board thickness in the rafter length?

No. The rafter length reported is measured to the theoretical roof centerline, not to a specific ridge board thickness. If your rafters bear on a ridge board, you typically add half the ridge thickness to each rafter when marking layout so the cut faces meet correctly at the ridge.

How do I convert the roof pitch into a saw angle?

The calculator outputs the roof angle in degrees by taking the arctangent of rise divided by run. You can use that angle as the bevel setting on a miter saw for plumb cuts or as a reference when setting adjustable bevel gauges in the field.

Can I use this tool for metric dimensions?

The inputs and outputs are defined in feet and inches, following typical North American framing conventions. If you work in metric, convert your span and overhang to feet (or treat the numbers as meters consistently) and interpret the results in the same unit, keeping the rise/run ratio consistent.

Does the calculator account for birdsmouth seat cuts or heel height?

Not directly. Birdsmouth cuts, raised heel trusses, and other framing details change how much of the rafter actually bears on the wall and can slightly reduce effective span. Use the calculator for geometric layout, then adjust your cut marks to match the details shown on your plans or engineering drawings.

Is this enough information to size rafters structurally?

No. This tool helps with length and angle layout only. Structural sizing depends on species, grade, spacing, snow and wind loads, and local building code requirements. Always consult span tables, manufacturer literature, or a licensed engineer when choosing rafter sizes and spacing.

What if my roof has hips, valleys, or multiple pitches?

This calculator assumes a single-pitch gable roof with common rafters. Hips, valleys, and intersecting roofs use different geometry and may require specialized calculators, layout guides, or professional design support to determine exact member lengths and angles.

construction calculator

Rafter Length Calculator

Calculate rafter length and angle from span, pitch, and overhang.

Results

Rafter length (ft): 13.42
Length incl. overhang (ft): 14.42
Roof angle (degrees): 26.57

Overview

This rafter length calculator helps you turn roof span, pitch, and overhang into a cut length and bevel angle you can actually use on site. Instead of flipping through rafter tables on a framing square or doing Pythagorean math by hand, you enter a few dimensions and get a clean length from wall plate to ridge plus the extra overhang for your eaves.

How to use this calculator

Measure or read the total roof span from exterior wall to exterior wall and enter it in feet. If you are working from a set of drawings, use the clear span dimension between bearing points.
Enter the roof pitch as inches of rise per 12 inches of run. For example, a 6/12 roof has a pitch of 6, meaning it rises 6 inches for every foot of horizontal run.
Add the planned overhang measured along the rafter, in feet. This typically represents the distance from the outside of the wall to the end of the rafter tail at the fascia.
Review the calculator outputs: rafter length from wall plate to ridge, total length including overhang, and the bevel angle in degrees that matches your chosen pitch.
Compare the computed rafter length against stock lumber sizes at the yard so you know whether you need 12s, 14s, 16s, or longer members, and plan for a bit of extra length so you can trim to final layout on sawhorses.
If you change span, pitch, or overhang during design, rerun the calculator with the new values to see how quickly board length and angle change before committing to framing details.

Inputs explained

Roof span (ft): The total distance from outside bearing wall to outside bearing wall across the building, measured horizontally. The calculator automatically divides this value by two to find the run for a simple gable roof, so you do not need to enter the half-span yourself.
Pitch (rise per foot of run): Roof pitch expressed as inches of rise for every 12 inches of run. A value of 6 means a 6/12 pitch (6-inch rise per foot), 4 means 4/12, and so on. Steeper pitches increase the rise and therefore the rafter length and angle.
Overhang (ft): The length of rafter tail that projects beyond the outside face of the wall plate. This covers your eave or fascia and can also support gutters or soffit details. Enter this in feet (for example, 1.0 for a 12-inch overhang, 1.5 for an 18-inch overhang).

Outputs explained

Rafter length (ft): The slope distance from the outside wall plate to the ridge centerline based on your span and pitch. This is the core length you need before adding overhang or adjusting for ridge thickness, birdsmouth depth, or heel height.
Length incl. overhang (ft): The rafter length plus the overhang you entered, giving you an overall board length from wall plate to tail. Use this to choose stock lengths, order material, and rough-cut pieces before making precise layout marks.
Roof angle (degrees): The pitch converted into a bevel angle measured between the rafter and the horizontal plane. Many miter saws accept angle settings directly in degrees, so you can use this value to set up plumb cuts and verify the roof slope during layout.

How it works

The calculator first converts the total roof span into a run by dividing by two. In a simple gable, each rafter carries half the span from exterior wall to ridge, so Run = Span ÷ 2. That run is measured horizontally, not along the slope of the roof.

Next, it turns your pitch into a rise for that run. Roof pitch is commonly expressed as inches of rise per 12 inches of run (for example, 6/12 or 4/12). We treat the pitch input as inches of rise per foot of run, so Rise = Run × (Pitch ÷ 12). The result is a vertical rise in feet over the half-span.

With run and rise in hand, the calculator applies the Pythagorean theorem to get the plumb rafter length from wall plate to the theoretical centerline of the ridge: Rafter length = √(Run² + Rise²). This gives you the slope distance of the rafter along its top edge before any overhang is added.

The overhang input is added linearly to the rafter length to represent the projection beyond the wall plate for your eaves or fascia. Total length = Rafter length + Overhang, which is the board length you should plan to cut, before trimming for ridge thickness or birdsmouth depth.

Finally, the tool computes the roof angle in degrees using the arctangent of rise over run: Angle = arctan(Rise ÷ Run) converted from radians to degrees. That angle is what you can dial into a miter saw for plumb cuts or use to verify that the pitch matches a printed plan.

All calculations are done in feet for length and degrees for angles. Internally, tiny safeguards prevent divide-by-zero errors if you experiment with very small spans or flat pitches, but for practical framing you’ll typically be working with standard spans and pitches in the 3/12–12/12 range.

Formula

Let Span = total distance from outside wall to outside wall (ft).\nRun = Span ÷ 2 (ft)\nPitch = inches of rise per 12 inches of run (for example, 6 for a 6/12 roof).\nRise = Run × (Pitch ÷ 12) (ft)\nRafter length = √(Run² + Rise²) (ft)\nTotal length = Rafter length + Overhang (ft)\nAngle (degrees) = arctan(Rise ÷ Run) × 180/π

When to use it

Estimating rafter board lengths and counts for a gable roof before ordering lumber, so you avoid buying pieces that are too short for your design pitch and overhang.
Checking how a change in roof pitch—from 4/12 to 6/12, for example—affects rafter length, material cost, and waste when you are still in the planning or budgeting phase.
Confirming the bevel angle to set on a miter saw for plumb cuts, especially if you do not have a framing square with rafter tables or you are more comfortable working in degrees than in pitch ratios.
Helping DIY builders translate a printed plan that specifies span and pitch into concrete cut lengths and angles for rafters, without needing to memorize trigonometry or dig through reference charts.
Exploring how extending eaves or reducing overhang affects overall board length when designing for shading, water management, or aesthetic proportions at the roof edge.
Creating quick comparison scenarios for different building widths and pitches to see which combinations keep rafter lengths within standard stock sizes while still meeting design goals.

Tips & cautions

Always allow a bit of extra length when ordering rafters so you have room to square up the ends and make clean plumb and seat cuts on site. The calculator gives a theoretical length; real lumber and layout usually need a little trimming.
Remember that ridge boards and engineered ridges have thickness. If your rafters meet at a ridge board, you may need to add half the ridge thickness to each side when laying out cuts so the actual meeting point aligns with the design span.
If you use a birdsmouth to seat the rafter on the wall plate, plan for the depth of the seat cut and plumb cut when marking your layout. This calculator does not subtract birdsmouth depth, so leave enough material to make those cuts safely.
Keep units consistent. Enter span and overhang in feet and pitch as inches of rise per foot of run. Mixing inches and feet without converting can produce rafter lengths that do not match real-world framing.
Use the angle output to double-check that your chosen pitch matches other details such as roofing manufacturer minimums, snow load considerations, and local code requirements for water shedding.
This tool models simple geometry for a single common rafter on a basic gable roof. It does not size structural members, account for snow load, wind load, or deflection limits, or replace engineered span tables.
Ridge thickness, heel height, birdsmouth cuts, and other framing details are not modeled explicitly. You must adjust cut lengths and layout to reflect the actual connection details in your design or construction drawings.
Hips, valleys, jack rafters, and complex roof intersections require different geometry and layout rules. This calculator does not produce lengths or angles for those members and should not be used for multi-directional framing without additional design work.
Building codes, engineering requirements, and manufacturer installation instructions can all impose minimum pitches, maximum spans, and specific framing details that this simple calculator does not enforce.

Worked examples

Example 1: 24 ft span, 6/12 pitch, 1 ft overhang

Span = 24 ft → Run = 24 ÷ 2 = 12 ft.
Pitch = 6/12, so Rise = 12 × (6 ÷ 12) = 6 ft.
Rafter length = √(12² + 6²) = √(144 + 36) = √180 ≈ 13.42 ft.
Total length including 1 ft overhang = 13.42 + 1.0 ≈ 14.42 ft.
Roof angle = arctan(6 ÷ 12) ≈ 26.6°, which you can use as the plumb-cut bevel angle.

Example 2: 20 ft span, 4/12 pitch, 1.5 ft overhang

Span = 20 ft → Run = 20 ÷ 2 = 10 ft.
Pitch = 4/12, so Rise = 10 × (4 ÷ 12) ≈ 3.33 ft.
Rafter length = √(10² + 3.33²) ≈ √(100 + 11.09) ≈ √111.09 ≈ 10.54 ft.
Total length including 1.5 ft overhang ≈ 10.54 + 1.5 ≈ 12.04 ft.
Roof angle = arctan(3.33 ÷ 10) ≈ 18.4°, suitable for setting the saw bevel for these rafters.

Example 3: 28 ft span, 8/12 pitch, 2 ft overhang

Span = 28 ft → Run = 28 ÷ 2 = 14 ft.
Pitch = 8/12, so Rise = 14 × (8 ÷ 12) ≈ 9.33 ft.
Rafter length = √(14² + 9.33²) ≈ √(196 + 87.06) ≈ √283.06 ≈ 16.82 ft.
Total length including 2 ft overhang ≈ 16.82 + 2.0 ≈ 18.82 ft.
Roof angle = arctan(9.33 ÷ 14) ≈ 33.5°, a noticeably steeper roof that demands longer stock lengths.

Deep dive

Use this rafter length calculator to convert roof span, pitch, and overhang into a practical rafter length and bevel angle for gable framing. It applies the standard rise–run geometry and Pythagorean theorem so you can move from plans to cut lists without manual trig.

The tool is ideal for framers, builders, and homeowners who want to check board lengths, choose stock sizes, and verify saw angles before cutting. Pair it with official span tables and local code guidance when selecting actual rafter sizes and spacing.

FAQs

Does this calculator include ridge board thickness in the rafter length?: No. The rafter length reported is measured to the theoretical roof centerline, not to a specific ridge board thickness. If your rafters bear on a ridge board, you typically add half the ridge thickness to each rafter when marking layout so the cut faces meet correctly at the ridge.
How do I convert the roof pitch into a saw angle?: The calculator outputs the roof angle in degrees by taking the arctangent of rise divided by run. You can use that angle as the bevel setting on a miter saw for plumb cuts or as a reference when setting adjustable bevel gauges in the field.
Can I use this tool for metric dimensions?: The inputs and outputs are defined in feet and inches, following typical North American framing conventions. If you work in metric, convert your span and overhang to feet (or treat the numbers as meters consistently) and interpret the results in the same unit, keeping the rise/run ratio consistent.
Does the calculator account for birdsmouth seat cuts or heel height?: Not directly. Birdsmouth cuts, raised heel trusses, and other framing details change how much of the rafter actually bears on the wall and can slightly reduce effective span. Use the calculator for geometric layout, then adjust your cut marks to match the details shown on your plans or engineering drawings.
Is this enough information to size rafters structurally?: No. This tool helps with length and angle layout only. Structural sizing depends on species, grade, spacing, snow and wind loads, and local building code requirements. Always consult span tables, manufacturer literature, or a licensed engineer when choosing rafter sizes and spacing.
What if my roof has hips, valleys, or multiple pitches?: This calculator assumes a single-pitch gable roof with common rafters. Hips, valleys, and intersecting roofs use different geometry and may require specialized calculators, layout guides, or professional design support to determine exact member lengths and angles.

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This rafter length calculator provides geometric estimates for planning and layout only. It does not perform structural engineering, verify compliance with building codes, or account for local snow, wind, or seismic requirements. Always confirm rafter sizes, spacing, framing details, and roof pitches with applicable code, manufacturer specifications, and a qualified design professional before construction.