Rafter Length Calculator
This rafter calculator sets out a single common rafter. Enter the span or the run and the roof pitch, and it reads back the line length, the rise, the total length including the eaves tail, the plumb and seat cut angles, the birdsmouth seat cut and height above plate (HAP), and the ridge deduction. Every length is available in feet-inch-fraction for the tape measure or in millimetres and metres.
The maths is the classic framing-square identity: the rafter length per unit of run is the square root of one plus the slope squared, so a 6:12 pitch travels about 13.42 inches of rafter for every 12 inches of run. The tool applies that multiplier to the adjusted run and adds the sloped tail for the overhang.
Everything runs in your browser: no sign-up and nothing uploaded. Switch between metric and imperial at any time. The birdsmouth and ⅓-depth advisory are set-out guidance, not a structural sizing — size your rafters from a span table or an engineer.
How to use the rafter calculator
Choose whether you are entering the full span or the run (half-span), then type the roof pitch as x:12 or in degrees. The calculator shows the line length, rise and total length live as you type, along with the plumb and seat cut angles.
Add the eaves overhang and pick whether you measured it horizontally or along the slope, then set the ridge thickness and reference. Finally, pick a rafter stock size (or type the depth) and the seat bearing width to read the birdsmouth seat cut, heel plumb cut and HAP. Enter the roof length and rafter spacing to get an optional rafter count and timber total.
How to set out a rafter in 5 steps
- Choose unitsPick metric (mm / m) or imperial (feet-inch-fraction) at the top of the page. Every input and result converts as you switch.
- Enter the span or runChoose Span to enter the full building width, or Run to enter the horizontal distance to the ridge, and type the figure.
- Enter the pitchType the roof pitch as a rise per 12 (x:12) or in degrees. The line length, rise and cut angles appear immediately.
- Add the overhang and ridgeEnter the eaves overhang and the ridge thickness, and choose the ridge reference so the run is deducted correctly.
- Read the cut listRead the total length, line length, plumb and seat cut angles, and the birdsmouth seat cut, heel and HAP. Print or export the cut list.
The parts of a rafter
A common rafter runs from the ridge down to the wall plate and past it to form the eaves. The run is the horizontal distance from the ridge centre to the outside of the wall plate; the rise is the vertical height the rafter climbs over that run. The line length is the sloping distance along the rafter's measuring line for that run.
The plumb cut at the top meets the ridge board, and half the ridge thickness is deducted from the run so both rafters meet on the centre line. The birdsmouth is the notch that seats the rafter on the wall plate — a level seat cut and a plumb heel cut. Beyond the plate the tail carries the overhang to the eaves, finished with a tail cut that is usually plumb.
The birdsmouth and HAP
The birdsmouth is cut so the rafter sits flat on the wall plate. The seat cut is a level line as deep as the plate bearing; the heel cut is the plumb line that meets it. Height above plate (HAP), or stand-off, is how much of the rafter's depth remains above the plate at the building line — it sets the height of the roof plane and is measured plumb from the seat to the top edge of the rafter.
A deeper seat cut gives more bearing but removes more of the rafter. Building codes limit the seat notch to no more than one third of the rafter's actual depth (IRC R802.7.1) so enough sound timber remains. The calculator flags when your seat depth pushes the notch past that third.
Plumb and seat cut angles
Both cut angles come straight from the pitch. The plumb cut — used at the ridge, at the birdsmouth heel and at the tail — equals the pitch angle measured from horizontal: the arctangent of the slope. The seat (level) cut is its complement, 90 degrees minus the pitch angle, because the seat and heel of the birdsmouth meet at a right angle.
On a speed square, dial the pitch angle on the common scale to scribe the plumb line, and cut the seat at the complementary reading. On a circular saw the bevel is set off square by the seat cut. Because the angles depend only on the pitch, this tool shows them as soon as you enter a pitch, even before the lengths are complete.
Ridge deduction
Two rafters meet over the ridge, so the run is normally measured to the ridge centre line and half the ridge thickness is deducted before computing the rise and line length. For a 1½ inch (38 mm) ridge board that is a ¾ inch (19 mm) deduction from each side.
Choose Ridge centre to apply the half-thickness deduction, Ridge face if your run is already measured to the near face of the ridge, or No ridge for a layout with no ridge board. Face and none apply no deduction; only the diagram annotation differs.
Rafter length per foot of run
The framing-square rafter table gives the common-rafter line length for every 12 inches of run at each common pitch — the same multiplier this calculator uses. Multiply the figure by the run in feet to get the line length, then add the tail for the overhang.
For example, at 6:12 the length per foot of run is 13.42 inches, so 12 feet of run gives about 13 feet 5 inches of line length before the tail.
| Pitch | Length per 12 run | Angle | Multiplier |
|---|---|---|---|
| 2:12 | 12.17 | 9.5° | 1.0138 |
| 3:12 | 12.37 | 14.0° | 1.0308 |
| 4:12 | 12.65 | 18.4° | 1.0541 |
| 5:12 | 13.00 | 22.6° | 1.0833 |
| 6:12 | 13.42 | 26.6° | 1.1180 |
| 7:12 | 13.89 | 30.3° | 1.1577 |
| 8:12 | 14.42 | 33.7° | 1.2019 |
| 9:12 | 15.00 | 36.9° | 1.2500 |
| 10:12 | 15.62 | 39.8° | 1.3017 |
| 12:12 | 16.97 | 45.0° | 1.4142 |
Frequently asked questions
How do I calculate rafter length?
Multiply the run by the rafter length per unit of run for your pitch — the square root of one plus the slope squared. For a 6:12 pitch that multiplier is about 1.118, so a 12-foot run gives roughly 13 feet 5 inches of line length. Add the sloped tail for the overhang to get the total length. This calculator does it for you from the span or run and the pitch.
What is the line length of a rafter?
The line length is the sloping distance measured along the rafter's measuring line for the adjusted run — from the ridge centre line down to the outside of the wall plate. It excludes the tail. The total rafter length is the line length plus the sloped tail that forms the overhang.
What is a birdsmouth cut?
A birdsmouth is the notch that lets a rafter sit flat on the wall plate. It has a level seat cut, as deep as the plate bearing, and a plumb heel cut that meets it at a right angle. Keep the seat notch to no more than a third of the rafter's actual depth so enough sound timber remains above the plate.
What is HAP, or height above plate?
HAP (height above plate), also called the stand-off, is the amount of the rafter's depth remaining above the wall plate at the building line, measured plumb from the seat cut to the top edge of the rafter. It sets the height of the roof plane and must match on every rafter for a flat roof surface.
What are the plumb and seat cut angles?
The plumb cut equals the pitch angle from horizontal — the arctangent of the slope — and is used at the ridge, at the birdsmouth heel and at the tail. The seat (level) cut is its complement, 90 degrees minus the pitch angle. On a 6:12 roof the plumb cut is about 26.6° and the seat cut about 63.4°.
Why deduct half the ridge thickness?
Two rafters meet over the ridge board, so the run is measured to the ridge centre line and half the ridge thickness is subtracted from each side. For a 1½ inch (38 mm) ridge that is a ¾ inch (19 mm) deduction, which keeps both rafters meeting exactly on the centre line.
Is the overhang horizontal or along the slope?
By default the overhang is the horizontal projection past the wall, and the calculator multiplies it by the pitch multiplier to get the sloped tail length. If you measured the sloped tail directly, switch the overhang to along-slope and the value is used as the tail length with no multiplier.
Does the calculator work in metric and feet-inches?
Both. Switch units at the top: imperial lengths are shown in feet-inch-fraction at your chosen tape denominator (1/8, 1/16 or 1/32), and metric lengths in millimetres and metres. The engine works in SI internally and only converts for display, so switching never changes the underlying set-out.
Is the rafter calculator private?
Yes. Every calculation runs entirely in your browser. Nothing you enter is sent to a server, and your inputs are saved only in your own browser's local storage.