Rafter calculator for pitch, run, rise, and cut references
Calculate common rafter length, ridge height, tail length, birdsmouth checks, and cut angles with a cleaner workflow than the old-school utility calculators.
Quick reference
What this tool helps you work out
Run helper
(Building width − ridge thickness) ÷ 2
Useful when you know the full span but need the common rafter run
Rise
Run × tan(angle)
Use this to estimate ridge height above the top plate
Rafter length
(Run + overhang) ÷ cos(angle)
Gives the line length of the full common rafter
Built for roof framing
This tool is ideal for quick geometry checks and cut planning. Always confirm structural sizing, spans, spacing, and local code requirements before building.
Try Metres.ai →1. Enter the roof geometry
Set the roof angle, run, overhang, member sizes, birdsmouth dimensions, and wall reference values.
2. Use the run helper if needed
If you know the building width and ridge thickness, the tool can calculate the run to the outer wall for you.
3. Read cuts and lengths
Get rise, ridge height, full rafter length, seat and plumb references, and quick cut checks in one place.
Common rafter calculator with run helper and cut checks
A cleaner take on the classic rafter calculator: better layout, better scanning, and the same practical geometry you need on site.
Inputs
Set up the roof geometry
Common pitch angle in degrees
Half span adjusted for ridge thickness
Horizontal tail overhang
Actual rafter depth
Actual rafter thickness
Top plate / wall thickness
Vertical plumb cut depth
Horizontal seat cut length
Top plate to finished floor reference
Run helper
Calculate rafter run from building width
Overall building width
Actual ridge board thickness
Formula: (building width − ridge thickness) ÷ 2
Live controls
Tune the drawing
Main canvas
Roof and rafter geometry
Cut detail canvas
Birdsmouth and cut reference
Results
Rafter lengths and cut references
Vertical rise from top plate to ridge line
Rafter length from ridge to outside wall line
Length along the rafter for the overhang
Full rafter line length excluding extra trim allowance
Top plate height plus calculated rise
Roof angle for ridge and birdsmouth plumb reference
Angle relative to the plumb cut
Approx. controlling birdsmouth depth from your inputs
Cut checks
Difference vs input plumb: 0.011 m
Difference vs input seat: 0 m
Useful for checking if the notch is too deep for the member
Vertical rise added by the horizontal overhang