About the Rafter Length Calculator
A common rafter runs from the top of the wall to the ridge. Its length depends on how far it runs horizontally and how steep the roof is. This calculator takes the run, the pitch as the rise per 12 units of run and the overhang beyond the wall, and gives the length of the rafter along the run, the extra length for the overhang, the total length and the height that the roof gains. Type the run, the pitch and the overhang and the rafter length appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the Rafter Length Calculator
- 1Enter the run, the pitch and the overhang.
- 2Pick the units or options from the menus if the rafter length tool offers them.
- 3Read the rafter length in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the rafter length
Carpenters use it to cut rafters and order the right length of timber. Designers use it to check that a loft will have enough headroom, and DIY builders use it to plan a shed or a garage roof.
The run is half the span of a simple gable roof, measured from the outside of the wall to the centre line of the ridge. Subtract half the thickness of the ridge board from the run for an exact cut.
How the rafter length is worked out
Rafter length = run × √(1 + (pitch ÷ 12)²). The overhang is multiplied by the same factor. Height gained = run × pitch ÷ 12. With a run of 12 feet and a 6 in 12 pitch, the rafter along the run is 13.416 feet. A horizontal overhang of 1 foot adds 1.118 feet, so the total is 14.534 feet. The roof rises 6 feet over the run.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- Rafter, overhang and total
- Height gained
- Any unit
Mistakes to avoid with rafter length
The length found is to the centre line of the rafter on the ridge and to the end of the overhang along the top edge. A birdsmouth seat and the plumb cut at the ridge need extra layout that this tool does not do. Buy timber a little longer for trimming, and check local codes for sizes and spacing.
On phone and computer
On a phone, type into the boxes one after another and read the rafter length underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the rafter length move. The rafter length calculator works the same on both. Rafters depend on the pitch of the roof, so check that too. Use the Roof Pitch Calculator for the slope, the Roofing Calculator to plan the covering, and the Wall Stud Calculator to frame the walls.
Features
- Run is to the ridge centre
- Overhang is horizontal
- Cut layout not included
- Clear messages when the rafter length inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate rafter length?
Rafter length = run × √(1 + (pitch ÷ 12)²). The overhang is multiplied by the same factor. Height gained = run × pitch ÷ 12. With a run of 12 feet and a 6 in 12 pitch, the rafter along the run is 13.416 feet. A horizontal overhang of 1 foot adds 1.118 feet, so the total is 14.534 feet. The roof rises 6 feet over the run.
What is the formula for rafter length?
Rafter length = run × √(1 + (pitch ÷ 12)²). The overhang is multiplied by the same factor. Height gained = run × pitch ÷ 12. The rafter length calculator applies it exactly and rounds only the displayed answer to six significant figures.
What is the run of a roof?
The horizontal distance from the outer edge of the wall to the centre of the ridge.
What is a birdsmouth cut?
A notch in the rafter where it sits on the wall plate.
Does the overhang count horizontally?
Yes, enter the horizontal projection beyond the wall. The tool adds the slope.
Is this rafter length calculator free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.