Calculate rafters with free app

Calculate the length of common rafters in feet, inches, and fractions – faster, easier, and more reliably than ever before.

Install now:

iOS download

Android download

The three things you need are:

Roof width

This dimension is the distance from the outside of the wall to the outside of wall where the HAP stops. This could be either the outside of the wall plate, or the outside of the wall sheathing (wherever the birds mouth hooks). This measurement does not need to include the overhang as this is done separately.


This value is easily found on the blueprints by finding whatever corresponding view relates to the rafters. This value is always represented in mid-air right above the roof plane with a half of rectangle where the horizontal value (Run) is 12″ and the number you are looking for is the vertical measurement (Rise). The larger the pitch, the steeper the roof.

Ridge thickness

Most situations will be simply inch and a half. With the advent of more advanced building materials such as laminated beams and more complex engineering such as three ply ridges, this is not always the case. The thicker the ridge, the shorter the rafter.

Open app

Diagonal calculator

Click on the Menu button

Select rafter

Rafter calculator menu

Click on rafter, the second one down.

Roof width

Roof width

Use feet and inches then click equals.



Enter a value for the inches per foot rise, then click equals.

Ridge thickness

Ridge thickness

Enter a dimension in inches or just use a decimal value, then click equals.



Rafter length

From the long point at the top to the top of the HAP at the bottom, then click equals for more.



The distance from the top of the HAP to the top of the ridge. Click equals for more.



The distance from the outside of the wall to the inside of the ridge.

My Technique

Make sure you are accurate for the three criteria you need.

Use a framing square and a sharp pencil for the layout.

Make a rafter tail from the same lumber first.

Write on the tail the length for the rafter, the rise and the run.

There are fundamental basics you should be familiar with before you should attempt to tackle a set of rafters. If you are not comfortable with the challenge, then hire somebody.

Keep everything accurate and safe.