Online Rafter Length Calculator

Calculate the length of common rafters with this free and easy online tool by entering a dimension for the roof width, selecting a pitch for the roof, and the thickness of the ridge. The results are automatically fired. They include :

  • The length of the rafter.
  • The rise of the rafter.
  • The run of the rafter.
  • Height of the scaffold

Rafter Calculator Instructions

Enter the value for the roof width (outside to outside) in feet, inches, and fraction, inches and fraction, or use the inches box to enter a value in decimals if that is your preference. See the detailed instructions below for any further questions.

  • If there will be no internet connection at the job site, then go ahead and open this page on a device such as a laptop or a smartphone in a location where there is an internet connection. Once this page is opened, the calculator will work without an internet connection.
  • Take a direct measurement of the width of the roof. This must be from walls that are of equal heights. If they are not, then click here for the unlevel wall rafter calculator.
  • Use the number boxes to enter the value in feet, inches, and fraction, inches and fraction.
  • Then use the drop down to select a value for the fraction.
  • For advanced users simply use the inches box to enter a decimal value.
  • Use the blueprints to establish a pitch for the roof. Select a roof pitch.
  • Determine a thickness for the ridge and use the drop down to select it.

The results are automatically fired as soon as any of the values are entered or altered. The length of the rafter is given from the long point of the ridge cut at the top down along the top edge the board to the same plumb cut on the rafter’s notch. The next result is the run. This is the horizontal measurement from the outside of the wall to the inside of the ridge. Then the rise is the vertical measurement from the top of the rafter on the outside edge of the plumb cut on the notch. This is also known as the HAP (height above plate). The scaffold is a value in inches that represents the vertical distance from the top of the HAP to the top of the walk board.

Parts of a Rafter

The top of a a rafter usually joins a ridge. The miter is simply a plumb cut. There must be no gaps in the precision of the fit. If there is a noticeable gap, then it should be a red flag for investigation.

The part of the rafter that sits on the wall plate is known as the bird’s mouth. It is a combination of two cuts. There is a plumb cut that is placed at distance returned by the calculator measured directly along the top of the board from the ridge cut, and there is a level slope (back slope) which should sit entirely on a wall plate with little or none of the wall plate left. What is left of the plumb cut is called the HAP.

The rafter tail has no considerations for the calculator. It should be laid out entirely with a framing square. There should be no part of the tail that is run “wild” and will have to be trimmed later.

The length of the rafter is marke along the top edge of the board.


Rafters are the sloping beams that support the weight of a roof. Typically they stretch from a wall plate to the ridge. They have limits on how far they can span and rules on how they must be spaced and fastened. Click here for 2012 IRC span charts and building codes.

Laying out a Calculated Rafter

  • Place the board on a set of sawhorses with the crown away from you.
  • Use a framing square to mark a plumb cut as close to the end of the board as possible. I prefer using the right hand side of the board for this ridge cut. Go ahead and saw this mark.
  • Hook the long point with a tape measure, stretch it out past the length, and make a mark along the same top edge of the board.
  • Use a framing square again to mark another plumb cut. It should be parallel with the ridge cut. This is the beginning of the bird’s mouth.
  • Mark the seat cut of the bird’s mouth so that intersects the plumb cut by 90º and cover most or all of the wall plate.
  • Lay out the tail with the same framing square.