Outdoor Stair Railing: How to Take the Right Measurements

Your deck is finally finished and it looks great, but you and the fam can't fully enjoy it safely until you get that railing up.  Almost all of the measurements are made, but those darn stairs are the tricky part.  You're not sure exactly what measurements you need to make or how to make them.  Sound familiar?  Don't worry, we hear this all the time; even from professionals.  Measuriing for outdoor stair railing can be tricky.  That's why we've put together the following guide below with step by step instructions to help you get it right.


Calculate The Stair Angle

  1. Measure the rise/run of your stairs. 

  • The rise is the vertical distance that your stairs cover.  You can measure 1 step from bottom to top (a) if your stairs are all a uniform height.  If not, measure the entire rise from the ground to the top of the top step (A). 
  • The run is the horizontal distance that your stairs span.  Again, you can measure 1 step tread from front to back (b) if your stairs are uniform.  If not, measure from the bottom edge of the first step to the point on the ground under the edge of the top step (B). 


2) Plug the rise/run into an angle calculator.


stair railing angle calculator (external site)
Click the screenshot to go to the stair railing angle calculator
  • No need to be overly complicated here and work out the trigonometry ourselves.  There are plenty of free tools online to calculate angles.  Try this one, which can be accessed through the web, or downloaded as an app.  Make sure to hit the angle button before entering the rise & run, then hit calculate.

Calculate The Stair Railing Length 


 1) Let Pythagoras Be Your Friend


  • In the drawing above, the rise is the distance from the surface of your deck to the ground.  The run is roughly the distance from the spot on the ground where your railing post will be anchored to the spot on the ground under where your top railing post will be anchored.  That's a little different measurement then the rise & run in the angle calculation above.  The reason is that the length of the railing depends on where you set your posts.
  • Remember in high school when you slept through geometry on the premise that you would never use it?  Well, think again.  Calculating the length of your stair railing actually is actually based on a fundamental relationship in geometry called the Pythagorean Theorem, which basically states (in terms of railing) that the squared length of your stair railing is equal to the squared rise of your stairs + the squared run of your stairs, or A^2 + B^2 = C^2. 

    For example,  suppose your rise & run (A & B in the drawing above, respectively) are 40" & 60".

    Then:  40^2 + 60^2 = C^2

    working through this problem gives us the following steps:

    1)  1600 + 3600 = C^2
    2) 5200 = C^2
    3) C = the square root of 5200
    4) C = 72.11 inches

    So the length of stair railing that you need is 72.11 inches.  Which brings us to another important point.  Since most outdoor stair railings come in whole sizes like 3ft., 4ft., 5ft., etc. what do you do when your measurment is between 2 sizes?  The answer is that you probably need to round up to the next size.  In this case though, .11 inches isn't very much and we could probably move our posts closer together by that much and stick with the 72" or 6' railing.

So, that's it.  it might seem like a lot of complicated steps, but it's actually pretty easy once you get started.  Of course, if you need help, you can always email us and we'll be happy to help you.

Once you have all of your measurements in hand, you're ready to order your outdoor railing system.  Check out the fantastic Westbury Aluminum Railing or Key-Link Aluminum Railing Systems  Both come in multiple styles, sizes, and colors and it's easy for do-it-yourself installers.


    1. Very well explained but you should also include the recommended height of railings

    2. Thank you for the great suggestion. You're right, that would be helpful.

      The IRC (Residential) code states that "Porches, balconies or raised floor surfaces located more than 30” above the floor or grade below shall have guards not less than 36” in height"

      The IBC (Commercial and Multi-Unit Residential) states that "Required guards shall be not less than 42 inches (1067 mm) high, measured vertically above the adjacent walking surfaces, adjacent fixed seating or the line connecting the leading edges of the treads." Guards are required on any open sided surface more than 30" above ground.

      These codes are the reason that most railings, including those sold in our online store are either 36" or 42" tall.

    3. Should also mention that not every municipality follows the same codes, so please with your local building office to make sure that the railing you're going to purchase meets your local code.

    4. Just an observation on your page, I think your measurement points are wrong. To Rise is fine but theRun is not. To calculate the length of hanrail you would need to measure from similar pitches on the stairs, e.g. from the leading edge of the first step to the leading edge of the top step. What you have done here is taken a measurement from the bottom of the riser on the first step to the top of the riser on the last step which gives you both wrong lengths and wrong angles. if you wanted to use Pythagarous' theory then you would need to add the length of one more step to the run and the square it and square the total rise.That will give you a proper pitch length and will give you an accurate angle of the steps.

    5. If my math teacher had given me real world examples as above I'd have understood pythagorean's theorem. Numbers don't mean a thing but when you put it to an application it makes more sense (no figuring out the height of the school based on its shadow length and the height and shadow length of a fire hydrant is not a real world application.

      1. Thank you Bill. It's definitely easier to learn with real world examples.