Stair Formulas

There are all kinds of different stair formulas, but we're going to focus on some of the simpler ones, so that we don't overwhelm your building skills. If you have a problem with basic math, you could have a problem with these basic math formulas and should plan on getting some assistance.

Basic Stair Formula For Treads

Divide the total amount of the individual stair treads, into the total stair run.

If I have a total stairway run of 130 inches and divide that number by 10,(this number will represent the width of each individual stair tread) it will give me 13 individual, 10 inch stair treads.

You don't need to use 10 either, feel free to use other numbers, if you need to adjust the stairway, in any direction. You can use any number in between 9 and 12 inches. 9 1/4 inches - 9 1/2 inches -9 3/4" and so on.

Simple Stair Formula For Risers

Divide the total amount of individual stair risers into the total stair rise.

If I have a total stairway rise of 105 inches, I can start out by dividing 7 1/2 inches into 105 inches, giving me 14 individual - 7 - 1/2 inch risers. If I divide that same number by 15, I will get 15 individuals stair risers at 7 inches.

Now for the ultimate stair formula, I refer to it as the 17 1/2 inch rule and believe it or not, it works great and creates comfortable stairways.

All you need to do is add one of your stair treads together with one of your stair risers and see if you come up with something close to 17 1/2 inches. The key here is to be as close to 17 1/2 inches as possible, it doesn't have to be exactly 17 1/2 inches.

Here's an example of what I'm talking about.

I have a 10 inch stair tread and a 7 1/2 inch riser, if I add these numbers together, I will have 17 1/2 inches, putting me exactly where I want to be, to create a comfortable stairway, to walk up and down.

What if I have a 10 inch stair tread and a 7 3/4-inch riser, even though this isn't going to be perfect, it's going to be close enough. This will still provide you with a comfortable stairway.

Now what if you have a 9 inch stair tread and 8 inch riser?

This is going to create an uncomfortable stairway, but could be fixed by adding a half-inch to the stair tread width, if possible. However, if this is all you have to work with and you can't extend the length of the stairway, then don't worry about.

This stair building formula has been used by stair builders for years and really does create a comfortable stairway.

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