Stair Calculator
From a total rise and a target tread run, get the number of steps, the exact riser height, the total horizontal run, and the stringer length — all kept under the code maximum riser so your stairs are even, legal, and comfortable to climb.
How to Use
- Enter the total rise — the finished floor-to-floor height in inches.
- Enter the target run (tread depth) per step, commonly 10 to 11 in.
- Set the maximum riser height allowed by your code (7.75 in for residential under the IRC).
- The tool divides the rise into enough steps to keep each riser under your maximum, all equal.
- Read the step count, riser height, total run, and stringer length, then check the Show Work below.
Show Work
Code reference
About the Stair Calculator
The Stair Calculator is a free tool for building, materials estimation and site work. It runs right in your web browser, so there is nothing to download. From a total rise and a target tread run, get the number of steps, the exact riser height, the total horizontal run, and the stringer length — all kept under the code maximum riser so your stairs are even, legal, and comfortable to climb.
How it works
Type your numbers into the boxes. The answer shows up right away — you do not have to press a button. If you change a number, the answer changes too. So you can try different numbers and watch what happens, or check an answer you worked out yourself. Just make sure each box has the right kind of number in it.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
What is the maximum riser height by code?
The IRC limits residential risers to 7-3/4 in and requires a minimum tread depth of 10 in. Commercial codes are tighter, often 7 in max riser and 11 in min tread. The biggest rule beyond the maximums is consistency: risers in a flight must be within 3/8 in of each other, which is why you divide the rise into equal steps rather than eyeballing it.
How do I get an even, comfortable stair?
Use the rule of thumb that riser plus run should total about 17 to 18 in, or that two risers plus one run lands near 25 in. A 7.5 in riser with a 10.5 in tread is a classic comfortable combination. This tool keeps every riser equal automatically; you just nudge the target run until the pair feels right.
How is stringer length figured?
The stringer is the diagonal, found with the Pythagorean theorem on the total rise and total run: square each, add them, and take the square root. So a stair with 108 in of rise and 110 in of run needs a stringer of about 154 in along the cut line. Always add length for the top and bottom connections and buy stock a bit long.
Why does the step count round up?
The number of risers must be a whole number, and the riser height has to stay under your maximum, so the tool rounds the count up. Rounding up makes each riser slightly shorter, which is the safe direction. If you rounded down you would exceed the code maximum and end up with a steeper, illegal stair.
What is the difference between rise, run, and tread?
Total rise is the full vertical height the stair climbs; the riser is the height of one step. Run (or tread depth) is the horizontal depth of one step, and total run is all the treads added together. Get the total rise right first — measure floor surface to floor surface — because everything else divides out from it.
How do I use the Stair Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Building interior stairs
Size risers and treads for a floor-to-floor stair that meets the IRC and feels even underfoot.
Cutting deck or porch steps
Lay out a short exterior flight from grade to deck with equal risers and the right stringer length.
Framing basement stairs
Fit a stair into a fixed total rise while keeping each riser under the code maximum.
Buying stringer stock
Get the diagonal stringer length so you order lumber long enough with room to trim.
Checking existing stairs
Verify that a current stair meets riser and tread limits before a remodel or inspection.
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