Always confirm with your local building department; many areas amend these rules.
How the stair calculator works
Stairs start with one measurement: the total rise from the finished lower floor to the finished upper floor. That height is divided by your maximum riser height and rounded up, which gives the number of risers. The total rise is then divided by that number so every riser is exactly the same height. A flight with uneven steps is a trip hazard, so codes only allow a tiny variation.
When the top step is the upper floor itself, there is one fewer tread than risers. The total run is the number of treads multiplied by the tread depth, and the stringer length is the diagonal of the total rise and total run. StairCalc rounds the stringer up to the next standard lumber length so you know which board to buy.
Laying out a stringer
Set stair gauges on a framing square at the riser height on the tongue and the tread depth on the blade. Mark the first notch at the top of the board, slide the square along and repeat for every step. Cut the bottom of the stringer short by the thickness of one tread so the first step is not taller than the rest. Cut one stringer, test it in place and use it as a template for the others.
Comfort rules
A long-standing rule says two risers plus one tread should be about 24 to 25 inches, or 600 to 640 mm, which matches a normal walking stride. The UK uses a wider band of 550 to 700 mm. A steep, short-tread stair saves floor space but feels harder to climb, especially going down. Everything is calculated in your browser.
Frequently asked questions
What is the maximum stair riser height?
7 3/4 inches under the IRC for homes, 7 inches under the IBC.
What is the minimum headroom over stairs?
6 feet 8 inches in US homes and 2 metres in the UK.
What is the steepest stair allowed in the UK?
42 degrees for a private stair in a home.
Is StairCalc free?
Yes, with no sign-up or ads.
Are my measurements saved?
Yes, in this browser only.