Total rise divided by the max riser, rounded up.
Free Fabora tool
Stair Stringer Calculator
Enter the total rise and the tread depth. You get the number of risers, the exact riser height, the total run, the stringer length and the stair angle, in inches or millimeters.
The short answer
Divide the total rise by the tallest riser allowed (7 3/4 in for a US home) and round up. That is the number of risers. Divide the rise by that number for the exact riser height. There is one fewer tread than risers when the upper floor is the top step, and the stringer length along the slope is the square root of the rise squared plus the run squared.
Work out risers, treads and stringer length
Your stair
Enter the rise and tread
7 3/4 in is the tallest riser the IRC allows in a US home, with 10 in minimum treads. Measure the rise between finished floor surfaces.
Results
Risers, treads and stringer
7.71 in exactly, every riser the same.
One fewer than the risers, because the upper floor is the top step.
13 treads at 10 in.
Along the slope, before the top and bottom cuts. Buy longer stock.
The pitch of the stair, measured from the floor.
Fabora Steel Tools
Need the drawing and the cut file?
The Stair Stringer Designer in Fabora Steel Tools takes the same numbers much further. It is part of Fabora Pro.
- Checks the rise, tread and pitch against the stair rules for your region: US codes (IRC, IBC, OSHA and ADA, with state amendments), the UK, Europe, Australia and India.
- Compares the best riser counts side by side and marks which ones pass.
- Draws the stair in 3D and as a dimensioned side view, with tread and end details.
- Exports a 1:1 DXF of the steel stringer plate, a set-out PDF and every tread position for the brackets.
- Handles rest landings, nosings, treads from plate to grating, and notched or plumb foot stringer ends.
How to calculate stair stringers
Six steps, the same for a timber stair or a steel one.
- Measure the total rise from the top of the lower finished floor to the top of the upper finished floor. Allow for flooring that is not down yet.
- Divide the rise by your maximum riser height and round up to the next whole number. That is the number of risers.
- Divide the total rise by the number of risers for the exact riser height.
- Count the treads. When the upper floor forms the top step, there is one fewer tread than risers.
- Multiply the number of treads by the tread depth for the total run.
- Work out the stringer length with Pythagoras: the square root of the rise squared plus the run squared. Then allow extra for the cuts at each end.
Worked example
A 108 in rise with 10 in treads: 108 divided by 7.75 is 13.94, so 14 risers. 108 divided by 14 is 7.71 in, just over 7 11/16 in per riser. 13 treads at 10 in make a 130 in run (10 ft 10 in). The square root of 108 squared plus 130 squared is 169 in, so the stringer is about 14 ft 1 in along the slope, and the stair sits at 37.6 degrees.
Riser and tread limits in the US and UK
These are the main geometry limits. Local codes can amend them, and they are not the only rules: headroom, handrails, guards and width apply too.
| Rule | Max riser | Min tread | Also |
|---|---|---|---|
| US homes (IRC R311.7.5) | 7 3/4 in | 10 in | Nosing of 3/4 to 1 1/4 in needed when the tread is under 11 in |
| US commercial (IBC 1011.5.2) | 7 in (4 in min) | 11 in | The 7-11 stair |
| US workplaces (OSHA 1910.25) | 9 1/2 in | 9 1/2 in | Standard stairs between 30 and 50 degrees |
| England, private stair (Approved Document K) | 220 mm (150 mm min) | 220 mm (300 mm max) | 42 degrees max, 2R + G between 550 and 700 mm |
| England, general access stair | 170 mm (150 mm min) | 250 mm (400 mm max) | 2R + G between 550 and 700 mm |
Sources: the 2021 International Residential Code and International Building Code, OSHA 29 CFR 1910.25 and Approved Document K (England). Wales, Scotland and Northern Ireland set their own limits.
Steel stair stringers
The geometry is the same whether the stringer is a 2x12 or a steel plate. What changes is how the stringer is made and fixed.
The stringer
Flat plate or flat bar is the simplest. A closed stringer carries the treads on brackets fixed to its face, and a sawtooth stringer is profile cut to the steps. Channel is common on heavier stairs.
The treads
Checker plate, open grating or timber on steel. Keep every finished riser the same height by allowing for the tread thickness at the bottom step and the finished floors at the top and bottom.
The ends
The top usually notches over or bolts to the landing trimmer, and the foot bolts to the floor through a plate or a plumb cut. Size and weigh the plate with the free steel weight calculator.
Stair stringer questions
How do you calculate stair stringers?
Divide the total rise by your maximum riser height and round up to get the number of risers, then divide the rise by that number for the exact riser height. Multiply the treads, usually one fewer than the risers, by the tread depth for the total run. The stringer length is the square root of the rise squared plus the run squared.
How many steps do I need for my stairs?
Divide the floor-to-floor rise by the tallest riser you can use and round up. For a 108 in rise and the 7 3/4 in limit for US homes, 108 divided by 7.75 is 13.9, so you need 14 risers and 13 treads.
What is the 7-11 rule for stairs?
It is shorthand for a 7 in maximum riser and an 11 in minimum tread. Those are the limits for commercial stairs under the International Building Code, and many builders use them as a comfortable target for any stair. US homes under the IRC can go up to 7 3/4 in risers with 10 in treads.
How long does a stringer need to be for 5 steps?
It depends on the rise and tread. Five risers of 7 1/2 in (a 37 1/2 in rise) with four 10 in treads give a 40 in run, so the stringer is about 54 13/16 in (4 ft 6 13/16 in) along the slope. A 6 ft board or length of steel leaves room for the end cuts.
What is the maximum riser height for stairs?
7 3/4 in for US homes under the IRC, 7 in for commercial stairs under the IBC and 9 1/2 in for OSHA standard stairs in workplaces. In England a private stair can have risers up to 220 mm and a general access stair up to 170 mm. Some states and countries amend these limits, so check your local code.
What is the minimum tread depth for stairs?
10 in for US homes (IRC), 11 in for commercial stairs (IBC) and 9 1/2 in for OSHA standard stairs. In England the going must be at least 220 mm on a private stair and 250 mm on a general access stair.
What angle should stairs be?
No single angle is right. A stair built to the IRC limits for homes, a 7 3/4 in riser over a 10 in tread, comes out at about 37.8 degrees. England caps private stairs at 42 degrees, and OSHA standard stairs must sit between 30 and 50 degrees.
What is 2R + G?
Twice the riser height plus the going (the tread depth). It is the UK and European comfort check: in England it must fall between 550 and 700 mm, and machinery access stairs to EN ISO 14122-3 use 600 to 660 mm. US codes do not use it.
Why is there one fewer tread than risers?
Because the upper floor or deck forms the top step. If the stair has its own top tread below the landing edge, the treads and risers are equal.
Does this work for steel stairs?
Yes. The rise, run and stringer length are the same for timber and steel. A steel stringer also needs a plate size, tread brackets and end details, which the Stair Stringer Designer in Fabora Steel Tools draws and exports as a 1:1 DXF cut file.
Does this calculator check my stair against building codes?
No. It works out the geometry only. Headroom, handrails, guards, width and landings have rules of their own, so check with your local building department or building control before you build.
Design the whole stair in Fabora Steel Tools
The Stair Stringer Designer checks your stair against the rules for your region, draws it, and exports a 1:1 DXF cut file and a set-out PDF. It is part of Fabora Pro, with 20 more tools for the bench, on iPhone, Android and in your browser.
