Stair stringer calculator
Enter the total rise and the tread depth. You get the number of risers and their height, the treads, the total run and the stringer length, checked against the IRC limits.
Enter your measurements
Risers
15
Exact geometry
- Riser height
- In the app
- Treads
- In the app
- Total run
- In the app
- Stringer length
- In the app
Every result is in Truemark
The free web version shows the main result and the drawing. Truemark gives you every mark and measurement, and exports a cut sheet. It is free to download.
- Meets the IRC R311.7.5 riser and tread limits.
How to lay it out
Measure the total rise
Measure from finished floor to finished floor, where the stair starts and where it lands.
Set the tread depth
Enter the tread depth you want. The IRC minimum is 10″.
Lay out the stringer
Set your framing square to the riser height and tread depth and step it down the stringer stock once for every tread.
Check the stringer length
Check the stringer stock is at least the stringer length shown before you cut.
Good to know
- Meets the IRC R311.7.5 riser and tread limits.
- FabLayout runs the same code as Truemark; the result is exact geometry.
- Source: International Residential Code 2021, R311.7.5.1 (riser height, 7¾″ max) and R311.7.5.2 (tread depth, 10″ min)
Method and formula
The stair is built around a comfortable riser of about 7″. The calculator picks the whole number of risers whose height comes closest to that, then checks it against the code.
- Riser height = total rise ÷ number of risers.
- Treads = risers − 1, for a stair between two floors.
- Total run = treads × tread depth.
- Stringer length = √(total rise² + total run²).
The check uses the 2021 International Residential Code: risers no taller than 7¾″ (R311.7.5.1) and treads no shallower than 10″ (R311.7.5.2). If the closest riser count would break the riser limit, the calculator uses the fewest risers that meet it.
Worked example
The page defaults: 108″ of total rise with 10″ treads. 15 risers give 108 ÷ 15 = 7.2″, closer to 7″ than 16 risers would. That makes 14 treads and 140″ of total run. The stringer is √(108² + 140²) = 176.8163″. A 7.2″ riser and 10″ tread meet the IRC limits.
Questions
Why is there one less tread than risers?
The top riser steps onto the upper floor, which acts as the last tread. A stair between two floors has one tread fewer than it has risers.
What are the IRC riser and tread limits?
The 2021 IRC allows risers up to 7¾″ (R311.7.5.1) and treads no shallower than 10″ (R311.7.5.2). Local codes can differ, so check with your building department.
Is the stringer length the board length to buy?
It is the length along the line of the stair. Allow extra for the top and bottom cuts and any connection to the floor.
Similar calculators
More in TruemarkOffsetOffset from end3-point saddleSee what the app does