Building / DIY

Stair Calculator: Rise, Run, Treads & Stringer Length

This stair calculator turns a total rise into equal riser height, step count, tread depth, and an approximate stringer length for a straight flight.

Total rise (ft)

Floor-to-floor height. Leave blank if you use inches only.

Total rise (in)

Added to the feet. 9 ft is 108 in.

Target riser (in)

Often about 7 in. Steps are rounded, then every riser is equal.

Run

Comfort rule (2R + T = 25)Tread depthTotal run

The comfort rule sets tread depth to 25 − 2 × riser height. Treads are one fewer than risers.

Examples

Stair layout

7.2 in riser

15 steps · 14 treads · 10.6 in tread

Stringer length: 183.539 in (15 ft 3.539 in)
Total run: 148.4 in (12 ft 4.4 in)
Total rise: 108 in (9 ft 0 in)
2R + T: 25 in
Angle: 36.05°

Inside the common planning ranges on this page: riser up to 7.75 in, tread at least 10 in, and 2R + T from 24 to 25.

Total rise: 9 ft 0 in = 108 in. Steps: round(108 ÷ 7) = 15. Equal riser: 108 ÷ 15 = 7.2 in. Treads: 15 − 1 = 14. Tread depth: 25 − 2 × 7.2 = 10.6 in. Total run: 14 × 10.6 = 148.4 in. Stringer length: √(108² + 148.4²) = 183.539 in.

Planning aid only. Confirm dimensions with your local building code and a qualified builder. Landings, nosing, and headroom are not included.

How to use

Enter the total rise in feet and inches. Leave feet blank and put the whole rise in the inches box if that is easier.
Enter a target riser height in inches. About 7 in is a common starting point.
Leave the run on the comfort rule, or switch to a tread depth or a total run.
Read the step count, equal riser, tread depth, total run, and stringer length.

Formula

Number of risers = round(total rise ÷ target riser), and it has to be at least 2. Equal riser height = total rise ÷ number of risers. Number of treads = number of risers − 1, because the upper floor is the last riser. On the comfort rule, tread depth = 25 − 2 × riser height. Otherwise the tread depth is what you enter, or total run ÷ number of treads. Total run = tread depth × number of treads. Stringer length = √(total rise² + total run²).

Example. A 9 ft rise (108 in) with a 7 in target riser has 15 equal risers of 7.2 in and 14 treads. The comfort rule gives a 10.6 in tread, a 148.4 in total run, and a 183.539 in stringer.

Example. The same 108 in rise with a 10 in tread depth has a 140 in total run and a 176.816 in stringer. 2 × 7.2 + 10 = 24.4.

Planning aid, not a code check

Many residential codes keep risers near 7 to 7.75 inches and treads at least 10 inches. A common comfort check is 2R + T between 24 and 25. This page is a straight flight only. Landings, nosing, and headroom need their own measurements. Confirm the dimensions with your local building code and a qualified builder.

Frequently asked questions

What is stair rise and run?

Rise is the vertical height of one step. Run (tread depth) is the horizontal depth of one step.

How many steps do I need?

Divide total rise by a target riser height (often about 7 in), then adjust so all risers are equal.

What is a comfortable riser height?

Many codes land near 7–7.75 inches. Always check your local building code.

Does this include landings?

This version is for a straight flight. Landings need separate runs.

Is this code-certified?

No. It is a planning aid. Confirm dimensions with code and a qualified builder.