Deck stair calculator
A stair is the one part of a deck where the numbers are not negotiable. Riser height and tread depth are set by code, and the comfort formula that experienced builders use is narrower still. Start from the total rise — never from a guessed riser height.
Your stair
How this is calculated
The rise is fixed by the site. Everything else follows from dividing it into equal steps:
- Risers = round(total rise ÷ target riser).
- Actual riser = total rise ÷ risers. Every riser must be identical — this is the number that matters, not your target.
- Treads = risers − 1, because the deck surface acts as the top tread.
- Run = treads × tread depth.
- Stringer = √(rise² + run²), the hypotenuse the stringer is cut along.
Worked example
A 36″ total rise with a 7.5″ target riser and 10″ treads:
- Risers = round(36 ÷ 7.5) = 5
- Actual riser = 36 ÷ 5 = 7.2″ — under the 7.75″ maximum
- Treads = 5 − 1 = 4
- Run = 4 × 10 = 40″ = 3.33 ft
- Stringer = √(36² + 40²) = √2896 = 53.8″ = 4.5 ft
- 2R + T = 2(7.2) + 10 = 24.4″ — inside the 24–25″ comfort band
Note that the answer landed on 7.2″, not the 7.5″ you asked for. You cannot have a 7.5″ riser and a whole number of steps in a 36″ rise. The tool rounds the count, then reports the real riser — which is the number you cut to.
The 2R + T rule
Riser height doubled plus tread depth should land between 24 and 25 inches. It is not code, it is a comfort and safety convention that shows up in carpentry manuals going back a century, and it encodes something real: a stair that satisfies the code minimums can still feel wrong. A 7.75″ riser with a 10″ tread gives 25.5″ — legal, but steep enough that people trip going down.
What this calculator does not look at
- It does not check headroom. IRC requires 6′8″ of clearance above the stair slope. Measure the space above before you build.
- It does not design the stringer. Whether a 2×10 stringer is adequate depends on the span, the tread material and the load. Cut stringers are notched, which removes material — this is a place to consult a span table or an engineer.
- It does not handle landings. A stair with a mid-landing is two flights; calculate each separately.
- It does not read your local amendments. The IRC limits quoted are the published ones. Some jurisdictions are stricter.
Questions people actually ask
What is the maximum riser height for a deck stair?
7.75 inches under IRC R311.7.5.1 for residential stairs, with a minimum of 4 inches. The riser must also be consistent — the code allows a 3/8″ variance across the whole flight, which is small enough that you should aim for zero.
How many steps for a 36 inch rise?
Five risers at 7.2″ each, which gives four treads. You cannot hit exactly 36″ with 7.5″ risers — 4.8 steps is not a step count, so the calculator rounds to 5 and reports the real riser height.
How long should a stair stringer be?
It is the hypotenuse of the rise and run. For a 36″ rise and 40″ run that is 53.8″, so you buy a 6 ft board and cut it down. Always cut the stringer from stock longer than the calculated hypotenuse.
Do I need a handrail on a deck stair?
IRC requires a handrail on at least one side of a stair with four or more risers, at 34–38″ above the tread nosing. A stair with four risers falls right at that threshold — check your local amendment, because interpretations differ.
Why does the tread count differ from the riser count?
Because the deck surface is the top tread. Five risers mean you cut four treads into the stringer; the fifth step is stepping onto the deck itself.