Free tool · IRC and IBC 2021

Stair calculator

Enter the floor-to-floor rise. Get the risers, treads, run, landings and stringer length, checked against the code.

Total rise, floor to floor

Finished floor to finished floor. 9 ft = 108 in.

Riser height you’d like (optional)

Never more than the 7 3/4″ maximum. Lower means more, gentler steps.

Tread depth

Stair width

Code check

Riser 7 11/16″: at most 7 3/4″
Tread 10″: at least 10″
Width 36″: at least 36″

Also required

Headroom at least 6′ 8″
Handrail 34″ to 38″ above the nosings

These are the model code numbers. Some states change them, so check the code your city has adopted.

Comfort check: two risers plus a tread = 25 7/16″. Between 24″ and 25″ (610–635 mm) feels most natural.

How to calculate stairs

Four steps, from the floor-to-floor height to the length of the stair.

Count the risers

Divide the total rise by the tallest riser allowed and round up.

Find the riser height

Divide the total rise by the number of risers, so every step is the same.

Count the treads

A straight flight has one tread fewer than risers: the upper floor is the last step.

Add up the run

Multiply the treads by the tread depth, and add a landing wherever one flight would climb too high.

Stair rules, residential and commercial

The main dimensions from the 2021 IRC (Section R311.7) and IBC (Section 1011).

Dimension	Residential (IRC)	Commercial (IBC)
Riser height	at most 7 3/4″	4″ to 7″
Tread depth	at least 10″	at least 11″
Stair width	at least 36″ above the handrail	at least 44″, or 36″ for fewer than 50 people
Headroom	at least 6′ 8″	at least 6′ 8″
Rise of one flight	at most 12′ 7″	at most 12′ 0″
Handrail height	34″ to 38″	34″ to 38″
Landing depth	at least 36″ on a straight run	the stair width or 48″, whichever is less

Questions

Why is there one tread fewer than risers?

Because the floor at the top is the last step. Twelve risers climb to the upper floor with eleven treads in between.

When do I need a landing?

When one flight would climb more than 12′ 7″ in a house or 12′ 0″ in other buildings. The calculator splits the stair into flights with near-equal risers and adds the landings.

What is the 2R + T rule?

A rule of thumb, not code: two risers plus one tread between 24 and 25 inches feels natural to climb. The calculator shows it as a comfort check.

Which rules apply to stairs inside a home in a commercial building?

The building code lets stairs inside a dwelling unit use residential dimensions, 7¾″ risers and 10″ treads (IBC 1011.5.2, exception 3). The 2024 edition no longer allows this in units that must be accessible.

Does my state use these numbers?

They are the 2021 model codes, and many states change them. Virginia and New Jersey, for example, allow 8¼″ risers with 9″ treads in homes. Check the code your city has adopted.