Stair calculator: risers, treads and code checks
Enter the floor-to-floor rise to get the number of risers, riser height, tread depth, pitch and a scaled side section, checked against IBC 2021 and IRC 2021. Then model it with our Revit stair and railing families.
Millimetres or feet-inchesIBC 2021 and IRC 2021 checks2R + T comfort ruleHeadroom and floor openingScaled side section
Units and code
Riser 4"–7", tread at least 11", headroom at least 80", width at least 44" (36" for under 50 occupants), up to 12' rise per flight. IBC 2021 §1011.
Lowers the minimum width to 36" (914 mm), IBC 2021 §1011.2.
Leave empty to skip the width check.
Stair dimensions
Type 9' 6", 9-6, 114" or 9.5'. A plain number is read as inches.
Finished floor to finished floor.
Total going. Sets the tread depth.
Clear width.
The riser count is picked to get close to it, never above the code maximum.
Stair type
In the direction of travel. Empty uses the code minimum.
Headroom (optional)
Finish to soffit. Gives the minimum opening.
From the top nosing back. Checks the headroom.
Your stair
Enter the total rise to see your stair.
Number of risers
–
Code checks
Checks cover riser, tread, flight rise, width, landing and headroom only, not handrails, guards, nosings or winders. Local amendments and other codes differ: check against your local code and have the design verified by a qualified professional.
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Pick units and code
Choose millimetres or feet-inches and the IBC 2021, IRC 2021 or your own custom limits. Feet-inches accepts 9′ 6″, 9-6, 114″ or 9.5′.
Enter the rise
Type the finished floor-to-floor height. Add the available run, stair width, preferred riser or floor thickness if you know them.
Read and adjust
You get the riser count, riser and tread sizes, pitch, run and a pass/fail list per code rule. Use + and − to try one riser more or fewer.
How to calculate stair risers and treads
Start from the total rise H, floor finish to floor finish, and the maximum riser your code allows.
Riser height R = H ÷ N
Treads = N − 1 (straight flight)
The top floor acts as the last tread, so a straight flight always has one tread fewer than risers. With a mid landing, the landing replaces one more tread.
Pitch = atan(R ÷ T)
Floor opening ≥ (headroom + floor thickness) × T ÷ R
Worked example: 3,000 mm rise with a 180 mm maximum riser gives 3,000 ÷ 180 = 16.7, so 17 risers of 176.5 mm and 16 treads. Aiming for 2R + T = 625 mm gives a 270 mm tread, a 4,320 mm run and a 33.2° pitch.
The calculator rounds the recommended tread to 5 mm (or 1/4″) and never below the code minimum.
IBC 2021 and IRC 2021 stair requirements
| Rule | IBC 2021 (commercial, egress) | IRC 2021 (dwellings) |
|---|---|---|
| Riser height | 4″–7″ (102–178 mm), §1011.5.2 | 7 3/4″ (196 mm) max, §R311.7.5.1 |
| Tread depth | 11″ (279 mm) min, §1011.5.2 | 10″ (254 mm) min, §R311.7.5.2 |
| Headroom | 80″ (2,032 mm), §1011.3 | 6′ 8″ (2,032 mm), §R311.7.2 |
| Width | 44″ (1,118 mm); 36″ (914 mm) under 50 occupants, §1011.2 | 36″ (914 mm), §R311.7.1 |
| Rise per flight | 12′ (3,658 mm), §1011.8 | 151″ (3,835 mm), §R311.7.3 |
| Landing depth | Stair width, need not exceed 48″ (1,219 mm) on a straight run, §1011.6 | 36″ (914 mm), §R311.7.6 |
| Uniformity in a flight | 3/8″ (9.5 mm), §1011.5.4 | 3/8″ (9.5 mm), §R311.7.5.1–.2 |
Summary of the ICC model codes. Your jurisdiction may amend them, and IBC §1011.5.2 has exceptions, for example inside dwelling units. Always check your local code.
Stair calculation questions
Divide the total floor-to-floor rise by the maximum riser height and round up. For a 3,000 mm rise and the IBC 7″ (177.8 mm) limit, 3,000 ÷ 177.8 = 16.9, so you need 17 risers of 176.5 mm. A straight flight then has 16 treads, because the upper floor is the last step.
IBC 2021 §1011.5.2 allows risers from 4″ to 7″ (102–178 mm) for commercial and egress stairs. IRC 2021 §R311.7.5.1 allows up to 7 3/4″ (196 mm) in one- and two-family dwellings. In both codes the largest and smallest riser in a flight may differ by no more than 3/8″ (9.5 mm).
It is Blondel’s comfort rule: twice the riser plus the tread should match an average stride, 600–650 mm or 24″–25″. With a 175 mm riser, 2 × 175 = 350 mm, so a tread of 250–300 mm feels natural. Codes set the minimum tread; the rule tells you whether the stair will feel right.
Under IBC 2021 §1011, stairs need risers of 4″–7″, treads of at least 11″, headroom of at least 80″ and a width of at least 44″ (36″ where fewer than 50 people use the stair). A single flight may rise no more than 12′ between landings, and landings must be at least as deep as the stair is wide, up to 48″ on a straight run.
Both IBC 2021 (§1011.3) and IRC 2021 (§R311.7.2) require 80″ (2,032 mm), measured vertically from the line of the tread nosings. The floor opening above must be at least (headroom + floor thickness) × tread ÷ riser long. For a 7.5″ riser, 10″ tread and 10″ floor that is 90 × 10 ÷ 7.5 = 120″ (10′).
Between 30° and 37° feels most comfortable. The steepest IBC stair (7″ riser, 11″ tread) is about 32.5° and the steepest IRC stair (7 3/4″ riser, 10″ tread) about 37.8°. Above roughly 42° a stair starts to feel like a ladder.
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