Comfortable, code-compliant stairs balance riser height and tread depth. The U.S. International Residential Code allows risers up to 7¾ in and treads of at least 10 in, with no more than ⅜ in variation between steps. A classic comfort rule says twice the riser plus the tread should be about 24–25 in (600–640 mm).
How to use the Stair Calculator
- Total rise (floor to floor)
- Maximum riser height
- Tread depth (run)
- Stair width
- Maximum stringer spacing
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the total rise from finished floor to finished floor, the maximum riser height, the tread depth (run), the stair width and stringer spacing. The calculator rounds up the risers, recalculates the exact riser height and gives the run, stringer length, angle and stringers.
Code limits
| Code | Max riser | Min tread |
|---|---|---|
| IRC (US residential) | 7¾ in | 10 in |
| IBC (US commercial) | 7 in | 11 in |
| UK Part K (private stair) | 220 mm | 220 mm going |
| NBC Canada (dwellings) | 200 mm | 255 mm run |
Tips for an accurate estimate
- Take measurements at floor level and at the ceiling – walls and rooms are often slightly out of square.
- Use the manufacturer’s coverage, spacing and installation data when it differs from the typical values here.
- Keep a little extra of every finish material for touch-ups and repairs later.
- Check local building codes and permits before structural, electrical, plumbing or gas work.
Common mistakes to avoid
- Sizing heating or cooling equipment by area alone when insulation, windows and climate make a big difference.
- Buying too little paint, stain or flooring and ending up with mismatched batches.
- Overlooking the extra material for steps, railings, gables and trim.
Inputs explained
| Field | What to enter |
|---|---|
| Total rise (floor to floor) | Units: in, cm, mm, ft, m |
| Maximum riser height | Units: in, cm, mm, ft, m |
| Tread depth (run) | Units: in, cm, mm, ft, m |
| Stair width | Units: in, cm, mm, ft, m |
| Maximum stringer spacing | Units: in, cm, mm, ft, m |
What each result means
- Risers = ceil([Total rise (floor to floor)]÷[Maximum riser height] – 0.000001)
- Riser height = [Total rise (floor to floor)]÷[Risers]
- Riser height (fraction) = [Riser height]
- Treads = [Risers] – 1
- Total run = [Treads]×[Tread depth (run)]
- Stringer length = sqrt([Total rise (floor to floor)]^2 + [Total run]^2)
- Stair angle = deg(atan([Total rise (floor to floor)]÷[Total run]))
- Comfort check 2R + T = 2×[Riser height] + [Tread depth (run)]
- Stringers = ceil([Stair width]÷[Maximum stringer spacing] – 0.000001) + 1
Formula
Risers = ceil(total rise ÷ maximum riser); riser height = rise ÷ risersTreads = risers − 1; total run = treads × tread depthStringer = √(rise² + run²); angle = atan(rise ÷ run); comfort: 2R + T ≈ 24–25 inWorked example
Inputs — Total rise (floor to floor): 108 in; Maximum riser height: 7.75 in; Tread depth (run): 10 in; Stair width: 36 in; Maximum stringer spacing: 16 in.
Results — Risers = 14; Riser height = 7.714 in; Riser height (fraction) = 7 11/16; Treads = 13; Total run = 130 in; Stringer length = 169.01 in; Stair angle = 39.7 °; Comfort check 2R + T = 25.43 in; Stringers = 4.
Key terms
- Riser
- The vertical face of a step.
- Tread
- The horizontal part you step on.
- Stringer
- The inclined board that supports the treads and risers.
Frequently asked questions
How many steps for a 9 ft rise?
108 in ÷ 7¾ in = 13.9, so 14 risers of 7.71 in and 13 treads.
What is the code for stair risers and treads?
IRC: risers ≤ 7¾ in, treads ≥ 10 in, headroom ≥ 6 ft 8 in, width ≥ 36 in.
What is the ideal stair angle?
30–37°; above about 42° stairs feel like a ladder.
What formula does the Stair Calculator use?
Risers = ceil(total rise ÷ maximum riser); riser height = rise ÷ risers.