Stair Calculator
Work out riser height, tread depth and stringer length to code.
What you'll need to order
- 2x12 stringer stock
- tread material
- riser boards
- stair hardware
- handrail
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
risers = round(total rise ÷ target riser) · actual riser = total rise ÷ risers
Every riser in a flight must be within 3/8 inch of every other — the most common code violation in DIY stairs. Check that 2 × riser + tread falls between 24 and 25 inches, the classic comfort rule.
Worked example
With total rise 108 in, tread depth 10 in, target riser height 7.5 in, this calculator returns 7.71 in.
Stair risers by total rise
Figures below assume tread depth 10 in × target riser height 7.5 in. Change any measurement in the calculator to get your own numbers.
| Total rise | Number of risers | Actual riser height (in) | Stringer length (in) |
|---|---|---|---|
| 84 in | 11.00 | 7.64 | 130.6 |
| 96 in | 13.00 | 7.38 | 153.7 |
| 105 in | 14.00 | 7.50 | 167.1 |
| 108 in | 14.00 | 7.71 | 169.0 |
| 117 in | 16.00 | 7.31 | 190.2 |
| 120 in | 16.00 | 7.50 | 192.1 |
| 132 in | 18.00 | 7.33 | 215.2 |
Assumes a 10 in tread and a 7.5 in target riser. Total rise is finished floor to finished floor, not subfloor to subfloor.
Stair dimensions for common floor heights
| Job | Size | Actual riser height |
|---|---|---|
| Deck, 3 steps down | 21 in × 11 in × 7 in | 7.00 in |
| Split-level landing | 48 in × 10 in × 7.5 in | 8.00 in |
| Basement stair | 96 in × 10 in × 7.5 in | 7.38 in |
| Standard 8 ft ceiling | 105 in × 10 in × 7.5 in | 7.50 in |
| 9 ft ceiling | 117 in × 10 in × 7.5 in | 7.31 in |
| 10 ft ceiling | 129 in × 11 in × 7 in | 7.17 in |
Ceiling heights include the floor structure above, which is why a nominal 8 ft ceiling gives about 105 in of total rise rather than 96.
Stair code limits at a glance
| Type | Limit (in) | Notes |
|---|---|---|
| Maximum riser height (IRC) | 7.75 | Commercial codes are usually 7 in |
| Minimum tread depth (IRC) | 10.00 | 11 in under most commercial codes |
| Maximum variation between risers | 0.38 | Across the whole flight, not per step |
| Minimum stair width | 36.00 | Measured above the handrail |
| Minimum headroom | 80.00 | Measured vertically from the nosing line |
Figures follow the International Residential Code. Local amendments vary, and existing stairs are often grandfathered — always confirm with your inspector before cutting stringers.
Frequently asked questions
How many stairs for an 8 foot ceiling?
Fourteen risers at about 7.5 inches each, giving 13 treads. An 8 ft ceiling gives roughly 105 inches of total rise once the floor structure above is included.
How do I calculate stair stringer length?
It is the hypotenuse of the total rise and the total run. For 105 in of rise and 130 in of run, the stringer is the square root of 105 squared plus 130 squared, about 167 inches.
What is the ideal riser height?
Between 7 and 7.5 inches for comfort. Code allows up to 7.75 in residential, but anything near the limit feels steep, especially going down.
How much headroom do stairs need?
Eighty inches minimum, measured vertically from the nosing line to the ceiling above. This is the constraint that most often forces a stair to be relocated rather than redesigned.
What is the maximum riser height allowed?
The IRC caps residential risers at 7.75 inches and requires a minimum tread depth of 10 inches. Commercial codes are stricter at 7 inches maximum riser and 11 inch minimum tread.
Why is there one fewer tread than riser?
The top riser lands on the upper floor, which serves as the final tread. A 14 riser stair has 13 treads, and forgetting this is the most common stair layout mistake.
What is the 2R + T rule?
Twice the riser height plus the tread depth should fall between 24 and 25 inches. It matches the stair to a natural stride — a 7.5 inch riser with a 10 inch tread gives 25 inches, right at the limit.
Can risers vary in height?
Only by 3/8 inch across the whole flight, and that tolerance exists for construction error, not design. Uneven risers are the leading cause of stair falls because the foot expects the previous spacing.