Stair Calculator
Calculate the number of risers and treads, actual riser height, total run, stair angle, stringer length and material quantities for a straight stair flight.
Stair formulas
Number of risers = ceiling(total rise ÷ maximum preferred riser height).
Actual riser height = total rise ÷ number of risers.
Treads = risers − 1 when the upper floor is the final tread; otherwise treads = risers.
Total run = number of treads × tread depth.
Stringer length = √(total rise² + total run²). Angle = arctangent(total rise ÷ total run).
Tread material area = treads × tread depth × stair width. The allowance affects material quantities and estimated cost, not the stair geometry.
Scope: this calculator models one straight flight with equal risers and equal treads. It does not design landings, turns, winders, spiral stairs, headroom, guards, handrails, connections or structural capacity. Verify every dimension against the locally adopted building rules and the project design before cutting material.
Six calculated examples
| Total rise and planning inputs | Calculated result |
|---|---|
| 108 in rise, 7.5 in maximum, 10 in tread; upper floor is final tread | 15 risers at 7.2 in; 14 treads; 140 in run |
| 96 in rise, 7.5 in maximum, 10 in tread; upper floor is final tread | 13 risers at 7.385 in; 12 treads; 120 in run |
| 108 in rise, 7.5 in maximum, 10 in tread; separate top tread | 15 risers; 15 treads; 150 in run |
| 2,800 mm rise, 190 mm maximum, 250 mm tread | 15 risers at 186.667 mm; 14 treads; 3,500 mm run |
| 3,000 mm rise, 180 mm maximum, 280 mm tread | 17 risers at 176.471 mm; 16 treads; 4,480 mm run |
| 84 in rise, 7 in maximum, 11 in tread; upper floor is final tread | 12 risers at 7 in; 11 treads; 121 in run |
How to use the calculator
- Measure total rise vertically from the lower finished surface to the upper finished surface.
- Enter the maximum riser height you want the calculation to use. This is an input, not a statement of code compliance.
- Enter the horizontal tread depth and choose whether the upper floor serves as the final tread.
- Optionally compare the calculated run with available horizontal space.
- Use stair width, stringer count and allowance for preliminary material quantities. Confirm actual board sizes, nosing, joints and supports separately.
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