Stair Calculator

How to use it: enter the total floor-to-floor rise, the riser height you'd like to aim for, your tread depth (the front-to-back going of each step), and - optionally - the horizontal run you have available, so the tool can flag whether the flight fits.

Formula. The number of risers is the total rise divided by your target riser height, rounded to the nearest whole step (never less than one); the actual riser height is then recalculated so every step in the flight is exactly equal:

Riser count = round(total rise ÷ target riser), Actual riser height = total rise ÷ riser count

There is always one fewer tread than risers (the top floor itself is the final "step"), so Tread count = riser count − 1 and Total run = tread count × tread depth. Stair angle and stringer length (the diagonal board the treads sit on) come from the same right triangle: Angle = atan(total rise ÷ total run), Stringer length = √(total rise² + total run²).

Worked example. With the defaults above (2800 mm rise, 175 mm target riser, 260 mm tread, 4200 mm available run): riser count = round(2800 ÷ 175) = round(16) = 16; actual riser height = 2800 ÷ 16 = 175.0 mm (an exact match here, since 2800 divides evenly by 175); tread count = 15; total run = 15 × 260 = 3900 mm, which fits within the 4200 mm available; angle = atan(2800 ÷ 3900) ≈ 35.7°; stringer length = √(2800² + 3900²) = √23,050,000 ≈ 4801 mm.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Total rise

2800 mm

Target riser

175 mm

Tread depth

260 mm

Available run

4200 mm

Primary result: Risers: 16.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

This tool calculates geometry only - it is not a code check. Maximum riser height, minimum tread (going) depth, minimum headroom, winder and landing rules, and handrail requirements all vary by country, state or province, and by whether the stair is residential or commercial. Always verify riser height, tread depth, and headroom against your local building code before building.

As a general sense of scale only - not a rule this tool enforces - many US residential codes commonly cite a maximum riser around 7-3/4 in (about 196 mm) and a minimum tread depth around 10 in (254 mm), but exact figures differ by jurisdiction and code edition. Treat any such number as a starting point to confirm locally, not a pass/fail result from this calculator.

The stringer length is the straight diagonal from the first riser to the last tread; it does not add the extra material needed for stringer notching, a starting or landing tread, or nosing overhang.