stairs Stair Calculator (Rise & Run) star

How Stair Dimensions Are Calculated

Start from the total rise (floor-to-floor height) and divide it by an ideal riser height (commonly around 17.5 cm / 7 in) to get a step count, then round to a whole number — every riser in the flight must then come out identical, since even a 1-2 cm difference between steps is a leading cause of trips. The core safety guideline used worldwide is the comfort formula: 2 × rise + run ≈ 63-65 cm (24-25.5 in) — the proportion that matches a typical human stride most comfortably. Most building codes also set hard limits on top of that, commonly a maximum rise around 18-20 cm and a minimum run around 25-28 cm.

Worked example: for a 2.70 m floor-to-floor height, dividing by 17.5 cm gives 15.4, which rounds to 16 steps at an actual rise of 16.875 cm. Applying the comfort formula, 2 × 16.875 + run ≈ 63-65 cm means a run of about 29-31 cm — comfortably inside the typical 25-28 cm code minimum. The stair's total horizontal run is then tread depth × (steps − 1), which is what determines whether it physically fits the available space.

What You Should Know

• Why 2 × rise + run ≈ 63-65 cm matters: this ratio approximates the length of an average adult stride on level ground, split between the vertical and horizontal component of each step — stairs built well outside this band feel awkward regardless of whether they meet code.
• Exceeding the maximum rise makes steps tiring and increases trip risk — each step demands more lift from the leg than the body expects, which compounds fatigue over a full flight and is especially hazardous for children and older adults.
• Too little run does not give the foot room to land properly — a tread shallower than about 25 cm forces part of the foot to overhang the step edge, which is exactly how missteps happen, particularly descending.
• Code limits and the comfort formula are not the same thing — a stair can be fully legal (within code maximums/minimums) yet still sit at the harsh edge of the comfort band, which is why this calculator checks both separately.
• Headroom needs roughly 2 m of clearance above the walking line, and if the horizontal run does not fit the available space, a landing with a turn is generally more comfortable than simply steepening every riser.