Stair riser calculator

People walk stairs by feel, not by looking down at every step. Your foot learns the riser height in the first two or three steps and then repeats that motion automatically. A riser that's even ½ in different from its neighbors catches a foot expecting the old height, and that's how falls happen.

This is why riser consistency, not riser height alone, is treated as the top safety item in stair inspections. A flight of slightly-too-tall but perfectly even risers is safer than a flight with one oddball step.

Commonly cited IRC reference values allow no more than ⅜ in of variation between the tallest and shortest riser in a single flight, and that figure gets checked with a story pole or a batch of measurements after the stringers are cut, not before.

That variance mostly shows up from small framing errors: a stringer notched slightly off layout, a finished floor added on one level but not the other, or a bottom step that didn't account for the thickness of finish flooring. Codes vary by jurisdiction, so treat this as a design target, not a guarantee this calculator provides.

When total rise divided by your target riser lands close to the midpoint, you get to choose whether to round the riser count up or down, and that choice changes the riser height in opposite directions.

Total rise 100 in at a 7.5 in target gives 100 ÷ 7.5 = 13.3. Round down to 13 risers and each one becomes 100 ÷ 13 = 7.69 in, taller and steeper. Round up to 14 and each one becomes 100 ÷ 14 = 7.14 in, shorter and gentler but the stair needs one more tread of floor space.

1. Divide total rise by your target riser height.
2. If the result is under about 7.75 in per riser at the lower rounding, round down for a steeper, more compact stair.
3. If the lower rounding pushes the riser height over commonly cited limits, round up instead.
4. Recalculate riser height from the rounded count and confirm it still meets your target comfort range.

Riser height is measured floor to floor, not stringer notch to stringer notch. If you're adding finished flooring, tile, or a thick underlayment on either level after the stringers are cut, that thickness has to be baked into your total rise measurement up front.

A common error is measuring to the subfloor, cutting the stringers, and then adding ¾ in of hardwood on the upper floor. That quietly shrinks the top riser by ¾ in relative to every other step, exactly the kind of variation the ⅜ in rule is meant to catch.

This table uses a 7¼ in target riser, then rounds and recalculates for a range of typical floor-to-floor heights.

Outdoor stairs give you a little more flexibility because you're often working against grade rather than a fixed floor-to-floor number, but the same equal-riser rule applies. Measure total rise from the deck surface to the finished grade or landing pad, not to raw dirt that will settle or get covered in mulch later.

Once your riser height is locked in, add a tread depth on the stair rise and run calculator to get total run, or move straight to the stair stringer calculator to lay out the actual stringer cuts.