Stair calculator

Common questions

How do I calculate how many stairs I need?

Divide your total floor-to-floor rise by a comfortable target riser height — about 7 in (175 mm) — and round to the nearest whole number. That is how many risers (steps up) you need, and the actual riser height is the total rise divided by that count. A 108-inch rise ÷ 7 ≈ 15.4, rounded to 15 risers, gives an actual riser of 108 ÷ 15 = 7.2 in. A standard flight has one fewer tread than risers, so 15 risers means 14 treads.

What is the 27 rule for stairs?

The 27 rule is a comfort guideline: two riser heights plus one tread depth should total roughly 24–27 inches, matching a typical adult stride. It extends the classic Blondel rule of 24–25 inches. A 7-inch riser with an 11-inch tread gives 2 × 7 + 11 = 25 inches — right in the middle of the band. Use it together with the 17-inch rule as a quick comfort sanity check.

What is the 17 inch rule for stairs?

The 17-inch rule says one riser height plus one tread depth should add up to about 17–18 inches for a comfortable stride. A 7-inch riser and an 11-inch tread sum to 18 inches; a steeper 7.5-inch riser pairs with a shallower 10-inch tread (17.5 inches). If riser + tread falls well outside 17–18 inches, the stairs feel cramped or over-stretched.

How many stairs to get up 10 feet?

Ten feet is 120 inches. At a 7-inch target riser, 120 ÷ 7 ≈ 17.1, so you would use 17 risers at an actual height of about 7.06 in, with 16 treads. Always round to whole, equal risers — uneven risers are a trip hazard. Enter 120 inches above to see the run, stringer length and angle for that flight.

Is this a staircase or stair stringer calculator?

Both. From your total rise it gives the full staircase layout — risers, riser height, treads, total run and angle — and it sizes the stringer (the sloped board you cut) by returning its exact length, so you can use it as a staircase calculator and a stair stringer calculator in one.

Want the full walk-through? Read the stair calculation guide →