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DatumSpot

Stair Calculator

Enter a floor-to-floor rise to size the flight — risers, riser height, tread run, total run, and pitch, checked against IRC R311.7.

The finished floor-to-floor height the stair has to climb.

Number of risers16
Riser height6-13/16" · 173 mm
Number of treads15
Tread depth11" · 279.4 mm
Total run13' 9" · 4.191 m
Stair pitch31.8°
2R + T (comfort)24.6" — comfortable
HandrailRequired (4+ risers)
  • 16 risers at 6.81" with 15 treads at 11" (riser ≤ 7¾" R311.7.5.1, tread ≥ 10" R311.7.5.2).
  • 2R + T = 24.6" is within the comfortable 24–25" range (rule of thumb).
  • 4 or more risers — at least one handrail required, 34–38" high (R311.7.8).
  • Provide ≥ 6'-8" headroom (R311.7.2) and ≥ 36" stair width (R311.7.1).

How the stair is sized

Given the total rise (the finished floor-to-floor height), the calculator targets a comfortable ~7" riser, then divides the rise into equal risers and adds one if needed so each stays within the 7¾" maximum (IRC R311.7.5.1). A flight always has one more riser than tread, so the total run is the tread depth times the number of treads.

The tread depth (run per step) must be at least 10" (R311.7.5.2). As a comfort check, the old carpenter's rule 2 × riser + tread ≈ 24–25" (the Blondel rule) flags stairs that will feel too steep or too shallow — it's a rule of thumb, not code.

Other R311.7 limits to keep in mind: at least 6'-8" of headroom (R311.7.2), a minimum 36" stair width (R311.7.1), a handrail 34–38" high where there are 4 or more risers (R311.7.8), and no single flight rising more than 151" between landings (R311.7.3). These are residential (IRC) values — commercial stairs follow the IBC, and local amendments vary.

Frequently asked questions

What is the maximum stair riser height?

7¾ inches for residential stairs under the model IRC (R311.7.5.1), with a minimum tread depth of 10 inches (R311.7.5.2). Some states amend these values, so verify against your local code and AHJ.

How many risers do I need for a given floor height?

Divide the total floor-to-floor rise by a target riser height around 7 inches, round to a whole number of risers, then divide the rise by that count to get the actual riser height. This calculator does it and checks the result against the IRC limits.

What is the difference between a riser and a tread?

The riser is the vertical face of each step; the tread (or run) is the horizontal surface you step on. The IRC caps riser height at 7¾" and requires at least a 10" tread depth.

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