Calculator

Stair calculator

Enter the floor-to-floor height, the maximum riser and the tread depth. The calculator works out the number of risers, their height, the treads, the total run and the angle, draws the stair to scale and checks it against the US IRC or the UK's Approved Document K.

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Stair calculator

Feet and inchesMetres

Building code

IRC (US)UK Part K

Floor-to-floor height (total rise)

Maximum riser

7¾ in (196 mm) under IRC R311.7.5.1.

Tread depth

At least 10 in, nosing to nosing; 11 in needs no nosing.

Stair width

At least 36 in clear (R311.7.1).

Measure the rise from finished floor to finished floor, including the floor structure above. Optional, for the headroom:

Ceiling height below the floor above

Opening length in the floor above

For 6 ft 8 in of headroom the opening must run at least 11′ 9″ back from the top riser.

Risers

15 × 7¼ in

14 treads of 11 in, 12′ 10″ of run

Each riser 7.27 in exactly

Riser height: 7¼ in
Treads: 14
Total run: 12′ 10″
Angle: 33.4°

Side view, to scale

IRC 2021

Pass: Riser 7¼ in Maximum 7¾ in, 196.85 mm (IRC R311.7.5.1).
Pass: Tread 11 in Minimum 10 in, nosing to nosing (IRC R311.7.5.2).
Pass: Width 36 in Minimum 36 in clear above the handrail (IRC R311.7.1).
Pass: Headroom 6 ft 10⅛ in Minimum 6 ft 8 in above the nosing line (IRC R311.7.2).
Pass: Flight rise 9 ft 1 in At most 12 ft 7 in (3835 mm) between floors or landings (IRC R311.7.3).
Pass: 2R + T 25 9⁄16 in (65 cm) A comfort rule, not code: FloorGen flags stairs outside 60 to 66 cm (23⅝ to 26 in).
Pass: Pitch 33.4° The IRC sets no angle; its limits allow up to about 37.8° (7¾ in over 10 in).
Pass: Handrail needed 15 risers: a handrail on at least one side, 34 to 38 in above the nosings (IRC R311.7.8).

Open as a floor plan

The formula

Risers: divide the total rise (finished floor to finished floor) by the maximum riser and round up. An exact fit is not rounded up again.
Riser height: the total rise divided by the number of risers. Every riser in a flight is the same.
Treads: one fewer than the risers on a straight stair, because the top step is the upper floor.
Total run: treads × tread depth, measured from the face of the bottom riser to the face of the top one.
Angle: the arctangent of riser ÷ tread. 2R + T is twice the riser plus the tread.

Where the limits come fromUS figures are the 2021 IRC, Section R311.7 (ICC, checked 11 October 2026): risers up to 7¾ in (R311.7.5.1), treads from 10 in (R311.7.5.2), 36 in of width (R311.7.1), 6 ft 8 in of headroom (R311.7.2), at most 12 ft 7 in (3835 mm) of rise in one flight between floors or landings (R311.7.3), a handrail on at least one side from four risers, 34 to 38 in high (R311.7.8). The 2024 IRC moved these to R318.7 without changing them. The 60 to 66 cm band for 2R + T is FloorGen's own comfort check, not code. UK figures are Approved Document K, 2013 edition (gov.uk PDF, checked 11 October 2026), Table 1.1 for a private stair: rise 150 to 220 mm, going 220 to 300 mm, 2R + G between 550 and 700 mm, and a pitch of no more than 42°; 2 m of headroom (Diagram 1.3); handrails 900 to 1000 mm high, on both sides from 1000 mm wide (para 1.34); and a change of direction of at least 30° once consecutive flights pass 36 risers (para 1.17). Its 12 and 16 riser limits per flight (para 1.18) are for buildings other than dwellings, so a private stair has none. Codes are adopted and amended locally: confirm with your building department.

Worked example

An 8 ft ceiling with about 13 in of joists, subfloor and finish above it gives 109 in from floor to floor. 109 ÷ 7.75 = 14.06, so the stair needs 15 risers, each 109 ÷ 15 = 7.27 in (7¼ in). With 14 treads of 11 in the run is 154 in, 12 ft 10 in, at 33.4°. 2R + T is 2 × 7.27 + 11 = 25.5 in (65 cm), inside the comfort band. With the 10 in minimum tread the run shrinks to 11 ft 8 in and the stair steepens to 36°.

For headroom, the opening in the floor above has to start far enough back. Under that 8 ft ceiling, 6 ft 8 in of headroom needs an opening at least 11 ft 9 in long, measured back from the top riser. A 12 ft opening leaves 6 ft 10 in at its edge.

Straight stairs at the IRC's 7¾ in maximum riser (worked with the calculator)
Floor to floor	Risers	Riser	Run, 11 in treads	Angle	Run, 10 in treads
8 ft 9 in (105 in)	14	7½ in	11 ft 11 in	34.3°	10 ft 10 in
9 ft 1 in (109 in)	15	7¼ in	12 ft 10 in	33.4°	11 ft 8 in
9 ft 9 in (117 in)	16	7 5⁄16 in	13 ft 9 in	33.6°	12 ft 6 in
10 ft 1 in (121 in)	16	7 9⁄16 in	13 ft 9 in	34.5°	12 ft 6 in
10 ft 9 in (129 in)	17	7 9⁄16 in	14 ft 8 in	34.6°	13 ft 4 in

At 121 and 129 in, 11 in treads put 2R + T just over 66 cm; a 10½ in tread brings it back into the band. In the UK, a private stair at the 220 mm maximum rise and a 250 mm going needs 12 risers of 217 mm for 2.6 m floor to floor, a run of 2.75 m at 40.9°, and 13 risers of 208 mm for 2.7 m.

Before you build

Measure finished floor to finished floor. Leaving out the finish on either floor is the usual reason the top or bottom riser ends up different from the rest; the IRC allows only ⅜ in of variation in a flight.
Allow for landings. The IRC asks for a landing at the top and bottom at least as wide as the stair and 36 in long (R311.7.6), and one part way up any flight that would climb more than 12 ft 7 in (R311.7.3); Approved Document K asks for one at least as long as the stair is wide (para 1.20).
Check the headroom on a section, not just on the plan: the floor framing at the edge of the opening is where stairs fail.
A turn saves length. A dog-leg with a half landing fits the same rise in about half the length, as in the two-story template below.

Questions

How many steps do I need for a 9 ft ceiling?

With about 12 in of floor structure, the floor-to-floor height is around 117 to 121 in, which takes 16 risers at the IRC's 7¾ in maximum, and 15 treads. Measure your own rise: an inch either way can change the count.

What is the maximum riser height?

7¾ in under the IRC (R311.7.5.1) and 220 mm for a private stair in England under Approved Document K. Canada's National Building Code allows 200 mm.

How much space does a staircase take?

A straight stair for an 8 ft ceiling runs about 12 to 13 ft with 11 in treads, plus a 36 in landing at each end under the IRC. A typical width is 36 to 42 in.

What angle should a staircase be?

The IRC sets no angle, but its 7¾ in riser and 10 in tread limit a stair to about 37.8°; the 109 in example above, with 11 in treads, is 33.4°. In the UK a private stair may be up to 42°.

What is the 2R + T rule?

Twice the riser plus the tread, a measure of how natural the step feels. In the UK, 2R + G must be 550 to 700 mm. In the US it is a comfort rule of thumb; FloorGen flags stairs outside 60 to 66 cm (about 23⅝ to 26 in).