LayoutWise

Stair Calculator

Enter the height from floor to floor. You get equal risers under the code limit, the treads and total run, how long the stringers are, and a side view drawn to scale.

Your stairs

1. Height
Total rise (finished floor to finished floor)
ft
in
2. Steps
3. Framing

Risers

15

7 3/16" each

Treads

14

10 1/2" run

Total run

12' 3"

34.4° angle

Stringers

3

16 ft boards

Side view

2R + T = 24.9"
9' 0" rise12' 3" total run15 x 7 3/16" risersopening starts here
  • Stringer length: 15' 11"
  • Bottom riser on the stringer: 6 3/16" (minus tread thickness)
  • Wood left below the cuts: 5 5/16"
  • Stairwell opening (6' 8" headroom): 11' 4 1/2"

Shopping list

  • ✓Stringers: 3 x 2x12 x 16 ft (pressure-treated outdoors)
  • ✓Treads: 14 at 3' 0" wide x 11 1/2" deep (2 boards each)
  • ✓Risers: 15 at 3' 0" x 7 3/16"
  • ✓Construction adhesive and 261 screws (3 per tread or riser at each stringer)
  • ✓Handrail: about 16' 2 7/16" on at least one side (required with 4 or more risers)

Check your local code for guards, handrails and lighting. Outdoor stairs follow the same riser and tread rules.

How to calculate stair risers

Divide the total rise by the tallest riser you want, and round up — that's the number of risers. Then divide the rise by that number for the exact riser height. A 9-foot (108") rise with a 7 1/2" target needs 108 ÷ 7.5 = 14.4, so 15 risers of 7.2" each.

There's always one less tread than risers, because the top "step" is the upper floor. Fifteen risers means 14 treads; at 10 1/2" each the stairs run 147" — just over 12 feet.

Residential code limits

Rule (IRC R311.7)Limit
Riser height7 3/4" maximum
Tread depth10" minimum
Nosing (when treads are under 11")3/4" to 1 1/4"
Headroom6' 8" minimum
Width36" minimum
Difference between risers in a flight3/8" maximum

The comfort rule

Stairs feel natural when two risers plus one tread add up to about 24 to 25 inches. A 7.2" riser and 10 1/2" tread gives 24.9" — comfortable. Steeper stairs save floor space but tire people out faster.

Stringers and the stairwell opening

Stringers are usually cut from 2x12s, spaced no more than about 18" apart for 1"-thick treads, so a 36"-wide stair takes three. Keep at least 3 1/2" of solid wood below the notches. The bottom riser on the stringer is cut shorter by the tread thickness so the first step matches the others once the tread is on.

The opening in the floor above has to be long enough that anyone on the stairs gets 6' 8" of clearance. The calculator works this out from the riser height and the thickness of the floor above.

Building a deck to go with it? Use the deck board calculator and the baluster spacing calculator.

Sources