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Stair calculator

Enter the total rise. We split it into equal steps under the code limits, draw the stair to scale, size the stringers, add a landing when one is needed and send a deck stair straight to the deck plan.

A 36 in rise takes 5 risers of 7.2 in and 4 treads of 10.5 in. The stair runs 3'-6" out and climbs at 34.4 degrees. Cut 3 stringers from 2x12, 17.3 in on center, each about 5'-7" long: buy 8 ft boards.
Total run 3'-6"Rise 3'-0"7.2 in riser10.5 in tread
Side view of the stair, to scale. The shaded band is a 2x12 stringer; the line is the cut.
Two risers plus a tread
24.9 in, in the comfortable 24 to 25-1/2 in band
Cut the bottom of the stringer
1 in shorter, the tread thickness, so every finished step is the same
Handrail
Needed on one side, 34 to 38 in above the nosings
Guards
Needed on any open side: the top is more than 30 in up
The limits this stair is checked against (IRC R311.7, and DCA 6 for deck stairs).
RuleLimitYoursResult
Riser height7.75 in or less7.2 inPasses
Tread depth10 in or more10.5 inPasses
Width36 in or more36 inPasses
Rise of one flight12'-0" or less3'-0"Passes
Stringer left under the cuts5 in or more (2x12)5.31 inPasses
Layout for the stringer: mark each step with a framing square set to the riser and the tread.
StepHeight above the bottomDistance out from the first riser
10'-7 3/16"0'-0"
21'-2 3/8"0'-10 1/2"
31'-9 5/8"1'-9"
42'-4 13/16"2'-7 1/2"
5 (deck surface)3'-0"3'-6"
Put this stair on a deck plan, free

The deck designer opens at a 36 in deck height with a 36 in stair. It lays out its own stair at the fewest risers and 10 in treads (two 5-1/2 in boards), with the stringers, landing, stair footings, guards and handrail on the plan. Set the deck size there.

Planning figures from IRC 2021 R311.7 and, for deck stairs, the AWC DCA 6 guide. Not an engineer's design or a permit: your building department has the final word, and local rules may be stricter.

Stair sizes at a glance

Worked out with the calculator above, 36 in wide.
StairRisersRiserTotal runAngle
Deck, 24 in rise46 in2'-6"31 degrees
Deck, 36 in rise57.2 in3'-4"35.8 degrees
Deck, 48 in rise76.86 in5'-0"34.4 degrees
Deck, 72 in rise107.2 in7'-6"35.8 degrees
House, 108 in floor to floor147.71 in11'-4 1/2"36.3 degrees
House, 108 in, easier steps, 11 in treads166.75 in13'-9"31.5 degrees

Rise, run and the rules they meet

The rise is the height from the lower floor or landing to the upper floor or deck surface. The run is how far the stair reaches out across the floor. The IRC caps one riser at 7.75 in and asks for treads of at least 10 in, measured nosing to nosing, and every riser in a flight within 3/8 in of the others. That last rule is why the calculator divides the rise evenly instead of using full 7.75 in steps and a short one at the top.

Comfortable steps

A stair at the code limits is steep. Builders check two risers plus one tread: between 24 and 25-1/2 in feels natural to most people. Pick "Easier steps" and the calculator aims for about 7 in risers, then try an 11 in tread. A 108 in floor to floor stair goes from 14 risers and 11'-4 1/2" of run to 16 risers and 13'-9".

Headroom and the floor opening

Inside a house you need 6 ft 8 in clear above the nosing line all the way up. Enter the upper floor thickness and the calculator gives the shortest floor opening that keeps that headroom, measured back from the top riser.

Deck stairs

Deck stairs follow the same riser and tread limits, plus the DCA 6 framing rules: 2x12 stringers, at least 3 of them and no more than 18 in apart, 5 in of wood left under each cut, and a row of posts under any stringer that spans past 6 ft. Pick "Deck" and the calculator applies them, then sends the height and stair width to the free deck designer, which draws the stair, its footings and its guards on the deck plan. For a deck stair only, the stair stringer calculator gives the stringer layout on its own, and the concrete steps calculator counts the concrete for poured steps.

Where the numbers come from

The limits are read from the 2021 International Residential Code, section R311.7 (stairways) and R312.1 (guards), and for deck stairs from the AWC DCA 6 Prescriptive Residential Wood Deck Construction Guide, pages 20 to 22. The comfort band (two risers plus a tread) is a builders' rule of thumb, not a code limit, and the page says so wherever it shows.

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Questions

Anything else? Write to us or see every calculator.

Questions people ask

Divide the total rise by 7.75 in, the tallest riser the IRC allows, and round up. That is the number of risers. Divide the rise by that number to get one riser height, so every step is equal. There is one tread fewer than risers, because the top floor or deck is the last step. A 108 in floor to floor height takes 14 risers of 7.71 in and 13 treads.

Measure the total rise straight down from the top of the finished deck or upper floor to the spot where the bottom step will land, after the pad or landing is in. Use a level and a straightedge if the ground slopes. Then pick a tread depth, and the calculator gives the risers, the total run and the stringer length to buy.

For 7 risers of 7.75 in and 10 in treads the stringer is about 7'-9" with the cuts at each end, so an 8 ft 2x12 is enough. With 11 in treads it grows to 8'-1", and you need a 10 ft board.

Outdoors, pressure-treated southern pine 2x12 rated for ground contact: DCA 6 sets 2x12 as the least for a cut stringer and asks for 5 in of wood left under every cut. Indoors, a straight kiln-dried 2x12 in Douglas fir or southern pine, picked free of large knots along the bottom edge.

Most house stairs land between about 30 and 37 degrees. At the steepest the IRC allows, a 7.75 in riser on a 10 in tread, the stair climbs at 37.8 degrees. The calculator gives the angle of your stair, and a riser near 7 in on an 11 in tread brings it nearer 32 degrees.

When one flight would rise more than 12'-3" indoors (IRC R311.7.3), or 12'-0" on a deck stair built to DCA 6. The landing is at least 36 in deep and as wide as the stair. The calculator splits a taller rise into equal flights for you: a 160 in deck rise becomes 2 flights of 11 and 10 risers.