Stair calculator

How to measure total rise

Total rise is the height the stair climbs, finished floor to finished floor. Every other number comes from it, so measure it twice.

1. Hold a level off the edge of the upper floor and measure straight down to where the bottom step will land. Floors are rarely flat, so measure at that spot.
2. Add the thickness of any finish flooring still to come on the upper floor, and subtract any still to come on the lower floor.
3. Enter the result. The calculator splits it into equal risers, so the first and last steps match the rest.

If a tread is thicker or thinner than the finished floor, cut the difference off the bottom of the stringer (dropping the stringer) so the first step matches (Fine Homebuilding).

Deck stairs: deck surface to the landing

For a deck, total rise runs from the top of the deck boards to the landing pad or ground where the bottom step sits. Ground often slopes, so measure where the stairs will land, not at the deck edge (Trex).

1. Rest a straight board on the deck, sticking out past the edge, and level it.
2. Measure from the board down to the ground where the stairs will end, about one tread depth out per tread.
3. Run the calculator, then measure again at the total run it gives. If the ground drops, enter the new rise (Canadian Woodworking).

How the stair calculator works

The calculator divides your total rise by your target riser and rounds to whole risers. If a riser would come out over 7¾ in, it adds one, so it never breaks the code maximum. All risers are equal.

There is one tread fewer than risers, because the upper floor is the last step. Total run is treads times tread depth. Stringer length is treads times the diagonal of one step, the length of the cut board. Add 2 in to spare and take the next stock length; lumber comes in 2 ft steps from 6 to 24 ft (Archtoolbox).

For indoor stairs it also sizes the opening in the upper floor for 6 ft 8 in of headroom above the nosing line.

The math, with your numbers

These lines update as you type above.

The step diagonal is the square root of riser² plus tread². The throat is the board depth minus riser × tread ÷ step diagonal.

Worked example: a 108 in stair, drawn to scale

Say a basement stair climbs 108 in (274 cm) floor to floor, with a 7½ in target riser, 10 in treads and a 1 in nosing.

108 ÷ 7.5 = 14.4, so 14 risers of 7.714 in (7 11/16 in), 13 treads and a 130 in run. Each step's diagonal is 12.63 in, so a stringer is 13 × 12.63 = 164.2 in and fits a 14 ft 2x12. The angle is 37.6°.

One catch: 7 11/16 in is rounded. Mark every riser at 7 11/16 in and 14 of them add up to 107 5/8 in, so the top step comes out 3/8 in taller, the most the IRC allows. Spread the difference and check the last riser before you cut.

Stair code limits (IRC R311.7)

The International Residential Code sets these limits for house stairs, and deck and porch stairs follow the same rules. The calculator checks every row except open risers.

Sources: the ICC's 2021 IRC chapter 3, checked against city and county handouts that quote it: Knox County, TN, Timnath, CO, White Bear Lake, MN, Wells, ME (2021 IRC) and Gilroy, CA. Flight rise also from Washington WAC 51-51-0311 and Anchorage handout R.13; open risers from Viewrail.

The American Wood Council's deck guide, DCA 6, adds rules for deck stairs: 2x12 stringers at least, three cut stringers at least, no more than 18 in between stringers on stairs over 36 in wide, and a landing once a deck stair climbs more than 12 ft (AWC DCA 6). See handrail height and stair dimensions for more.

The comfort rule: 2 risers + 1 tread

Code limits keep stairs safe, not comfortable. The stair builder's rule: twice the riser plus the tread should come to about 25 in. Fine Homebuilding puts it as 2 risers plus 1 run within an inch of 25 in, and riser plus run within an inch of 18 in (Fine Homebuilding, 2 rules for comfortable stairs). ArchDaily gives the metric form: 2 risers plus 1 tread equals 63 to 65 cm (ArchDaily).

Tread depth is the run, nosing to nosing.

Stringers: board size and how many

Cut stringers are 2x12s, the DCA 6 minimum for deck stairs. A 2x12 is actually 1½ × 11¼ in (Archtoolbox, SPIB), and the notches eat into that. Leave at least 5 in of solid wood, the throat, below each notch (JLC, common defects in deck stairs, Decks.com). The calculator flags a throat under 5 in.

A cut 2x12 should span at most 6 ft horizontally; past that, add a post and footing under the middle. Solid (uncut) stringers can span up to 13 ft 3 in (Simpson Strong-Tie).

The number of stringers depends on width and treads. DCA 6 asks for at least three cut stringers, at most 18 in apart. Thin and composite treads need closer spacing: Trex's thicker boards span up to 12 in, and the North American Deck and Railing Association recommends 12 in (Trex, Simpson Strong-Tie). The calculator uses 18 in.

Outer stringers flush with both edges. Spacing: AWC DCA 6, Anchorage handout R.13.

What to buy

For the worked example, a 36 in wide stair with closed risers:

Actual board sizes from Archtoolbox. A 12 ft board gives four 36 in pieces.

• Treads: as deep as the run plus the nosing, 11 in here. Two 2x6s give 11 in; one 2x12 gives 11¼ in. DCA 6 Table 6 allows 2x4 or 5/4 boards as the smallest treads on cut stringers (AWC DCA 6).
• Risers: a riser board fills the riser height minus the tread thickness, so under a 1 in tread a 1x8 ripped to width covers any code riser. Open risers must stop a 4 in sphere.
• Waste: buy one spare tread board. Cut one stringer first, test-fit it, then use it as the pattern (Fine Homebuilding).

Stairs and decks more than 30 in above the floor or grade need guards on their open sides (Wells, ME 2021 IRC handout); see railing height and the baluster spacing calculator.

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