Stair stringer calculator (rise, run and stringer length)

You don’t choose the number of steps — measure the height and the calculator works out how many equal steps fit. The deck or upper floor is the top step, so a 36 in deck gets 4 steps you cut plus the deck = 5 step-ups. For stairs between two floors, pick Inside a house: the deck-only checks drop out and headroom comes in. New to this? Words explained.

How the stair math works

Every riser in a flight should be the same height, so start from the total rise and divide by the smallest whole number of risers that keeps each one under your limit. The number of treads is one fewer than the number of risers, because the top floor (or deck) acts as the last step.

risers = total rise ÷ max riser, rounded up
riser height = total rise ÷ risers
treads = risers − 1
total run = treads × tread depth
stringer length = √(total rise² + total run²)

Example: a deck 36 in above the patio. 36 ÷ 7.75 = 4.6, so 5 risers of 7.2 in (about 7-3/16 in). Four treads at 10½ in give a total run of 42 in, and the stringer measures √(36² + 42²) ≈ 55-5/16 in along the slope.

Worked examples by height: How many steps for a 2 ft deck (where three 8 in steps are tempting but too tall), How many steps for a 30 in deck (four even 7½ in risers), How many steps for a 3 ft deck, How many steps for a 4 ft deck (the height where the stair itself needs a guard), Stair stringers for an 8 ft rise, Stair stringers for a 9 ft rise, Stair stringers for a 10 ft rise and, in metric, Stair stringers for a 2.7 m rise.

The finer points (under “More options”)

• Finish floors (indoor stairs). Total rise is finished surface to finished surface. If flooring still has to go down, the stringer is laid out on the subfloor: drop the bottom by the tread thickness minus the bottom finish, and set the top level cut riser + tread thickness − top finish below the upper subfloor. Otherwise the first and last risers come out uneven.
• Tread stock. The tread depth in code is measured nosing to nosing. The board you buy has to cover that, plus the nosing overhang, plus any riser board it sits in front of — two 5/4×6 deck boards with a ¼ in gap are 11¼ in.
• Headroom. Pick Inside a house, enter the thickness of the floor at the top of the stair and the calculator gives the shortest opening, measured from the top riser, that keeps 6 ft 8 in of headroom above the nosing line (R311.7.2): (80 in + floor thickness) × tread ÷ riser.
• Shopping list. For deck stairs the tread line assumes 5½ in wide deck boards (5/4×6 or 2×6) with ¼ in gaps, cut to the stair width from 8, 10, 12 or 16 ft boards — whichever wastes least. It doesn’t include the handrail, guard posts or footings.
• Tight on space? Enter the room you have for the run, and the tread depth is worked out from it — with a warning if it drops below 10 in. The table under the result shows one riser fewer and one more.

Code limits for stairs (2021 IRC)

Deck stairs: what DCA 6 adds

The American Wood Council’s prescriptive deck guide (DCA 6), which many building departments accept for decks, is stricter than the IRC in a few places. With Deck picked, the calculator flags each one:

• Stringers are at least 2×12s, and a cut stringer keeps at least 5 in of wood below the notches (Figure 28).
• Use at least three cut stringers, no more than 18 in apart on center (Figure 29). The calculator starts at 16 in, the joist spacing 5/4 wood decking needs on a deck (2021 IRC Table R507.7) — a 36 in stair gets four stringers at 16 in, three at 18 in. Go to 18 in only if your tread boards are rated for that span.
• A cut stringer may span no more than 6 ft 0 in horizontally; beyond that, add a 4×4 post on a footing under it or a landing (Figure 28).
• A stair more than 12 ft tall needs an intermediate landing.
• At the bottom, stringers bear on footings below frost depth — not on pads or patios that aren’t founded that deep (Figure 34).

Tips for laying out stringers

• Measure total rise with the finished flooring or decking in place (or allow for it). A mistake here throws off every step.
• For a deck on sloping ground, measure rise at the point where the bottom step will land, not at the house.
• Use stair gauges on a framing square to mark the rise and run consistently; the stair stringer guide walks through it.
• Space cut stringers no more than 18 in apart (the DCA 6 maximum) for 5/4 or 2× wood treads, closer for thinner or composite treads — follow the tread manufacturer’s span table.