How to Calculate Stair Steps for a Deck in 5 Simple Steps

Stair steps in one line

Stair steps are simple: total rise, divided by the riser height, rounded up to whole steps, then the rise shared equally across those steps for the actual riser, with one fewer tread than risers for the total run. A 96 in rise at 7.5 in per riser needs 13 risers at 7.38 in each, 12 treads at 10 in, and 120 in of run. The rest of this guide is about getting those three inputs honest: the rise, the riser target, and the tread depth.

Step 1: Measure the total rise, floor to floor

Measure the total rise from the finished lower floor to the finished upper floor, in inches, at the staircase itself — not at a wall nearby, since floors can slope. The worked example is 96 inches, an 8 ft storey, typical of a deck to a raised porch. If the staircase lands on a deck, measure to the finished deck surface, including the decking boards, not to the joists. A rise measured to the subfloor under-counts by the flooring thickness on both floors, and every inch of error moves the step count or the riser height.

Step 2: Divide the rise and round up

Divide the total rise by the desired riser height, then round up to whole steps: 96 ÷ 7.5 = 12.8, rounded up to 13 risers. Steps are whole, so a fraction is not a staircase — 12.8 steps means 13 steps, with the rise shared across one more step than the division suggests. This is the step most estimates get wrong: rounding down would give 12 steps at 8 in each, a steeper staircase than planned.

Step 3: Share the rise equally for the actual riser

Divide the total rise by the rounded step count for the actual riser: 96 ÷ 13 = 7.38 in per riser. Every riser must be this height, because one odd step trips everyone who uses the stairs in the dark. The actual riser is rarely the target exactly — 7.38 in against a 7.5 in target — and that gap is the point: equal risers matter more than hitting the target. Write the actual riser on the stringer layout before cutting, so every step is marked from the same number.

Step 4: Count the treads and the total run

Count one fewer tread than risers, then multiply by the tread depth for the total run: 13 risers mean 12 treads, and at 10 in per tread the run is 120 in, or 10 ft. The top step lands on the upper floor itself, so it is not a tread. Check the run against the floor available: a staircase that fits the rise but not the run needs a landing or a turn, not a steeper riser.

Step 5: Order, cut and check the staircase

Cut the stringers to the layout — 13 risers at 7.38 in, 12 treads at 10 in — and check the staircase before fixing it: every riser equal, every tread level, the total rise matching the floor-to-floor measure. Then count what steps do not cover: handrail, balusters, landing boards and fasteners. For a deck staircase, the deck board calculator sizes the landing boards, so the deck and the stairs are ordered together.

Stair steps in numbers

Suppose the total rise is 96 in, the desired riser is 7.5 in, and the tread depth is 10 in. Risers: 96 ÷ 7.5 = 12.8, rounded up to 13. Actual riser: 96 ÷ 13 = 7.38 in. Treads: 13 − 1 = 12. Total run: 12 × 10 = 120 in, or 10 ft. Order: stringers cut for 13 risers at 7.38 in, 12 treads at 10 in, plus handrail, balusters and fasteners bought separately.

Common mistakes

The most common mistake is measuring the rise to the subfloor instead of the finished floor, which under-counts by the flooring thickness. The second is rounding the step count down instead of up, which builds a steeper staircase than planned. The third is using the desired riser as the cut height, which builds 12.8 steps in theory and uneven steps in practice. The fourth is counting a tread for the top step, which adds 10 in to the run that the landing already provides. The fifth is forgetting the run, which builds a staircase that hits the far wall.

Worked example, start to finish

A deck staircase, 96 in from the ground to the deck surface, 7.5 in desired riser, 10 in treads. Risers: 13. Actual riser: 7.38 in. Treads: 12. Total run: 120 in, or 10 ft. Result: cut stringers for 13 risers at 7.38 in and lay 12 treads at 10 in, with the top step landing on the deck itself, and order handrail and balusters separately. If the same rise used a 7 in riser instead, the need would rise to 14 risers at 6.86 in each, and the run to 130 in — the rise alone would be identical.

What changes the answer most

The rise changes the answer most: every inch of rise moves the count across a rounding, and a wrong rise rebuilds the whole staircase. The riser target is second: moving from 7.5 in to 7 in adds a whole step, since it divides every inch of rise. The tread depth is third, and it changes only the run, never the count — a deeper tread stretches the staircase without adding a step. The landing is fourth, and it changes only the layout, never the step maths.

Frequently asked questions