Loan Stair Estimator
Stairs fail on paper before they fail in wood. Enter the floor-to-floor rise and get the riser count, the exact riser height, a tread depth from the comfort rule, the total run and the stringer length — checked against the usual code ranges.
Total rise is the finished-floor to finished-floor height — for a standard 9-ft ceiling with typical joists it lands near 108–112 in. Measure it; do not guess it from the ceiling height alone, because the joist and flooring stack is part of the climb. The estimator targets a 7.5-in riser, then sets the tread by the comfort rule (2 × riser + tread ≈ 25 in), never letting the tread drop below the 10-in code minimum.
How it works
Risers = total rise ÷ 7.5, rounded to a whole step; the exact riser is then total rise ÷ risers — every riser identical, which is both the comfort rule and the code rule (variation over ⅜ in between risers is a trip hazard and a violation). Treads are risers − 1: the top floor itself is the last landing. Tread depth comes from the classic comfort rule 2R + T ≈ 25 in, floored at the 10-in residential minimum. Stringer length is the hypotenuse √(rise2 + run2) — a lower bound; buy the next standard board length up, since the stringer's ends are cut at angles and the bottom riser loses height to the tread stock.
Formula & worked example
Formula: Risers = total rise ÷ 7.5, rounded to a whole number; exact riser = rise ÷ risers. Treads = risers − 1. Tread depth = max(10 in, 25 − 2 × riser) from the comfort rule 2R + T ≈ 25. Run = treads × tread. Stringer = √(rise² + run²).
Worked example: A 108-in floor-to-floor rise: 14 risers of 7.71 in, 13 treads at 10.00 in, a total run of 130 in (10.83 ft) and a stringer hypotenuse of 169.0 in (14.08 ft) — so 16-ft stringer stock, with 2R + T = 25.43 in, inside the comfort band.
When to use this
- Framing a new staircase and cutting stringers that land exactly on both floors.
- Checking whether a planned stair fits the available run before framing the opening.
- Replacing basement or deck stairs to modern comfort dimensions.
- Reviewing a contractor's stair layout against code ranges.
How to use it, step by step
- Measure the total rise: finished floor to finished floor, flooring included.
- Read the riser count and exact riser height — never round risers individually.
- Check the tread and the comfort score; adjust expectations if the run doesn't fit.
- Take the stringer hypotenuse and buy the next board length up.
- Re-measure after subfloor and finish flooring choices are final — they change the rise.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Total rise | Finished-floor to finished-floor height in inches | 96–120 in per storey |
Another worked example
A basement stair with a lower ceiling stack: 96 in floor to floor. Risers: 96 ÷ 7.5 rounds to 13, at exactly 7.38 in each; tread from the comfort rule is 10.23 in (no clamping needed), giving a 122.8-in run (10.23 ft) and a 155.8-in stringer — 14-ft stock suffices here, and 2R + T lands at exactly 25.00.
Frequently asked questions
How many steps do I need for a 9-foot floor-to-floor rise?
108 in ÷ 7.5 in = 14.4, rounded to 14 risers of exactly 7.71 in each, with 13 treads at 10 in deep. Total run 130 in (10 ft 10 in), and the stringer hypotenuse is 169 in — buy 16-ft stringer stock.
Why is the riser height never a round number?
Because the rise rarely divides evenly, and identical risers matter more than pretty ones. Your legs learn the stair in two steps; a single riser ½ in taller than the rest is precisely where people fall. The math spreads the odd fraction across every step instead.
What is the 2R + T rule?
The comfort rule from centuries of stair building: twice the riser plus the tread should land between 24 and 26 in — roughly a natural stride. The default stair here scores 25.43 in. Outside the band, stairs feel either like a ladder or like a ramp with delusions.
What is the longest riser the code allows?
7¾ in is the common US residential maximum, with a 10-in minimum tread. This estimator targets 7.5-in risers, so typical floor heights stay inside the limit — a very tall floor-to-floor (over about 116 in) is where you should double-check against your local code.
How long a board do I need for the stringer?
The hypotenuse is the floor: 169 in for the default stair, so 16-ft (192 in) stock. The cuts at both ends, plus the bottom-riser adjustment for tread thickness, consume the extra. When in doubt, the longer board is the cheaper mistake.
🪵 Pricing the stringer lumber?
Once the geometry is set, the Board Foot Estimator turns your stringer and tread dimensions into board feet and a lumber cost.
What if my stairwell run is too short for the computed run?
Steeper is the temptation and the trap: risers may not exceed your code maximum and treads may not go under the minimum. Real solutions are a switchback (L or U stair), a winder turn, or relocating the opening — not shaving each tread until the stair becomes a ladder.
Are deck stairs calculated the same way?
Yes — total rise is deck surface to grade, and the same riser/tread rules apply, though outdoor codes sometimes allow a slightly taller riser. Stringers for decks are pressure-treated; buy length by the same hypotenuse rule.