PreConstruct AIFree calculator
Stair stringer calculator — rise, run and stringer length
Type the total rise, the number of risers and the tread depth the drawings show. Get each riser, the total run and the stringer's length up the slope, bought at the next even stock length — in inches and millimetres.
16 foot stringer stock, each riser 195.94 mm.
Stringer
16' stock
14.29 ft up the slope
Each riser
7.71"
195.94 mm
Total run
133.25"
11.1 ft · 3,384.55 mm
Tread
10.25"
260.35 mm
Each riser is within the 125–200 mm the Ontario Building Code allows for a private stair. Check the rest of Section 9.8 on your drawings.
The run is within the 255–355 mm the Ontario Building Code allows for a private stair.
The working
- riser 108" ÷ 14 risers = 7.71" (195.94 mm)
- run 13 treads × 10.25" = 133.25" (11.1 ft)
- stringer √(108² + 133.25²) ÷ 12 = 14.29 ft → 16' stock
A starting figure, not an engineered quantity. Sizes, reinforcing and code requirements come from your drawings, your specifications and your engineer — check them before you order.
Where a calculator stops
This does one run, from numbers you type. A bid is every run on every sheet.
- — It doesn't read the drawing set or its schedules
- — It doesn't measure on the PDF or keep the lengths
- — It doesn't show what an addendum changed
- — It doesn't price, add labour or build a proposal
Riser height
The total rise — finished floor to finished floor — divided by the number of risers: total rise ÷ risers = riser. 108 in over 14 risers is 7.71 in, or 196 mm.
Total run
A typical straight flight has one fewer tread than risers, because the top riser lands on the floor above: (risers − 1) × tread = run. 13 treads at 10¼ in is 133¼ in.
Stringer length
The stringer is the hypotenuse of the rise and the run: √(rise² + run²). 108 in and 133¼ in give 171.5 in, 14.3 ft — a 16 ft stringer. Add length for a kicker or the cut at the top if your detail needs it.
Checking it against the code
For private stairs, Ontario's Building Code allows a rise of 125 to 200 mm and a run of 255 to 355 mm; a public stair is limited to a 180 mm rise and needs at least a 280 mm run. The 255 mm minimum applies to permits applied for from 1 January 2022 (it was 210 mm before), so an older house can have steps that were legal then. Our guide to Ontario stair rise and run has the rest of Section 9.8. The drawings and the code govern — this does not design the stair.
Questions estimators ask
- How do you calculate a stair stringer's length?
- Square the total rise and the total run, add them, and take the square root. With the rise and run in inches, divide by 12 for feet, then buy the next stock length up.
- What is the maximum stair riser height in Ontario?
- For private stairs — inside a house, or outside serving one dwelling — the Ontario Building Code allows a rise from 125 mm to 200 mm. Public stairs are limited to 180 mm. The run for a private stair must be 255 to 355 mm for permits applied for from 1 January 2022.
- Why is there one less tread than riser?
- On a typical straight flight the last riser steps up onto the floor above, which is the landing, not a tread. Winders, landings partway and nosings change the count, so follow the drawings.
- How many stringers does a stair need?
- It depends on the stair's width, the tread material and the drawings; wide stairs need more. The stair section or the framing plan shows it.
Stop retyping sizes into a calculator
On PreConstruct AI you measure on the PDF, type the sizes the drawings print, and the concrete, forms, bars, board and stringers come out with the arithmetic beside every line — in imperial, metric or both. Your first bid is free, with no card.