Stair Stringer Calculator

US & Canada

Enter your floor to floor height to get the riser count, the exact riser height, the total run and the stringer board to buy.

Your stairs

1. Total rise

in
in

Measure finished floor to finished floor, including the flooring on both levels. 7 3/4 in is the IRC maximum; a few states allow 8 1/4 in.

2. Steps

in
in

Tread depth is measured nosing to nosing, not the width of the board you buy.

3. Stringers

in

Result

0

Your cut list

Cut one stringer, offer it up to the opening and check every riser before you cut the rest. Confirm against your local building department: the IRC only applies where it has been adopted, and some states amend the rise and run.

How the stair maths works

Everything follows from one decision: how many risers you split the height into. Round that up, never down, then divide back.

  • Risers = floor to floor height ÷ 7 3/4, rounded up
  • Riser height = floor to floor height ÷ risers
  • Treads = risers − 1, because the top tread is the floor you step onto
  • Total run = treads × tread depth
  • Stringer length = √(total rise² + total run²)

Dividing the height equally is what keeps every riser the same. The 3/8 in tolerance in the code is there for build error, not as an allowance to spend: a flight of equal risers and one odd one at the bottom is the classic failed inspection, and the classic trip.

Stairs by floor to floor height

At a 10 1/2 in tread, 7 3/4 in maximum riser and 36 in width. Stringer length is measured along the nosing line, so buy the next board up.

Risers, run and stringer length by floor to floor height
Floor to floor Risers Each riser Total run Stringer Board
96 in (8 ft 0 in) 13 7 3/8 in 126 in 158 3/8 in 14 ft
102 in (8 ft 6 in) 14 7 5/16 in 136 1/2 in 170 3/8 in 16 ft
108 in (9 ft 0 in) 14 7 11/16 in 136 1/2 in 174 1/16 in 16 ft
114 in (9 ft 6 in) 15 7 5/8 in 147 in 186 in 16 ft
120 in (10 ft 0 in) 16 7 1/2 in 157 1/2 in 198 in 20 ft
126 in (10 ft 6 in) 17 7 7/16 in 168 in 210 in 20 ft
132 in (11 ft 0 in) 18 7 5/16 in 178 1/2 in 222 in 20 ft

What the IRC allows

From the IRC 2021, Section R311.7. The 2024 edition moved these provisions to R318.7 without changing any of the numbers, so both section references are given. The IRC is a model code and only applies where your state or city has adopted it.

IRC residential stair limits
Item Limit 2021 2024
Maximum riser height 7 3/4 in (196 mm) R311.7.5.1 R318.7.5.1
Minimum tread depth 10 in (254 mm) R311.7.5.2 R318.7.5.2
Riser and tread uniformity Greatest may exceed smallest by at most 3/8 in (9.5 mm), within one flight R311.7.5.1, R311.7.5.2 R318.7.5.1, R318.7.5.2
Minimum width 36 in (914 mm) above the handrail height R311.7.1 R318.7.1
Minimum headroom 6 ft 8 in (2 032 mm), measured from the tread nosing line R311.7.2 R318.7.2
Maximum rise in one flight 12 ft 7 in (3 835 mm) between floor levels or landings R311.7.3 R318.7.3
Nosing Required where the tread is under 11 in (279 mm) deep; projects 3/4 to 1 1/4 in (19 to 32 mm) R311.7.5.3 R318.7.5.3
Nosing uniformity Greatest projection may exceed smallest by at most 3/8 in (9.5 mm) R311.7.5.3 R318.7.5.3

Source: International Residential Code, 2021 edition, Section R311.7. codes.iccsafe.org. Several states amend the rise and run, so confirm with your building department before cutting.

When you need a landing

A single flight may not rise more than 12 ft 7 in (151 in) between floor levels or landings, under R311.7.3. At a 7 3/4 in maximum riser that is 20 risers, so almost no house storey hits it: a 120 in height needs only 16. You run into the limit on tall stairwells, split levels and tall decks.

Where a landing does break the flight, work out each flight separately. The landing is the floor at the top of the lower flight and the bottom of the upper one. A landing must be at least as wide as the stair and at least 36 in in the direction of travel.

Cutting the stringers

  • Use a 2x12 unless the stair is tiny. After the notch, a 2x12 cut for 7 11/16 in risers and 10 1/2 in treads leaves about 5 1/16 in of timber behind the cut. The same cut in a 2x10 leaves only about 3 1/16 in.
  • Cut one and test it. Offer it up to the opening and check the top and bottom risers before you cut the rest.
  • Take the bottom riser down by the tread thickness. Otherwise the first step ends up taller than the others once the treads go on, which breaks the uniformity rule.
  • Buy the next board length up. The stringer is the diagonal, not the run, and the top and bottom cuts eat a little more.

Common questions

How many stairs do I need for 8 feet?
A 96 in floor to floor height needs 13 risers at 7 3/8 in each. That gives 12 treads and a total run of 126 in (10 ft 6 in) at a 10 1/2 in tread.
What is the maximum riser height for stairs?
7 3/4 in (196 mm) in the IRC, at R311.7.5.1. The minimum tread depth is 10 in. Both are model code figures: a handful of states amend the riser to 8 1/4 in over a 9 in tread, so check what your state adopted.
How do I calculate rise and run?
Divide the floor to floor height by 7 3/4 and round up. That is your riser count. Then divide the height by that count to get the riser height, which is how every riser comes out equal. Total run is the tread depth times one fewer than the riser count.
How long does a 2x12 stair stringer need to be?
For a 14 riser flight at a 10 1/2 in tread, the stringer measures 174 1/16 in along the nosing line, so you buy a 16 ft board. Measure the diagonal, not the run.
How many stringers do I need?
Enough that no gap exceeds your spacing. A 36 in wide stair at 16 in on centre takes 4 stringers. Wider treads and thinner decking want them closer together.
When do stairs need a landing?
When the flight would rise more than 12 ft 7 in (151 in) between floor levels, under R311.7.3. Below that a landing is optional, though a long straight run is still unpleasant. There must also be a floor or landing at the top and bottom of every stairway.
What is the throat on a stair stringer?
The timber left behind the notch, measured square to the cut line. On a 2x12 cut for 7 11/16 in risers and 10 1/2 in treads it comes to about 5 1/16 in. The IRC does not set a minimum, but the less you leave the weaker the stringer, which is why a 2x12 is normal and a 2x10 is tight.
Do deck steps follow the same rule?
Yes. A 3 riser step off a deck 21.5 in above grade still needs equal risers of 7 3/16 in. The uniformity rule is the one inspectors actually fail people on.