Stair calculator: rise, run and number of steps
A 9 ft (108") floor-to-floor height needs 14 risers of 7 11/16" and 13 treads. Enter your own height below to get the step count, total run, angle and a drawing.
One riser more or less
Measuring the total rise
Everything on this page starts from one number: the height from the finished floor at the bottom to the finished floor at the top. If either floor is still bare subfloor, add the thickness of what is going on it. Hardwood over plywood adds about 3/4"; tile on a mortar bed can add 1 1/4". Forgetting this is the most common reason the top or bottom step comes out taller than the rest.
Measure at the spot where the bottom of the stair will land, not at the wall. Hold a straight board level off the upper floor edge, run it out over the landing spot, and measure straight down. Garage slabs and decks over sloped ground can be off by an inch or more across a few feet.
How the calculator works it out
The calculator divides the rise by your maximum riser and rounds up. That gives the fewest risers that stay within your limit. It then divides the rise evenly by that count, so every step is identical.
riser = total rise ÷ risers
treads = risers − 1
total run = treads × tread run
angle = arctan(riser ÷ run)
stringer = treads × √(riser² + run²)
Worked example for a typical 9 ft floor-to-floor height: 108" ÷ 7.75" = 13.9, rounded up to 14 risers. 108" ÷ 14 = 7 11/16" per riser. That leaves 13 treads; at 10" each the stair runs 10' 10" across the floor at 37.6°. The layout line on each stringer is 13' 8 3/16" long.
Some calculators keep the riser fixed at 7 3/4" and put whatever is left into the first step. For 120" of rise that produces fifteen 7 3/4" steps and one 3 3/4" step at the bottom, which is a trip hazard and fails inspection. Splitting the rise evenly avoids that.
Risers, treads and how to count steps
A riser is the vertical distance from one walking surface to the next. A tread is the surface you step on. Riser height is measured from the top of one tread to the top of the next, so it includes the tread thickness. When someone says "a 13-step stair" they usually mean 13 risers, but suppliers sometimes count treads. Ask which one before ordering precut stringers.
The nosing is the front lip of a tread that overhangs the riser below it. It gives your foot a little more landing room on the way down. The IRC requires 3/4" to 1 1/4" of nosing on stairs with solid risers, unless the treads are at least 11" deep.
IRC stair rules at a glance
These are the residential limits from the IRC 2021 International Residential Code, section R311.7. Most US towns adopt the IRC with local edits, so read this as a starting point and confirm with your building department.
| Item | Limit | Section |
|---|---|---|
| Riser height | 7 3/4" max; risers in a flight within 3/8" of each other | R311.7.5.1 |
| Tread depth | 10" min, nosing to nosing; within 3/8" of each other | R311.7.5.2 |
| Nosing | 3/4" to 1 1/4", within 3/8" of each other; not required on treads 11" or deeper | R311.7.5.3 |
| Open risers | Where the stair is more than 30" above the floor or grade below, the gap must not pass a 4" sphere | R311.7.5.1 |
| Width | 36" min clear above the handrail | R311.7.1 |
| Headroom | 6' 8" min, measured up from the nosing line | R311.7.2 |
| Height of one flight | 151" max between floors or landings | R311.7.3 |
| Landings | At top and bottom; 36" min in the direction of travel | R311.7.6 |
| Handrail | Needed with 4 or more risers; 34" to 38" above the nosings | R311.7.8 |
Comfortable steps: the 2R + T check
Code limits keep a stair from being dangerous. They do not make it pleasant. A centuries-old rule of thumb says two risers plus one tread should come to roughly 24" to 26", which matches the length of a normal stride on a slope. A 7 1/4" riser with a 10 1/2" tread gives exactly 25". If your result sits near 27" or more, the steps will feel stretched; below 23" they feel choppy.
To get a gentler stair, lower the max riser under "Riser limit and nosing". Going from 7 3/4" to 7" on a 9 ft rise adds two risers and about 20" of run, so check that the floor space is there first.
Steps for common heights
Ready answers for the rises people ask about most, from a two-step porch to a 12 ft basement. All values use the 7 3/4" limit and even risers.
| Total rise | Risers | Riser height | Treads | Run at 10" treads | Run at 11" treads | Angle at 10" |
|---|---|---|---|---|---|---|
| 1' 6" | 3 | 6" | 2 | 1' 8" | 1' 10" | 31.0° |
| 2' 0" | 4 | 6" | 3 | 2' 6" | 2' 9" | 31.0° |
| 2' 6" | 4 | 7 1/2" | 3 | 2' 6" | 2' 9" | 36.9° |
| 3' 0" | 5 | 7 3/16" | 4 | 3' 4" | 3' 8" | 35.8° |
| 3' 6" | 6 | 7" | 5 | 4' 2" | 4' 7" | 35.0° |
| 4' 0" | 7 | 6 7/8" | 6 | 5' 0" | 5' 6" | 34.4° |
| 5' 0" | 8 | 7 1/2" | 7 | 5' 10" | 6' 5" | 36.9° |
| 6' 0" | 10 | 7 3/16" | 9 | 7' 6" | 8' 3" | 35.8° |
| 7' 0" | 11 | 7 5/8" | 10 | 8' 4" | 9' 2" | 37.4° |
| 8' 0" | 13 | 7 3/8" | 12 | 10' 0" | 11' 0" | 36.4° |
| 8' 6" | 14 | 7 5/16" | 13 | 10' 10" | 11' 11" | 36.1° |
| 9' 0" | 14 | 7 11/16" | 13 | 10' 10" | 11' 11" | 37.6° |
| 9' 6" | 15 | 7 5/8" | 14 | 11' 8" | 12' 10" | 37.2° |
| 10' 0" | 16 | 7 1/2" | 15 | 12' 6" | 13' 9" | 36.9° |
| 11' 0" | 18 | 7 5/16" | 17 | 14' 2" | 15' 7" | 36.3° |
| 12' 0" | 19 | 7 9/16" | 18 | 15' 0" | 16' 6" | 37.2° |
Stairs with a landing
A single flight can climb at most 151" (12' 7") before the IRC asks for a landing. Landings also solve tight floor plans: an L or U layout folds the run in half. Each flight is calculated on its own, using the height from the floor to the landing surface for the first flight and from the landing to the upper floor for the second.
Example: a 14 ft rise (168") needs 22 risers of 7 5/8". Put the landing after 11 risers, at 7' 0" above the floor, and both flights come out identical. The landing must be at least as wide as the stair and 36" deep in the direction you walk. Enter each flight's rise in the calculator separately to get its own stringer.
Deck stairs have a stricter trigger. The American Wood Council's DCA 6 deck guide calls for an intermediate landing once the total height of the stairway passes 12' 0" (144"), and that landing is built as a freestanding deck with its own posts and footings.
Deck stairs
Deck stairs follow the same rules, with two outdoor wrinkles. First, measure the rise to the finished surface where the bottom of the stair will land, not to the grass at the rim joist. Ground that falls away from the house adds rise. Second, outdoor treads are often two 2x6 boards, which gives an 11" tread and 1 1/2" of thickness.
For a 30" deck with two 2x6 treads (11 1/4" run, 1 1/2" thick): 4 risers of 7 1/2", 3 treads, 2' 9 3/4" of run. The stringer drops 1 1/2" at the bottom, so its first rise is cut at 6". Our stair stringer calculator lays out the cuts and tells you how many stringers to set.
Frequently asked questions
How many steps do I need for 8 feet?
An 8 ft (96") rise needs 13 risers of 7 3/8" and 12 treads. With 10" treads the flight runs 10' 0" horizontally. Twelve risers would be 8" each, which is over the 7 3/4" limit.
What is the formula for stair rise and run?
Risers = total rise ÷ maximum riser, rounded up to a whole number. Riser height = total rise ÷ risers. Treads = risers − 1. Total run = treads × tread run. Angle = arctan(riser ÷ run).
What is the best angle for stairs?
Most house stairs land between 30° and 38°. A 7" riser on an 11" tread gives 32.5°, which most people find easy. The steepest code stair, 7 3/4" on 10", is about 37.8°. Above 40° you are building a ship ladder, not a stair.
Why does my staircase have one fewer tread than risers?
The upper floor acts as the last step. You climb the top riser and step straight onto the landing or floor, so a straight flight of 14 risers has 13 treads.
Can the bottom step be shorter than the others?
Not under the IRC. All risers in a flight have to be within 3/8" of each other, and the same goes for tread depths. Re-divide the total rise instead of trimming the first or last step.
Do I count the nosing in the tread depth?
The code measures tread depth from the front edge of one nosing to the front edge of the next, so the nosing does not add to it. The tread board itself is deeper: run plus nosing, for example 10" + 1" = an 11" board.