How the stair calculator works
Every stair starts with one measurement: the total rise, from the finished floor at the bottom to the finished floor at the top. The calculator divides that by the tallest riser you're allowed, rounds up to a whole number of risers, then splits the rise evenly between them. That gives you the riser height. You'll have one fewer tread than risers, because the top floor is the last step.
risers = round up (total rise ÷ max riser)
riser height = total rise ÷ risers
treads = risers − 1
total run = treads × tread depth
Here's a typical house stair. Total rise is 108″ and the IRC maximum riser is 7 3/4″. 108 ÷ 7.75 = 13.94, so you need 14 risers. 108 ÷ 14 = 7.714″, which is 7 11/16″ on a tape. That's 13 treads, and at 10 1/2″ each the stair needs 136 1/2″ of floor, or 11′ 4 1/2″.
The plate redraws as you type. Check it against your space: the total run has to fit before the wall or the landing, and the dashed headroom line has to clear the floor framing above.
Measuring total rise without getting burned
Most stair mistakes start here.
- Measure finished to finished. If the subfloor is down but the 3/4″ oak isn't, add it. If the bottom gets tile over a mortar bed, add that too. Otherwise the first or last step comes out a different height from the rest, and that's the step people trip on.
- For decks, measure where the stairs land. Run a level or a laser out from the deck surface to the spot where the bottom tread will sit. Ground that drops 3 inches over 8 feet adds 3 inches of rise.
- Measure twice in different spots. Floors aren't flat. If two readings disagree, use the one at the actual stair location.
What the code requires
These are the residential limits in the 2021 International Residential Code, section R311.7. The calculator checks the first four automatically.
| Rule | IRC limit | Section |
|---|---|---|
| Riser height | 7 3/4″ max. | R311.7.5.1 |
| Tread depth | 10″ min. | R311.7.5.2 |
| Nosing, treads under 11″ | 3/4″ to 1 1/4″ | R311.7.5.3 |
| Stair width | 36″ min. | R311.7.1 |
| Headroom | 6′ 8″ min. | R311.7.2 |
| Riser or tread variation in a flight | 3/8″ max. | R311.7.5.1–2 |
| Handrail | 4 or more risers | R311.7.8 |
| Guard, open side over 30″ high | 36″ tall min. | R312.1 |
The 3/8″ variation rule is the one inspectors catch most. Even risers on paper turn uneven when the bottom landing is poured later or the flooring changes thickness. Lay out the stringer from the finished heights and you avoid it.
Making a legal stair comfortable
A stair can pass code and still feel steep. The old rule of thumb is that two risers plus one tread should land between about 24 and 25 inches, which matches a normal stride. A 7 1/2″ riser with a 10″ tread gives 25. A 7 3/4″ riser with a 10″ tread gives 25 1/2, which is legal but steep, the kind of stair you hold the rail on.
If you have the floor space, adding one riser is often the best upgrade you can make. Use the "other riser counts" table above to see how much extra run it costs.
Deck stairs and basement stairs
Outdoor stairs follow the same rules. Deck treads are usually two 5/4×6 boards with a gap, about 11 1/4″ deep, so the nosing is optional. At the bottom, the American Wood Council deck guide (DCA 6) attaches the stringers to guard posts on footings, with the stringers bearing on a 2×4 block. Keep them off bare dirt, and ask your building department before you swap that detail for a concrete pad.
For basement and interior stairs, headroom is the usual problem. The calculator works out the minimum stairwell opening from your upper floor depth: how far back from the top riser the floor framing has to stop so a 6′ 8″ person clears it. If you can't get that much opening, you need more run, a landing, or a winder.
Once the numbers work, take them to the stair stringer calculator, which lays out the cuts on a 2×12 and checks the throat.