Stair Calculator
Work out risers, treads, total run and stringer length from a total rise.
Last reviewed September 19, 2026
Result
Buy 14 risersEstimate
No allowance is added to the counts. Buy stringer stock longer than the diagonal: the cuts need the extra length, and a mis-marked stringer is scrap.
- Riser height
- 7.714 in
- Number of treads
- 13
- Total run
- 130 in (10.83 ft)
- Minimum stringer length
- 169.01 in (14.08 ft)Estimate
- Angle of the flight
- 39.719 °
https://calcgallery.com/lumber-carpentry/stair-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use it
- Measure the total rise from finished floor to finished floor.
- Set the riser height you are aiming for. Around 7.5 in, or 180 mm, is comfortable.
- Set the tread depth, measured nosing to nosing.
- Open the advanced options to set the maximum riser your local code allows.
- Read the riser count, the exact riser height, the total run and the stringer length.
How it is calculated
Stairs start from the one measurement you cannot change, the total rise between the two finished floors:
risers = round(total rise ÷ target riser) at least 1
riser = total rise ÷ risers
treads = risers − 1
run = treads × tread depth
stringer = √(rise² + run²)The target riser height does not set the riser height. It only decides how many risers there are. Once that is a whole number the rise is divided evenly among them, which is why asking for 7.5 in over a 108 in rise gives 7.714 in.
If dividing evenly would put the riser above the maximum, one more riser is added. That handles the ordinary case where rounding down makes the steps slightly too tall.
A target far above the maximum is not corrected. Only one riser is added, so asking for 9 in risers against a 7.75 in limit still returns 8.31 in. Check the riser height against your limit rather than assuming the calculator has enforced it.
Safety and building codes
Stair dimensions are regulated everywhere, and the limits are not the same in any two places. Maximum riser height, minimum tread depth, the permitted variation between risers in one flight, headroom, landing sizes, handrail heights and nosing projections are all specified, and they differ between countries, between states and often between a new build and an alteration.
Nothing on this page is a compliance check and no result here should be described as code compliant. The figures are a starting point for a design that you then check against the code that applies where you are building. Where a flight forms part of a means of escape, or serves a loft conversion, the rules are usually tighter again.
Stairs are also the most common cause of serious injury in the home. A flight with one riser a quarter inch out of step with the rest is genuinely dangerous, because people climb stairs without looking at them.
Practical tips
Measure finished floor to finished floor. Not to the top of the joists, and not to the subfloor. If tile is going down at the bottom and carpet at the top, both change the rise, and both have to be in the number before you cut a stringer.
Check the run fits before anything else. The total run is the number that ruins plans. A 9 ft rise needs nearly 11 ft of floor, and if you have 8 ft the answer is not steeper stairs, it is a landing and a turn.
Rise and going move together. Steeper treads need shallower steps to stay comfortable. The old rule of thumb is that twice the riser plus the going lands somewhere around 24 to 25 in, or 600 to 630 mm.
Cut one stringer and test it. Mark it, cut it, and offer it up before cutting the other two. An error repeated three times costs three boards.
Dry fit before fixing. Check every riser with the same gauge block. The first and last risers are the ones that drift, because the floor thicknesses at top and bottom are easy to forget.
With your numbers
- Riser height = 108 in ÷ 14 risers = 7.714 in
- Total run = 13 treads × 10 in = 130 in
- Stringer = √(rise² + run²) = 169 in
- Risers = total rise ÷ your 7.5 in target, divided evenly into 14
Quick reference
No allowance is added to the counts. Buy stringer stock longer than the diagonal: the cuts need the extra length, and a mis-marked stringer is scrap.
| Job | Number of risers |
|---|---|
| 8 ft rise, 7.5 in target | 13 |
| 9 ft rise, 7.5 in target | 14 |
| 10 ft rise, 7.5 in target | 16 |
| 3 ft deck steps | 5 |
| 9 ft rise, 7 in target | 15 |
| 2.6 m rise, 180 mm target | 14 |
| 2.7 m rise, 180 mm target | 15 |
| 1 m deck steps | 6 |
Common questions
- How many stairs do I need for a 9 foot rise?
- Fourteen risers and thirteen treads, at a target of 7.5 in. That divides 108 in into risers of 7.714 in each and needs 130 in of floor space for the run.
- Why is the riser height not the number I asked for?
- Because every riser in a flight has to be identical. The total rise is fixed by the two floor levels, so it is divided into a whole number of equal risers. Your target only decides how many. A flight with one odd riser is the classic trip hazard, which is why codes are strict about it.
- Why is there one fewer tread than riser?
- The top riser brings you up to the floor above, and that floor is the last tread. So a flight of fourteen risers has thirteen treads in between. Counting fourteen treads is a common mistake and it makes the run a full tread too long.
- How much floor space will the stairs take?
- The total run, which is the tread count times the tread depth. Fourteen risers at a 10 in tread need 130 in, just under eleven feet. This surprises people planning a loft conversion more than any other number here.
- How long a board do I need for the stringer?
- Longer than the diagonal the calculator gives. The diagonal is the straight line from the bottom of the flight to the top, but cutting the notches needs extra at both ends, and a stringer marked out wrong is firewood. Add at least a foot.
- Is a result from this calculator code compliant?
- No. It is arithmetic, not a compliance check. Maximum riser, minimum tread, headroom, handrails, landings and nosing overhang are all regulated and the limits differ by country and jurisdiction. Take these numbers to your local code or your building inspector before you cut anything.