Code check, not just arithmetic
Riser height, tread depth, the comfort ratio and stair width are each tested against residential limits and reported line by line, so you find out about a problem here rather than at inspection.
Stair Calculator
Enter your total rise and the calculator works out the riser count, exact riser height, total run, stringer length and stair angle, then checks the result against residential building code limits. It also sizes the stairwell opening and shows the riser counts either side of yours, so you can see the trade-off before you cut a single stringer.
Code compliance check
Scaled side elevation
Inches or centimetres
Runs in your browser
Your measurements
in
in
in
in
in
Meets residential code
Number of risers––
Actual riser height––
Total run–horizontal length
Stringer length–minimum board length
Stair angle––
Stairwell opening–for 80 in headroom
Tread stock needed–total linear length
2R + T comfort rule––
Compare nearby riser counts
| Risers | Riser height | Treads | Total run | Angle | Code |
|---|
Fewer risers means a steeper, shorter flight. More risers means a gentler stair that eats more floor space. The calculator marks which options stay legal.
Calculated locally in your browser; nothing is uploaded. Figures follow the International Residential Code as a general reference. Your local authority may adopt different limits, so confirm with your building department before construction.
The numbers behind it
Stair dimensions and code limits
Every figure the calculator checks, and why the limit exists. These are the residential values; commercial stairs under the IBC are governed separately and are generally more restrictive.
| Dimension | Residential limit | Why it matters |
|---|---|---|
| Maximum riser height | 7 3/4 in (196 mm) | Taller risers force an unnatural lift of the leg and are the single most common cause of trips going up. |
| Minimum tread depth | 10 in (254 mm) | Shallow treads leave part of the foot unsupported on the way down, which is when most serious falls happen. |
| Riser and tread variation | 3/8 in (9.5 mm) max | The body learns the rhythm of a flight after two steps. One odd step breaks it and is a classic fall cause. |
| Minimum headroom | 6 ft 8 in (2032 mm) | Measured vertically from the tread nosing line to the finished ceiling or opening above. |
| Minimum stair width | 36 in (914 mm) | Measured above the handrail; below handrail height a small reduction is permitted. |
| Nosing projection | 3/4 in to 1 1/4 in | Required where treads are less than 11 in deep, since it adds effective foot room without adding run. |
| Handrail height | 34 in to 38 in | Measured vertically from the nosing line, and required on at least one side of any flight with four or more risers. |
| Comfort rule | 2R + T = 24 to 25 in | Not code, but the ratio that separates a stair that feels natural from one that is technically legal and still awkward. |
Built differently
Why this stair calculator is better
Most stair calculators return a riser height and stop. Building a staircase needs several more answers before you buy timber.
Scaled side elevation
The flight is drawn to scale with dimension lines for total rise and total run and the angle marked, so a mistyped measurement is obvious immediately instead of hiding in a column of numbers.
Riser count comparison
Your result plus the counts either side of it, each with its riser height, run, angle and legality. Choosing between a steeper stair and one that fits the room becomes a decision instead of a guess.
Stairwell opening sizing
Almost no calculator tells you how long the floor opening has to be to keep 80 in of headroom. This one does, using your upper floor structure thickness, which is the number that decides whether the stair fits at all.
A usable cut list
Stringer length, tread stock in linear feet and the exact riser height to four decimal places, copyable in one tap for your notes, your lumber order or your framer.
Fast, private, site ready
Inches or centimetres, no accounts, no tracking, no heavy frameworks. A few kilobytes of vanilla JavaScript that loads instantly on a phone in a half-framed basement.
Step by step
How to use the stair calculator
Measure once properly and the rest takes about a minute.
1
Measure the total rise
From finished floor below to finished floor above, not subfloor to subfloor. If flooring is not down yet, add its thickness now or every riser will be wrong later.
2
Set your target riser and tread
7 in and 10.5 in is the comfortable default for a home. The calculator rounds to a whole number of risers and gives you the exact height each one must be.
3
Check the code panel and the drawing
Every failed check is named. If the flight is too steep, try one more riser in the comparison table and watch the run and angle change.
4
Confirm run and opening fit
Make sure the total run fits the floor space and the stairwell opening clears headroom, then copy the cut list before you order material.
Get it right
Mistakes that ruin a set of stairs
Almost every failed stair inspection traces back to one of these.
- Measuring subfloor to subfloor
Tile or hardwood added later shortens the top riser by its thickness, and a single odd riser is an automatic code failure.
- Forgetting the stringer drop
The bottom of the stringer must be cut down by the tread thickness so the first riser finishes the same height as the rest.
- Counting treads as risers
A flight landing on an upper floor always has one more riser than it has treads. Getting this backwards throws out the total run.
- Ignoring headroom until framing
The floor opening length is set by riser height and floor thickness. Discovering it is too short after the joists are cut is expensive.
- Adding nosing to the run
Nosing overhangs the riser below, so it adds effective foot room but not total run. Counting it twice makes the stair longer on paper than in the room.
- Rounding riser height
Spreading a fraction of an inch across every riser is fine. Rounding to the nearest eighth and dumping the remainder into the last one is not.
Questions
Stair calculator FAQ
What people ask before they cut the first stringer.
How do I calculate the number of stairs I need?
Divide the total rise by your target riser height and round to the nearest whole number. That result is the number of risers. Then divide the total rise by that whole number to get the exact height each riser must be.
For example, a total rise of 119 in with a 7 in target gives 17 risers, and 119 divided by 17 is exactly 7 in per riser. A flight landing on an upper floor has one fewer tread than it has risers, so 17 risers means 16 treads.
What is the standard rise and run for stairs?
In residential construction a riser of about 7 to 7.5 in with a tread of 10 to 11 in is the normal range, and code caps the riser at 7 3/4 in and sets a tread minimum of 10 in. That combination produces a stair angle of roughly 32 to 37 degrees, which is the range most people find comfortable.
What is the 2R plus T rule?
Twice the riser height plus the tread depth should land between 24 and 25 in. It approximates a natural walking stride on an incline, and it is the reason a 7 in riser pairs well with a 10.5 in tread but badly with a 14 in one.
A stair can pass every code limit and still fail this ratio, which is usually why a technically legal staircase feels wrong underfoot.
How long does a stair stringer need to be?
The stringer follows the hypotenuse of the triangle formed by total rise and total run, so its length is the square root of rise squared plus run squared. A 119 in rise with a 168 in run needs about 206 in, which is a little over 17 ft.
Add several inches for the top hanger and bottom cuts, and remember that cutting a stringer removes material: a 2x12 is the usual minimum for a cut stringer because roughly 3.5 in of solid timber must remain behind the notches.
How much space do stairs take up?
The floor space consumed is the total run, which is the number of treads multiplied by the tread depth. A typical 9 ft floor to floor with 17 risers and 10.5 in treads needs 16 treads, so 168 in or 14 ft of horizontal space, plus clear landing space at both ends.
If that does not fit, options are a switchback flight with a landing, a slightly steeper stair within code, or in some jurisdictions a spiral stair, though spiral stairs have their own dimensional rules.
How big does the stairwell opening need to be?
Long enough that anyone on the treads below the opening still has 80 in of clear headroom. It depends on riser height and how thick the upper floor structure is, since headroom is measured from the tread nosing line to the underside of the finished ceiling above.
The calculator estimates this for you from your floor thickness figure. Deeper joists or a thick ceiling assembly always push the opening longer.
Can all the risers be slightly different heights?
No. The largest and smallest riser in a flight may differ by no more than 3/8 in, and the same tolerance applies to tread depths. Your body memorises the rhythm of a flight within two steps, so a single odd riser, usually the bottom one after flooring goes down, is a genuine hazard and a common inspection failure.
Do I include the nosing in the tread depth?
Not for the run calculation. Tread depth for code and for total run is measured from nosing to nosing, which equals the horizontal cut on the stringer. The nosing overhangs the riser below, so it adds foot room without adding floor space. Enter the stringer cut dimension in this calculator, then add 3/4 to 1 1/4 in of nosing when you buy the tread boards.
Get the rise right before you cut
Stringers are unforgiving: one wrong riser height and the whole flight is scrap. Run your numbers, check the code panel, and take the cut list to the lumber yard.
Calculate your staircase