Stair Riser Layout Calculator

The riser rule: every riser within 3/8" of every other, nose to nose, none over 7". The leftover goes into the bottom nose — or the top nose, at the cost of a piecemark — never the treads between. The split between flights, not the riser height, is what moves the mid-landing. Finished floor to finished floor, not top of steel. Three elevations in; the layout that lands closest with the tightest spread out, and which noses it spent. Built by a working steel stair detailer, not a software company.

Layouts within 1'-0" of your landing

Closest landing first, then tightest spread. Riser count and the split between flights both move — the split is what moves the landing. Anything further than a foot off your target is not listed; clear the landing field to see the stair without that limit. Tinted rows have a standard riser below your window. Click or tab to a row and press Enter to load it.

Landing FF	Off target	Risers	Treads	Split	Standard riser	Adjusted	Variance	Run lo / up	Angle

The rules this page uses

The landing you type is a target, not a constraint. Layouts are sorted by how close they land to it, and every row tells you the miss in sixteenths. Leave it blank and the landing floats — then it sorts on the stair alone.
The split between flights is not fixed at half and half. The split is the thing that moves the landing, so every split that could reach your landing is tried. With no landing given it stays near even.
There are only two places to adjust on a run: the bottom nose and the top nose. Nothing in between ever moves — the second riser is a standard tread and stays one. That is 2 spots on a straight run, 4 on an out and back.
The bottom nose is the cheap one. That piece already carries its own piecemark, so adjusting it costs the shop nothing. The top nose does not — it can be a typical tread, so moving it creates a piecemark that would not otherwise exist. The selector decides whether the page may spend one, and every layout that does is priced in the Adds PCMK column.
Smallest variance wins among layouts that land equally close, because a tighter stair is never worse to walk, build or inspect. Fewest adjusted risers only breaks the tie.
Variance is 3/8" nose to nose across the whole stair — greatest riser minus smallest, every riser in the run, not just neighbours. 1/8" or under reads as comfortable; past that the page shows what fraction of the 3/8" you are spending.
Finished floor to finished floor. Not top of steel. The nosing is set to FF. If there is a nosing setback for field-added gripping pieces, this layout shifts.
Risers and treads are different counts. A flight of n risers has n − 1 treads. Out and back with N risers = N − 2 treads; straight run = N − 1.
The riser count is itself an adjustment. When a count outside your window closes the stair more quietly, the page says so instead of hiding it.
Tread is 11", locked. The shop standard this page is built around; a 12" tread is treated as wrong for an 11" nosing-to-nosing layout with a 1" nose and heel setback.
An adjusted riser may fall below your minimum, never above your maximum — and never above 7", the code cap.
No flight may rise more than 12'-0" between landings (IBC 1011.8). The page still shows the layout so you can see the numbers, but it says plainly when a flight is over and the stair needs another landing.

The math, in order

Solved in integer sixteenths of an inch. Anything typed is snapped to the nearest 1/16 on the way in. Total rise H = top FF − bottom FF; landing target T = mid FF − bottom FF.

For each riser count N and each split n₁ / n₂, the page tries every pair of standard risers R₁, R₂ on the 1/16 grid, from 1/2" below your minimum (never under 4") up to your maximum or 7", whichever is lower.
Leftover L = H − (n₁R₁ + n₂R₂), spread over a spots in as-equal-as-possible sixteenths. Every workable a is kept, not just the smallest — one more spot often halves the spread.
Bottom noses fill before top noses, and within that, the lowest riser first when L is positive and the highest first when negative, so the adjustment closes the spread instead of opening it.
A layout is kept only if every riser is ≤ the cap and max − min ≤ 3/8", and its riser heights are summed back against H before it can be shown.
Landing reached = bottom FF + Σ lower-flight risers. Miss = that − your target.
Ranking: standard riser inside your window, then landing miss in half-inch bands, then an even split when the landing floats, then smallest variance, then fewest added piecemarks, then fewest adjusted risers.

Run per flight = (risers in flight − 1) × 11". Angle = atan(standard riser / 11).

A worked example

A 16'-0" floor-to-floor out-and-back stair, bottom finished floor at 100'-0", top at 116'-0", architect's mid-landing shown at 108'-0". Riser window 6 3/4" to 7", tread 11".

28 risers at 6 7/8", split 14 up / 14 up. Landing lands exactly on 108'-0".

Straight arithmetic first: 28 × 6 7/8" = 16'-0 1/2" — half an inch over the opening. That half inch has to go somewhere.

Put it in the two bottom noses and each drops to 6 5/8", giving a 1/4" nose-to-nose spread and costing the shop nothing, because a bottom nose already carries its own piecemark. Allow the top noses as well and the leftover splits four ways instead of two: four risers at 6 3/4", spread down to 1/8" — at the price of two piecemarks that would not otherwise be cut.

That trade is the whole job. The calculator shows you both and prices each one; it does not pick for you.

What this calculator does not do

Headroom. The 6'-8" minimum is on you. Nothing here knows what is over the stair.
Landing depth. Not calculated. A mid-landing must be at least as deep as the inside-stringer width, and that is a separate check.
Stringers, connections and framing. This is a riser layout, not a design. No stringer sizing, no bolt groups, no reactions.
Winders, spirals, curved and alternating-tread stairs. Straight runs and out-and-back with one mid-landing only.
IBC commercial only. Not IRC residential, not OSHA industrial — the limits checked are 7" maximum riser, 11" minimum tread, 3/8" maximum nose-to-nose variance.
Nosing setback. The layout assumes the nosing is set to finished floor. If gripping pieces are field-added with a setback, the layout shifts.

Questions detailers ask

What is the riser rule for stairs?

Every riser in a stair within 3/8" of every other, measured nose to nose across the whole stair, with no riser over 7". Set the standard riser from total rise divided by riser count, then put the leftover only where it is cheap: the bottom nose of each run, which already carries its own piecemark, or the top nose if you accept the extra piecemark. Nothing between them moves. The split between flights is what moves the landing; the riser height is not. Work finished floor to finished floor, not top of steel.

How do you calculate stair risers with a mid-landing?

Total rise divided by riser count gives the average, but the landing is what makes it hard. The split between the two flights is what moves the landing elevation, so you solve for both together: pick a riser count, try every split that could reach the landing you were given, and take the one that lands closest with the tightest spread. An even 50/50 split is a coincidence, not a rule.

Do all the risers in a stair have to be the same height?

Not identical, but close. The greatest riser may not exceed the smallest by more than 3/8" — and that is measured nose to nose across the whole stair, not between neighbouring risers. In practice you want most risers identical and the leftover absorbed in as few as possible.

What is the maximum riser height for a commercial stair?

7" under IBC, with a minimum 11" tread. This calculator treats 7" as a hard cap and will not return a layout above it no matter what maximum you type.

Why does this work from finished floor instead of top of steel?

Because the nosing is set to finished floor, and that is what a person's foot lands on. Steel elevations fall out of the layout afterwards once the finish at each level is known. A mid-landing carrying 3" of concrete is part of the layout from the start, not a correction later.

How many treads does a stair with 28 risers have?

Fewer than you might expect — risers and treads are different counts. A flight of n risers has n − 1 treads, because the floor or landing is the last tread. An out-and-back with 28 risers has 26 treads; a straight run of 28 risers has 27.

Can the two flights of an out-and-back stair have different riser heights?

Yes, and it is often the better answer. Two flights carrying two different standard risers can close a rise that no single riser height closes cleanly, provided every riser in the stair still sits inside the 3/8" nose-to-nose spread. This calculator tries both and prefers one shared standard only when it can get it.

Which risers should be adjusted to absorb the leftover?

The bottom nose and the top nose of each run, and nothing between them. The bottom nose is the cheap one — that piece already carries its own piecemark, so adjusting it costs the shop nothing. The top nose can be a typical tread, so touching it creates a piecemark that would not otherwise exist.

Why is the tread locked at 11"?

It is the shop standard this page is built around, and 11" is the IBC minimum. Architects sometimes draw a full 12"; on this page that is treated as the wrong tread for an 11" nosing-to-nosing layout with a 1" nose and heel setback.

Verify with the project engineer, contract documents, and the governing code. Layout aid only — not a design tool, and not a code compliance certification.