Stair riser height:
dividing a floor-to-floor gap into equal steps

Start from the total rise, not the step

A staircase is designed from the top down. The first number you need is the vertical distance between the finished surface of the lower floor and the finished surface of the upper floor. That distance is the total rise, and everything else on the stair is fitted into it.

Measure it after screed, tiles or timber have been laid, or subtract their thickness from a raw structural measurement. A flight designed on a bare slab level will end up with a top riser that is too tall or too short once the finish goes down, and a single odd riser is the classic cause of tripping on an otherwise sound staircase.

The risers are the vertical faces. Each one must lift you by the same amount, within a few millimetres, because the body learns the rhythm of a stair within two or three steps and misjudges anything that breaks it.

• Total rise: finished floor to finished floor, in one unit, ideally centimetres or millimetres.
• Number of risers: always a whole number, because you cannot build 15.6 steps.
• Riser height: the result, which you then compare with comfort and code limits.

The riser height formula

Once the total rise is known, the single division below gives the height of each riser.

The count of treads is one fewer than the count of risers in a straight flight that lands on the upper floor. That detail matters later, when you work out how much floor length the stair eats up.

Choosing how many risers to use

You rarely get to pick the riser height directly. In practice you pick a target height that feels comfortable, divide the rise by it, and round to the nearest whole number of risers. Then you divide again to get the true height.

For a 280 cm rise and a target near 17 to 18 cm, the options are close together, and the table shows how much each extra riser changes the step.

Choosing 16 here keeps the riser under the roughly 19 cm ceiling that many residential rules set, while keeping the staircase from sprawling across the room. A calculator that handles general quantity sums is handy for testing several counts quickly, though the division itself is simple enough for a phone.

Checking comfort against the going

A good riser height only works if the tread depth, called the going, suits it. The long-standing comfort rule says twice the riser plus the going should land between about 55 and 70 cm, with 60 to 63 cm being the sweet spot for a stride.

That 375 cm run is the horizontal floor length the straight flight needs, before any landing. If the room only has 300 cm, the options are a steeper stair, a turn, or a winder section, and each of those has its own rules.

When the floor space is tight

Sometimes the division gives a good riser but the run does not fit. A straight 15-tread flight at a 25 cm going needs 3.75 m of floor length, plus a margin at top and bottom for the landing. If the plan only offers 3 m, a few options exist, and each trades comfort or space for fit.

• Use more risers with a shallower going. The riser falls, but so does the tread, and 2R + G can slip below 55 cm.
• Add a quarter or half landing and turn the flight, which splits the run into two shorter pieces at the cost of floor area for the landing.
• Replace a few treads at the corner with winders, whose narrow ends are subject to their own minimum depth rules.
• Move the stair opening in the floor above, which is usually the cheapest change if it is caught at design stage.

Whichever route you choose, redo the division with the new riser count and repeat the 2R + G check. The numbers interact, so changing one without the other is how designs end up with a riser that is comfortable and a tread that is too short.

Where the rules differ and what is not in the sum

Maximum riser height, minimum going and headroom are set by national and local building rules, and they differ between houses, flats and public buildings. Treat 19 cm and 25 cm as familiar residential ballparks, then confirm the figures that apply to your project before cutting any timber or pouring any concrete.

• Headroom is a separate vertical clearance measured above the nosing line, not part of the riser sum.
• Nosings overlap the tread below and change the visible going; use the actual tread plus nosing consistently.
• The first and last risers can be affected by the floor finish; set out the stair using finished levels.
• Open-riser stairs usually have a maximum gap size so a small sphere cannot pass through.

Mistakes that leave one odd step

Most stair faults come from an incorrect rise value rather than bad arithmetic. A tape measured from the structural slab, a different datum used upstairs and downstairs, or a floor build-up decided late all produce a total rise that is wrong by a few centimetres.

That error is then divided across 16 steps, so each one is only slightly off and looks harmless on paper. The problem appears when the finish is added and a carpenter has to absorb the difference into the first or last riser. Verify the finished heights on site, redo the division with the confirmed rise, and only then mark the stringers.

1. 1Confirm both finished floor levels.
2. 2Subtract the floor-to-floor gap carefully and write down the unit.
3. 3Divide by the planned number of risers and round to the nearest millimetre.
4. 4Check 2R + G and the headroom, then mark out one riser at a time from the bottom.