Stair Rise and Run Calculator

Start from the height and work backwards

There is only one measurement you cannot change: the total height from the finished floor below to the finished floor above. Everything else follows from it.

steps      = round(total height ÷ target rise)
exact rise = total height ÷ steps
treads     = steps − 1
total run  = treads × going

The exact rise will be an awkward number like 185.7 mm, and that is correct. What matters is that every riser is identical, not that the figure is round. Rounding the rise and absorbing the error in the last step is the classic beginner mistake and it produces exactly the hazard the regulations exist to prevent.

Measure the height in three places if the floor is at all uneven, and use the largest. Also make sure you are measuring to finished floor levels: adding 20 mm of tiles at the top after building the stair changes every step.

The 2R plus G rule

Two rises plus one going should come to between 550 mm and 700 mm, with around 600 to 640 mm considered ideal.

The rule derives from the mechanics of a walking stride. On the flat, an average pace is roughly 600 mm; climbing converts some of that horizontal movement into vertical, at roughly two units of horizontal per one of vertical. A staircase that respects the relationship feels natural to climb without anyone thinking about why.

Get it wrong and the result is immediately noticeable even to someone who could not say what is wrong. Too low a total and the steps feel cramped and hurried. Too high and you are stretching for each one.

This is a comfort rule rather than a legal one, and it is the check most worth doing, because a staircase can satisfy every regulation and still be unpleasant.

What the regulations require

Numbers vary by country and by whether the stair is private, public or in a workplace. Broadly, for a private dwelling in England and Wales:

• Maximum rise: 220 mm.
• Minimum going: 220 mm.
• Maximum pitch: 42 degrees.
• Minimum headroom: 2,000 mm measured vertically above the pitch line.
• Consistency: every rise and every going in a flight must be the same.
• Handrail: required on at least one side, at 900 to 1,000 mm above the pitch line.

The US International Residential Code is different in ways that catch people out: a maximum rise of about 7¾ inches, roughly 196 mm, and a minimum tread depth of 10 inches, roughly 254 mm. A stair designed to UK numbers will often fail US inspection and vice versa.

Always check the current code for where you are building. These figures change, and a staircase is expensive to rebuild.

Headroom is what usually goes wrong

The rise and going are easy. Headroom is the constraint that ruins otherwise sensible designs, particularly in loft conversions.

You need 2,000 mm measured vertically from the pitch line, the imaginary line joining the nosings of the treads, to whatever is above: a ceiling, a beam, or the underside of the floor you are cutting through. The critical point is usually where the stair passes through the opening.

If headroom fails, the options are to lengthen the stairwell opening, make the stair steeper, which quickly runs into the pitch limit, or move the staircase. Discovering this after the timber is cut is expensive, so check it first.

Treads, risers and the off-by-one

There is always one fewer tread than there are risers, because the top riser brings you level with the upper floor and that floor serves as the final tread.

A 14-riser staircase needs 13 treads. Ordering 14 is one of the most common material mistakes, and the reverse, cutting 13 risers into a stringer for a 14-rise stair, is considerably worse.

One more subtlety: the bottom riser of the stringer must be cut shorter by the thickness of one tread. Otherwise the first step ends up taller than the rest once the treads are laid, which reintroduces exactly the inconsistency you were avoiding. The figure above accounts for it.

Why consistency matters so much

After the first two steps, people climb stairs on a learned rhythm rather than by looking. The foot is placed where the body expects the step to be.

A single step that differs by 10 mm breaks that prediction, and the result is a trip going up or a stumble going down. Falls on stairs are a leading cause of accidental injury in the home, and inconsistent risers are a well-documented contributor.

This is why codes limit the variation between the largest and smallest riser in a flight to around 5 mm, and why the awkward decimal rise is the right answer rather than a rounded one.