Stair Calculator

How to use the stair calculator

1. Fill in the fields — they start with a worked example.
2. Read the answer, and the arithmetic shown beneath it.
3. Copy the results, or adjust the inputs to compare scenarios.

About this tool

Stairs are governed by the relationship between rise and going, and the reason is safety rather than aesthetics: inconsistent or badly proportioned steps are a genuine and well-documented cause of falls.

The first rule is that every riser must be equal. The number of risers is therefore chosen first and the height divided by it, rather than picking a nice round rise and letting the last step absorb the remainder. A final step even ten millimetres different from the rest is a trip hazard, because people climb stairs by rhythm rather than by looking.

The checks here use figures common to UK domestic building regulations: a rise between 150 and 220 mm, a going of at least 220 mm, and a pitch no steeper than 42 degrees. There is also a long-standing rule of thumb that twice the rise plus the going should fall between 550 and 700 mm, which captures a comfortable stride better than either measurement alone.

This is a design aid, not an approval. Regulations differ by country and by whether the stair is domestic, and headroom, handrails, landings and width all have requirements this does not check.

Common uses

• Planning a staircase for a loft conversion or extension.
• Checking whether an existing stair meets current standards.
• Working out how much floor space a stair will take.

Frequently asked questions

How many steps do I need?

Divide the floor-to-floor height by a target rise of about 190 mm and round to a whole number, then divide the height by that count so every riser is exactly equal.

Why must every riser be the same?

People climb stairs by rhythm, not by looking. A final step differing by even ten millimetres is a recognised trip hazard.

What is the 2R + G rule?

Twice the rise plus the going should fall between about 550 and 700 mm. It captures a comfortable stride better than rise or going alone.