What is the stair step formula (2R + G)?

The stair step formula is a rule of thumb that shows whether a stair is comfortable to use, based on the length of an average stride. It is:

Step value = 2 × rise + 1 × going (2R + G)

The idea is that stepping up takes roughly twice as much effort as stepping forward. If the result matches a natural stride, the stair feels easy. In Belgium and the Netherlands this value is called the stapmodulus, and around 600 mm is commonly cited as comfortable. In England, Approved Document K requires 2R + G to be between 550 and 700 mm.

Worked example

• Rise 180 mm, going 240 mm: 2 × 180 + 240 = 600 mm. Comfortable.
• Rise 220 mm, going 220 mm: 2 × 220 + 220 = 660 mm. Within the range and at the limits for a private stair in England, but steep and noticeably less comfortable.

The formula alone is not enough: the individual dimensions must also be right.

Application

1. Start with the floor-to-floor height. Divide it by the number of risers so that every rise is equal.
2. Work out the going. Use the formula to find a going that suits the rise, then check the stair fits the stairwell opening.
3. Check the rules. For a private stair in England, Approved Document K allows a maximum rise of 220 mm, a minimum going of 220 mm and a maximum pitch of 42 degrees. Other countries and building types have their own limits.
4. On winding stairs, measure on the walking line, not at the narrow inner edge.
5. Keep all steps equal. One odd riser, often the top or bottom one, is a classic trip hazard.

Common mistake: designing the stair on the formula alone and forgetting that the going must be deep enough for your foot. A stair with a shallow going stays dangerous on the way down.

Related terms

• Riser height
• Tread depth
• Walking line on a stair
• Spiral staircase
• Stair run

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