What is the 27 rule for stairs?

1. Where the 27 rule comes from

The formula is credited to French architect François Blondel, who published it in 1675 after studying how the human stride relates to stair proportions. The logic is straightforward: walking on flat ground, an average adult stride is roughly 24–25 inches. Climbing stairs shortens the horizontal part of that stride for every inch gained vertically, so Blondel proposed a formula that trades tread depth for riser height in a fixed relationship — 2 x riser + tread = 24 to 27 inches in the commonly cited modern range.

A separate, related version of the same idea says riser + tread should equal about 17–18 inches — a simpler formula aimed at the same result. Both are ways of expressing the same underlying comfort principle: taller risers need shallower treads, and shorter risers can take deeper treads, without the stair feeling wrong underfoot.

2. The sweet spot — and the trade-off

A 7-inch riser paired with an 11-inch tread is a commonly cited sweet spot: 2 x 7 + 11 = 25, right in the middle of the 24–27 range. Push the riser higher with a shallow tread and the stair starts to feel steep, tiring on the legs, and less safe underfoot — a shorter step forces a shorter, more awkward stride on every step. Go the other way — a low riser and a very deep tread — and the stair takes up noticeably more floor space per step, and can feel oddly shallow, almost like walking rather than climbing.

3. This is a comfort rule, not your building code

The 27 rule predates every modern building code by centuries, and it isn't legally binding anywhere. Your local code sets its own minimum tread depth and maximum riser height for a legal, inspectable staircase, and those code limits don't always land neatly inside the 24–27 range — a code-minimum stair can still be a bit steeper than Blondel's comfort zone and still pass inspection. Use the 27 rule to judge how a staircase will feelto climb; use your actual local code, and a professional where required, to confirm it's legal to build. The two questions have different answers.

In practice, most comfortable residential stairs already satisfy both at once — a 7-inch riser and 10–11-inch tread tends to clear typical code minimums for interior stairs while also landing inside the 24–27 comfort range. The formula is most useful when you have some flexibility in the design (a new build or a major renovation) and want to choose the most comfortable proportions within what your code and available space allow.

Common questions

What is the 27 rule for stairs?

It's a design guideline — 2 x riser height + tread depth should land between 24 and 27 inches — used to keep a staircase comfortable for an average adult stride. It's not a building code requirement; your local code sets separate, legally binding minimums for riser height and tread depth.

Where does the 27 rule come from?

It traces back to French architect François Blondel's 1675 formula relating stair proportions to the human stride. Modern versions state it as 2R + T = 24 to 27 inches, with 25 inches (a 7-inch riser and 11-inch tread) often cited as the comfortable middle.

Is the 27 rule the same as a building code?

No. It's an ergonomic design rule of thumb used by builders and stair designers. Building codes (like the IRC in the US) set their own binding riser and tread minimums, which may or may not land inside the 24-27 inch comfort range — always check your code's actual rise/run limits for a legal build.

What is a comfortable stair riser and tread combination?

A 7-inch riser with an 11-inch tread (2x7 + 11 = 25) is a commonly cited sweet spot. Higher risers with shallower treads feel steep and tiring; lower risers with deeper treads take up more floor space per step and can feel awkwardly shallow.