What is the 27 rule for stairs?

The “27 rule” is not a standard building code term, so it may refer to a common stair-proportion rule of thumb. The most common guideline is that two risers plus one tread should equal about 24 to 25 inches, which keeps steps comfortable to climb.

Stair comfort depends on the balance between riser height and tread depth. Several related guidelines are widely used:

• Riser plus tread should equal about 17 to 18 inches

• Two risers plus one tread should equal about 24 to 25 inches

• Residential codes commonly cap riser height at about 7.75 inches and require treads of at least 10 inches

Our guide to stair riser height and stair tread depth standards explains these measurements in detail. Fabricators use them when planning a build, as shown in our how to build a metal staircase overview.

For Utah projects, confirm the code version your city has adopted, since requirements vary. A fabricator can design a metal staircase that meets local code and still feels comfortable to use.