How to Calculate Stair Stringers

Published 2026-09-04

Divide the total rise by 7 inches and round to a whole number of risers, then divide the rise back by that count for exact riser height. Total run is the tread count times tread depth, and the stringer is the square root of rise squared plus run squared.

A stringer is the sawtooth board that carries the treads. Cutting one correctly is entirely a matter of getting four numbers right before the saw comes out — and every one of them follows from the total rise.

Step 1: measure total rise

Measure from the finished lower surface to the finished upper surface. Not the subfloor, not the bare ground — if you are pouring a landing pad or laying decking later, account for its thickness now. A quarter-inch error here is multiplied across every riser.

Step 2: riser count and height

riser count = round(total rise ÷ 7)
riser height = total rise ÷ riser count

Seven inches is the comfortable target. Rounding to a whole number is what makes every riser identical, which is both a code requirement and the thing your feet notice.

For a 96-inch rise:

1. 96 ÷ 7 = 13.7
2. Round to 14 risers
3. 96 ÷ 14 = 6.86 inches per riser

Step 3: treads and total run

There is always one fewer tread than risers — the top riser lands on the upper floor, which is not a tread.

treads = risers − 1
total run = treads × tread depth

With 14 risers and a 10.5-inch tread: 13 × 10.5 = 136.5 inches of horizontal run, about 11 feet 4 inches. This is the number that decides whether the stair fits the space you have — and the reason a steep stair is sometimes the only stair that fits.

Step 4: stringer length

The stringer is the hypotenuse of the rise and run:

stringer = √(total rise² + total run²)

√(96² + 136.5²) = √(9,216 + 18,632) = √27,848 = 166.9 inches, just under 14 feet.

Buy stock at least a foot longer. (Working out the decking or concrete for the landing at the same time saves a second trip.) The top and bottom cuts, plus the plumb cut where the stringer meets the header, eat length you cannot recover, and a 2×12 is the minimum depth that leaves enough material behind the notches.

The code limits

The International Residential Code, section R311.7 sets the boundaries the layout has to live inside (headroom comes from R305.2.2):

Local amendments exist — some jurisdictions are stricter — so check with your building department before you cut. Our stair calculator flags a layout that falls outside these limits as you type.

The ⅜-inch tolerance is why the rounding step matters. Split a 96-inch rise into 13 risers instead of 14 and each is 7.38 inches — still legal, but noticeably steeper, and you lose a tread’s worth of run.

Reduce the top riser by the tread thickness

The classic stringer mistake: cut all 14 notches identically, then nail on 1-inch treads, and the bottom riser is now 1 inch taller than the rest — outside tolerance and a genuine trip hazard. Drop the bottom of the stringer by exactly the tread thickness before you cut, or plan for the landing pad to make up the difference.