How is the number of risers determined?

Divide the total rise by your target riser height and round to the nearest whole number, since every riser must be equal. The actual riser height is then the total rise divided by that whole number of risers.

Why is there one fewer tread than risers?

The top riser lands on the upper floor, which is not a tread, so a flight always has one more riser than treads. A stair with 14 risers has 13 treads, and the total run is 13 times the tread depth.

How is the stringer length calculated?

The stringer is the diagonal of the triangle formed by total rise and total run: length equals the square root of (total rise squared plus total run squared). This is the cut length of the stringer board before notching.

What are the IBC rise and run limits?

The International Building Code caps riser height at 7-3/4 inches and requires a minimum tread depth of 10 inches for most occupancies. Many builders also apply the comfort rule that twice the rise plus the run falls between about 24 and 25 inches.

Does this work in metric?

Yes. Switch units and the same geometry applies with millimetres. Common residential limits abroad differ, so the IBC checks shown are a guide; confirm against your local building regulations.

What is the Stair Stringer Calculator?

From a total rise height, compute the number of risers, the exact riser height, the tread depth, total horizontal run, and the diagonal stringer length and angle. Checks the bar against common IBC limits: 7-3/4 in max rise, 10 in min run, and the rise plus run rule. It runs free in your browser on Gera Tools, with nothing uploaded.

Stair Stringer Calculator

Name: Stair Stringer Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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A safe, comfortable stair depends on equal risers and a consistent tread depth within code limits. This calculator takes your total floor-to-floor height, finds the riser count and exact dimensions, then gives the diagonal stringer length and the stair angle, flagging anything outside common IBC limits.

How it works

The total rise is divided into equal risers, and the treads follow:

riser count = round(total rise / target riser)
riser height = total rise / riser count
tread count  = riser count − 1
total run    = tread count × tread depth
stringer len = √(total rise² + total run²)
angle        = arctan(total rise / total run)

Code checks compare the riser height against 7-3/4 in max, the tread depth against 10 in min, and twice the rise plus the run against the 24 to 25 in comfort band.

Worked example

A total rise of 108 in (9 feet, floor to floor) with a target 7.5 in riser:

riser count  = round(108 / 7.5) = round(14.4) = 14
riser height = 108 / 14 = 7.71 in
tread count  = 14 − 1 = 13
total run    = 13 × 10.5 = 136.5 in
stringer len = √(108² + 136.5²) = √(11,664 + 18,632) = √30,296 ≈ 174 in
angle        = arctan(108 / 136.5) ≈ 38.4°

At 7.71 in, each riser passes the IBC 7-3/4 in maximum. The tread depth of 10.5 in clears the 10 in minimum. The comfort check: (2 × 7.71) + 10.5 = 25.92 in, which sits at the upper edge of the 24–25 in comfort band — acceptable but on the steep side. Increasing tread depth to 11 in would bring it to 26.42 in and give a more comfortable climb.

Why equal risers matter

The single most important rule in stair layout is that every riser must be the same height. Code allows a maximum variation of 3/8 in between the tallest and shortest riser in a flight. When risers vary, the foot expects each step at a learned height and a single inconsistent riser causes trips — the most common stair-related fall. This is why the calculator divides the total rise by a whole number of risers rather than accepting whatever dimension happens to reach the floor.

The bottom riser reduction

When treads sit on top of the stringers (rather than dadoed into them), the bottom riser must be shortened by exactly one tread thickness. This is because the tread material adds height to every step except the first, which rises from the finished floor. Forgetting this cut creates an out-of-specification low bottom riser that code inspectors will flag.

For example: if your treads are 1 in thick, the bottom riser is cut to (riser height − 1 in). The stringer layout is marked with the full riser height; only the actual cut riser board is trimmed.

Stringer size and notch rules

The calculated stringer length is the diagonal measurement before any notching. The IBC requires a minimum net cross-section after notching: the remaining depth of the stringer above the notch must be at least 3-1/2 in for a nominal 2×12 stringer. Wide, shallow stairs with many risers can require deeper lumber or steel stringer support underneath.

Always lay out from the actual measured total rise on the day of installation. Floors are rarely exactly as shown on the architectural drawings, and a discrepancy of even 1/4 in in the total rise will divide unevenly across 14 risers unless you recalculate.