Stair Stringer Calculator

Understanding IBC stair code

The International Building Code (IBC) sets clear limits for residential stair dimensions. The maximum rise per step is 7¾ inches (7.75"), and the minimum tread depth is 10 inches. These limits ensure stairs are safe and comfortable for everyday use.

A practical guideline is the comfort rule: the sum of one rise plus one tread should fall between 17 and 18 inches. A staircase within this range feels natural to walk up and down. For example, a 7.5" rise with a 10.5" tread gives a sum of 18" — right at the top of the comfort zone.

This calculator automatically selects a step count that keeps the rise per step within the IBC maximum. If your tread depth is below the 10" minimum, or the comfort rule is violated, you'll see a warning.

How to calculate stair stringers

Here's the step-by-step process, using a 108-inch (9-foot) floor-to-floor rise with a 10.5-inch tread depth:

1. Determine step count. Divide total rise by the target rise of 7.5 inches: 108 / 7.5 = 14.4, which rounds to 14 steps. Verify that the actual rise (108 / 14 = 7.71") does not exceed the 7.75" IBC maximum.
2. Calculate rise per step. Divide total rise by step count: 108 / 14 = 7.714 inches per step. This is the measurement you'll mark on your framing square.
3. Calculate total run. Multiply tread depth by (steps − 1): 10.5 × 13 = 136.5 inches. There's one fewer tread than risers because the top step lands on the upper floor.
4. Calculate stringer length. Use the Pythagorean theorem: √(108² + 136.5²) = √(11664 + 18632.25) = √30296.25 ≈ 174.1 inches (about 14.5 feet).
5. Determine cut angle. The angle from horizontal is arctan(rise/run) = arctan(7.714/10.5) ≈ 36.3°. This is the angle your stringer makes with the floor.

The calculator above handles all this math automatically. Enter your total rise and tread depth, and it returns every measurement you need to lay out and cut your stringers.

Cutting your stringers

Once you have your rise and run measurements, follow these steps to lay out and cut the stringer:

1. Set up your framing square. Clamp stair gauges to the square at the rise measurement on the tongue (short side) and the run measurement on the blade (long side). These gauges ensure every notch is identical.
2. Mark the first step. Place the square on the stringer board with both gauges against the top edge. Draw a line along both the tongue and blade — this marks one rise and one run.
3. Walk the square down the board. Slide the square until the blade gauge aligns with the end of the previous rise line. Mark again. Repeat for all steps.
4. Adjust for the bottom riser. The bottom riser needs to be shortened by one tread thickness (typically ¾" for standard decking or 1" for hardwood treads). This accounts for the tread that sits on top of the first step. Measure and mark the reduction from the bottom cut line.
5. Cut the notches. Use a circular saw to cut along the rise and run lines, stopping just short of the inside corner. Finish the inside corners with a handsaw or jigsaw to avoid overcutting. Overcutting weakens the stringer.
6. Check remaining material. After cutting, the narrowest part of the stringer (the "throat") must be at least 3.5 inches wide for a 2×12. This ensures structural integrity.

Always cut one stringer first and test-fit it before cutting the rest. Use it as a template for consistent results.