Stair Stringer Calculator

A flight is a total rise split into equal steps, so careful measuring matters more than the arithmetic. If the stairs come off a deck, the deck step calculator is the page for that job.

How to use this calculator

1. Measure the total rise from the finished floor at the bottom to the finished floor at the top. Carpet or tile that goes down later adds height, so measure with it in place or add its thickness.
2. Enter the rise in the unit you measured in, and measure again before you type it, because every cut depends on that figure.
3. Set the maximum riser height. The starting value is the code limit of 7.75 inches. A lower value gives a gentler climb with more steps.
4. Enter the tread depth, measured from the front edge of one step to the front edge of the next. The starting value is the code minimum of 10 inches, and a deeper tread gives more foot room.
5. Read the riser count, riser height and stringer length, then lay out the stringer as described below. Cut a test riser from scrap before you cut the full set.
6. Use the stringer length with the board feet calculator to work out the lumber.

Worked examples

Total run for common floor-to-floor heights and tread depths

Each cell: Total run (in). Inputs not in the table use the calculator's starting values.

How the calculation works

The calculator divides the total rise by your maximum riser height and rounds up to a whole number of risers. That count is the fewest equal risers that keep each one at or under your limit, so the riser height never comes out above the maximum.

The top riser climbs straight onto the upper floor, so only the treads between risers count. Total run is that tread count times the tread depth. Stringer length is the diagonal across the total rise and the total run, and stair angle is the slope of one step, the riser height over the tread depth.

The calculator does not size lumber or flag code violations, so treat its output as an arithmetic check.

Code limits to check

The limits below come from the International Residential Code (IRC). Local codes can be stricter or differ in other ways, and your building department decides what passes inspection.

• Maximum riser height: 7.75 inches.
• Minimum tread depth: 10 inches.
• Largest difference allowed between risers, or between treads, in a flight: 0.375 inch.
• Minimum stair width: 36 inches.
• Minimum headroom: 80 inches.
• Handrail height above the nosings: 34 to 38 inches.
• Handrail threshold: 4 risers or more, with a handrail on at least one side.
• Minimum landing depth for a straight run: 36 inches.
• Minimum guard height on the open side of the stairs: 34 inches.

Laying out the stringer with a framing square

A stringer is the sloped board that carries the treads and risers. Set a framing square so the riser height runs along one leg and the tread depth along the other, mark both, then slide the square down the board and repeat for each step.

Check the top and bottom steps against the real floors before you cut the rest, since small errors are hardest to fix there. If the open side needs balusters, the spindle spacing calculator covers their spacing.

Mistakes to avoid

• Measuring to the subfloor. The top step ends up short by the thickness of the finish you add later.
• Cutting each stringer from its own measurements instead of copying one layout.
• Treating the result as final. Check every riser, tread and rail figure against the limits above.
• Ignoring the landing and the width. Check the floor space at the top and bottom before you commit to a layout.

Adjusting the bottom step for tread thickness

Treads sit on top of the stringer, so the first step is taller than the rest by the thickness of a tread. Cut that thickness off the bottom of the stringer so the first riser matches the others once the tread is on. Check this against the real floor before you cut the other stringers.

How to cut a stair stringer

Cut along your marks with a circular saw, and stop at each inside corner. Finish the corners with a handsaw so you do not cut past the line and weaken the board. Use the first stringer as a pattern and trace it onto the others, so every stringer matches.

When do stairs need a landing?

A landing is a flat platform that breaks up a tall flight. Under the IRC, one flight can rise no more than 151 inches between floors or landings, and a straight landing must be at least 36 inches deep. For decks, the AWC deck guide asks for an intermediate landing once the stairs climb past 144 inches.

Common questions