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Free stair calculator

Stair Calculator

Enter the floor-to-floor height to get riser height, steps and stringer length.

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Stringer layout

To scale. Marks along the top edge of the board, from the bottom.

StepHeight from floorRun from start

How to use the stair calculator

  1. Measure the total rise: from the finished lower floor to the finished upper floor, in inches (or type 9' or 2.7 m).
  2. Set the tread depth you want — 10" is the IRC minimum, 10 1/2"–11" is more comfortable.
  3. Read the riser height and step count, check the code notes, and use the layout table to mark the stringer.

The stair formulas

Risers = total rise ÷ max riser, rounded up

Riser height = total rise ÷ risers · Treads = risers − 1

Stringer = √(total rise² + total run²)

Example: a 108" rise needs 14 risers of 7 11/16". With 10" treads the 13 treads make a total run of 130", and the stringer is √(108² + 130²) ≈ 169" long, at an angle of about 37.7°.

Laying out a stringer

Set a framing square on the board with the riser height on the tongue and the tread depth on the blade (stair gauges help), mark each step, and slide along. Drop the stringer at the bottom by the tread thickness and check the first and last risers against the table. Stringers should be no more than 16" apart for 5/4 treads (12" is common for composite).

Related calculators

Building a deck with steps? Size it with the deck calculator. Railings need baluster spacing under 4".

Frequently asked questions

How do I calculate the number of stairs?

Measure the total rise from finished floor to finished floor, divide it by the tallest riser you allow (7 3/4" under the IRC) and round up. That is the number of risers; there is one tread fewer. A 108" rise gives 108 ÷ 7.75 = 13.9, so 14 risers of 7 11/16" and 13 treads.

What is the code for stair rise and run?

The International Residential Code (R311.7.5) allows risers up to 7 3/4" and requires treads of at least 10" (measured nosing to nosing), with no more than 3/8" difference between the tallest and shortest riser or deepest and shallowest tread in a flight. Headroom must be at least 6' 8". Local codes can be stricter.

How long does a stair stringer need to be?

The stringer runs along the hypotenuse of the total rise and total run: √(rise² + run²). For 108" of rise and 130" of run that is about 169", so buy a 16 ft board to leave room for the top and bottom cuts.

What is a comfortable riser and tread?

A common rule is 2 × riser + tread = 24–25". A 7 1/2" riser with a 10" tread gives 25"; a 7" riser with an 11" tread also gives 25". Stairs that fall outside this range feel either steep or oddly shallow.

How much wood should be left under the stringer notches?

At least 3 1/2", and 5" is better, measured square from the inside corner of each notch to the bottom edge of the board. A 2×12 usually leaves enough for standard stairs; a 2×10 often does not.

Why is the bottom riser shorter?

Because the tread material sits on top of the stringer. Cut the bottom of the stringer down by the thickness of one tread (called dropping the stringer) so the first step comes out the same height as the rest.