Stair Calculator With Detailed Plans

A stair calculator with detailed plans works out the number of treads, riser height, total run, and stringer length for a straight staircase, and it can also generate a cut list for the stringers. You enter the total rise (floor-to-floor height), the desired riser height or number of steps, and the tread depth (going), and it returns the exact dimensions you need to build a code-compliant stair. The calculator below uses the standard building-code formula (2 × riser + tread = 24–25 inches) to give you a plan you can transfer directly to your lumber.

Formula

The core stair formula is a comfort and safety guideline used by most building codes (e.g., IRC R311.7):

2 × R + T = 24 to 25 inches

• R = riser height (vertical distance between two treads), in inches
• T = tread depth (horizontal depth of each step, not including the nosing), in inches

For a straight stair, the number of risers is the total rise divided by the chosen riser height, rounded up to the next whole number. The actual riser height is then the total rise divided by that number. The total run is the tread depth multiplied by (number of risers − 1), because the top tread is at the upper floor level. The stringer length is the hypotenuse of a right triangle with legs equal to total run and total rise.

Worked Example

Using the default values: total rise = 105 in, desired riser height = 7.5 in, tread depth = 10 in.

1. Number of risers = ceil(105 / 7.5) = ceil(14.0) = 14
2. Actual riser height = 105 / 14 = 7.5 in
3. Total run = (14 − 1) × 10 = 130 in
4. Stringer length = √(130² + 105²) = √(16900 + 11025) = √27925 ≈ 167.1 in (13 ft 11 in)
5. Step rule check: 2 × 7.5 + 10 = 25 in (within the 24–25 range)

The calculator shows the same numbers: 14 risers, 7.5 in riser, 130 in run, 167.1 in stringer, and a step rule of 25 in.

How to Use the Stair Calculator

Start by measuring the total rise from the finished floor of the lower level to the finished floor of the upper level. If you’re building a deck, that’s from the ground or concrete pad to the deck surface. Then choose a desired riser height—most codes cap it at 7.75 in (IRC) or 8.25 in (some local codes), and a comfortable range is 7 to 7.75 in. The tread depth should be at least 10 in (IRC) or 11 in (some jurisdictions).

The calculator gives you:

• Number of risers – the number of vertical steps (including the top step).
• Actual riser height – the exact rise per step after rounding.
• Total run – the horizontal length the stair occupies, useful for layout and for checking headroom.
• Stringer length – the length of the diagonal board you need to cut (add extra for overhang or notches).
• Step rule check – confirms your riser/tread combo is within the comfort zone.

How to Read the Results and Build the Stringers

The stringer length is the hypotenuse of the triangle, but you don’t cut the board at that length; you lay out the steps along the board. Use a framing square with stair gauges: place the square so that the riser height is on one leg and the tread depth on the other, then mark the notches along the board. The first notch starts at the bottom, and you subtract the tread thickness from the bottom riser if you’re using a separate tread board.

For a 2×12 stringer, the notch depth (the vertical cut) is typically the riser height minus the tread thickness. The horizontal cut is the tread depth. The stringer length shown is the diagonal distance between the bottom of the first riser and the top of the last riser—not the total board length, which should be longer to allow for the top and bottom cuts.

Common Mistakes and How to Avoid Them

• Forgetting the top riser: The total rise includes the step up to the upper floor. Many people count one fewer riser and end up with a short top step.
• Ignoring tread thickness: If you add a 1-in tread on top of the stringer, the first riser must be shortened by that thickness, or the top step becomes too high.
• Using the wrong units: Mixing inches and feet is the most common error. Convert everything to inches before calculating.
• Not accounting for nosing: Tread depth is measured from the face of one riser to the face of the next, not including the overhang. A 1-in nosing does not count toward the minimum tread depth.
• Rounding too early: Always round the number of risers up, then divide to get the actual riser height. Rounding the riser height first can give you a stair that’s off by a fraction of an inch.

Building Code Basics (USA)

Most residential codes (IRC) require:

• Maximum riser height: 7.75 in
• Minimum tread depth: 10 in (measured from nosing to nosing)
• Minimum stair width: 36 in
• Maximum variation between risers or treads: 3/16 in
• Handrails on at least one side if there are 4 or more risers
• Headroom of at least 6 ft 8 in above the stair

Local codes may differ, especially for decks or commercial buildings. Always check with your local building department before cutting lumber.

Limitations of the Calculator

This calculator works for straight, single-flight stairs with uniform risers and treads. It does not handle landings, winders, spiral stairs, or multiple flights. It also assumes a simple 90-degree stringer cut—if you’re using a different stringer type (e.g., solid or steel), the dimensions change. The step rule (2R+T) is a guideline, not a guarantee of comfort; very long or very short stairs may need adjustment.

Frequently Asked Questions

What is the ideal riser height and tread depth?

Most building codes allow risers up to 7.75 in and treads as shallow as 10 in, but the most comfortable combination for most people is a 7-in riser with an 11-in tread. The 2R+T rule says the sum should be between 24 and 25 in, so 7+7+11 = 25 in works well.

How do I measure the total rise accurately?

Measure from the finished floor surface of the lower level to the finished floor surface of the upper level. If you have carpet, tile, or hardwood, include the thickness of the flooring. Use a laser level or a straight board and a spirit level to get a vertical measurement.

How long should my stringer board be?

The stringer length from the calculator is the diagonal from the bottom of the first riser to the top of the last riser. For a 2×12 stringer, add at least 12 in to allow for the top and bottom cuts and any overhang. It’s safer to buy a 16-ft board for most residential stairs.

Can I use this calculator for exterior deck stairs?

Yes, the math is the same, but outdoor stairs often use pressure-treated lumber and may have different code requirements (e.g., 36-in guardrails, slip-resistant treads). Also, the total rise is measured from the ground to the deck surface, and the ground may not be level—you’ll need to adjust the bottom step.

Why does the calculator show 14 risers when I have 105 inches?

Because 105 divided by 7.5 is exactly 14. If your total rise doesn’t divide evenly, the calculator rounds up to the next whole number, then adjusts the riser height slightly. For example, a 106-in rise with 7.5-in desired risers gives 15 risers (ceil(14.13) = 15) and an actual riser of 7.07 in.

What if my stair has a landing?

Treat each flight as a separate straight stair. Measure the rise from the lower floor to the landing, and then from the landing to the upper floor. The calculator works for each flight individually, but you must also account for the landing depth (usually at least the width of the stair).

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in total_rise | Total rise (floor to floor) | in | 105 | 0
in riser_h | Desired riser height | in | 7.5 | 1
in tread_depth | Tread depth (going) | in | 10 | 1
out num_risers | Number of risers | - | ceil(total_rise / riser_h) | 0
out actual_riser | Actual riser height | in | total_rise / num_risers | 2
out total_run | Total run (horizontal) | in | (num_risers - 1) * tread_depth | 2
out stringer_len | Stringer length (hypotenuse) | in | sqrt((total_run)^2 + (total_rise)^2) | 2
out step_rule | 2R+T check | in | 2 * actual_riser + tread_depth | 2