Stair Calculator

Frequently Asked Questions

How many steps for a total rise of 108 inches?

At 7.5 in desired riser: 108/7.5 = 14.4, round to 14 risers and 13 treads. Actual riser = 108/14 = 7.71 in. Within IRC 7.75 in maximum.

What is the maximum riser height per IRC code?

7.75 inches maximum per IRC R311.7. Minimum tread depth is 10 inches. The typical comfortable target is 7 to 7.5 inches.

How do I calculate stringer length?

Stringer length = sqrt(total_rise^2 + total_run^2). For 108 in rise with 14 risers and 13 treads at 10 in (130 in run): sqrt(108^2 + 130^2) = sqrt(11664 + 16900) = sqrt(28564) = 169.0 in (14.1 ft). Buy 16 ft stringers.

What is the rule of 17 for stairs?

A traditional rule: riser + tread = 17 to 18 inches. So with a 7-in riser, use a 10-11 in tread. Another rule: 2 x riser + tread = 24-25 inches.

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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

The calculator divides total rise by the desired riser height and rounds to the nearest whole number (never below 1) to get the riser count. Actual riser height is total rise divided by that count. A straight flight always has one fewer tread than risers - the top riser lands on the upper floor - so total run is (number of risers - 1) multiplied by tread depth. Stringer length uses the Pythagorean theorem: sqrt(rise^2 + run^2).

Formula: num_risers = round(total_rise / desired_riser). actual_riser = total_rise / num_risers. num_treads = num_risers - 1. total_run = num_treads x tread_depth. stringer = sqrt(total_rise^2 + total_run^2).

IRC Stair Code Requirements

The International Residential Code (IRC R311.7) sets minimum standards for residential stairs:

Maximum riser height: 7.75 inches

Minimum riser height: 4 inches (most jurisdictions)

Minimum tread depth: 10 inches (nosing not included)

Minimum stair width: 36 inches clear of handrails

Maximum variance between risers: 3/8 inch

Handrail required on one side when 4 or more risers

The calculator flags when the actual riser height falls outside the 6.75 to 7.75 inch comfort range commonly targeted by builders.

Stringer Cutting Notes

The stringer calculation gives the diagonal length before cuts. For a cut stringer (traditional sawtooth), you need boards at least 2x12 lumber. After cutting the stair profile the remaining depth (the run depth or throat) must be at least 3.5 inches per IRC to maintain structural integrity.

Add 10-15% to the calculated stringer length when ordering lumber to allow for the plumb cut at the top and the level cut at the bottom of the stringer.

FAQ

What is a comfortable riser height for stairs?

Most people find 7 to 7.5 inches the most comfortable. Steeper than 7.75 in is tiring; shallower than 6 in requires awkward short steps. The "rule of comfort" is: 2 x rise + run = 24-25 inches. With a 7-in riser, aim for a 10-11 in tread.

How many steps for a 9-foot ceiling?

A 9-ft ceiling with a 12-in floor structure gives a total rise of about 120 in. At 7 in per riser: 120/7 = 17.1, round to 17 risers. Actual riser = 120/17 = 7.06 in. 16 treads (one fewer than risers) at 10 in = 160 in total run (13 ft 4 in).

What is the difference between risers and treads?

Risers are the vertical faces; treads are the horizontal surfaces you step on. A staircase always has one more riser than tread because the top riser leads to the upper floor level which is the last tread.

How long should my stair stringers be?

Use this calculator's stringer length result and add a few inches for waste (the plumb cut at the top and level cut at the bottom). Standard 2x12 lumber is available in 10, 12, 14, and 16-ft lengths - pick the next size above your calculated length.

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