Stair Calculator

Residential stair dimensions in the US are governed by the International Residential Code (IRC). The table below lists the most commonly cited limits under the current IRC. Always confirm the exact figures with your local building department, since some jurisdictions amend these numbers. These are the residential IRC figures specifically. Commercial stairs fall under the separate International Building Code (IBC), which sets somewhat different numbers, so do not apply this table to a commercial project.

The IRC does not set a strict residential minimum riser height the way it sets a maximum, since a shorter riser is not itself a safety problem. In practice, most residential stairs stay at or above 4 inches for basic comfort and cost reasons, and some commercial or accessibility-focused codes do set their own explicit minimums.

The maximum rise without a landing has changed across IRC editions. The current edition allows 151 inches, while some older editions in use in certain jurisdictions still reference 147 inches (12 ft 3 in). Check which edition your local code has adopted before relying on either figure.

When Do You Need a Landing?

The Landing Required result above checks your entered total rise against the 151 inch limit directly. If a landing is required, this calculator, like every other single-flight stair calculator, cannot size both flights and the landing together in one pass. Instead, split your total rise at the landing, run this calculator once for the flight below the landing and once for the flight above it, and plan the landing itself at least as deep as the stairway is wide, with a minimum depth of 36 inches for a straight run.

Handrails and Guards

A guard, the barrier along an open side of a stairway, is generally required once the total rise is more than 30 inches above the floor below, with a minimum height of 34 inches measured from the tread. A handrail is generally required on any flight with 4 or more risers, at a height between 34 and 38 inches. Baluster and guard spacing is typically sized so that a 4 inch diameter sphere cannot pass through, a standard aimed specifically at preventing a young child from slipping through the gap.

Once you have your number of steps, actual rise, and step run from the calculator above, marking and cutting a stringer follows the same basic process regardless of the specific numbers.

1. Select a straight 2x12 board, long enough to cover the calculated stringer length with room to spare.
2. Using a framing square set to your actual step rise and step run measurements, mark the first riser and tread lines at one end of the board.
3. Slide the square down the board and repeat the same two lines for every remaining step, keeping each mark aligned to the previous one.
4. Cut along the marked lines with a circular saw, finishing tight inside corners with a handsaw so the cuts meet cleanly without overcutting into the board.
5. Check that at least 5 inches of uncut material remains at the narrowest point of the stringer, the minimum most codes require for structural strength.
6. Use the first cut stringer as a template to trace and cut the remaining stringers, so every one matches exactly.

If you plan to add solid risers rather than leaving the stair open on the riser side, install them before adding the treads, and double check whether your design needs the top tread depth adjusted for the riser thickness at the upper connection point.

Meeting code is the minimum bar, not necessarily the most comfortable one. A widely used rule of thumb, sometimes called the 2R + T rule, says twice your step rise plus your step run should land somewhere around 24 to 25 inches, close to a natural stride length. The Comfort Check result above tests your own numbers against this same range.

In practice, many stair designers consider a step rise around 7 inches paired with a tread run around 10 to 11 inches to be an especially comfortable combination for everyday residential stairs, well inside both the code minimums and the 2R + T comfort range.

This calculator works the same way for deck stairs as it does for interior stairs, since the underlying rise, run, and stringer geometry does not change based on where the staircase sits. A couple of details are still worth planning for specifically on a deck.

Deck stairs commonly end at a landing pad at ground level, a small paved or gravel surface that keeps mud and debris off the bottom step and gives a stable, level base for the stringers to rest on. Outdoor stringers are also more exposed to moisture than an interior staircase, so pressure treated lumber and proper fasteners matter more here than they do indoors.

These related tools cover other parts of stair and lumber planning:

• Staircase Rise and Run Calculator - a focused look at rise and run alone
• Spiral Staircase Calculator - for a curved, space saving staircase design
• Stair Carpet Calculator - estimate how much carpet your finished staircase needs
• Lumber Board Feet Calculator - work out how much lumber your stringers and treads require
• Square Footage Calculator - for the floor space your staircase opening will use