Stair Calculator

How to use

1. 1Set the units toggle to Imperial or Metric to match the tape measure you are working with.
2. 2Enter the Total rise: the vertical distance from the lower finished floor (or grade) to the upper finished floor.
3. 3Choose Auto from target riser to pick a comfortable riser height, or Fixed step count when you already know how many steps fit the opening.
4. 4Pick the Blondel comfort rule for an automatic tread depth, or switch to Pick the depth to enter a tread depth you already own.
5. 5Optionally enter the Ceiling clearance above the stair so the headroom check runs against the IRC minimum.
6. 6Read the geometry panel for the riser height, tread depth, total run, stringer length, and stair angle, then click Copy summary to share the result.

About this tool

Stair Calculator turns a known total rise (the floor-to-floor or grade-to-deck height) into a complete plan for a straight run of stairs: the number of risers and treads, the exact riser height and tread depth, the total horizontal run, the diagonal stringer length, and the stair angle. Two solver modes cover the most common workflows. Auto from target riser lets you pick a comfortable riser height (default 7 inches or 18 centimeters), then divides the total rise by that height, rounds to the nearest whole number of risers, and back-solves the precise riser needed to land flush on the upper floor. Fixed step count lets you supply the number of risers you already plan to build and reports the resulting riser height and tread. Tread depth can be set directly to match stock lumber, or derived automatically from the classical Blondel comfort rule 2R + T = 24.6 in (63 cm), which produces a stair that walks naturally at any speed. The stringer length is computed exactly as sqrt(rise^2 + run^2) so a carpenter can mark and cut the diagonal in one pass, and the stair angle is reported in degrees so it can be compared against the residential maximum of about 37.76 degrees. Every result is checked live against the International Residential Code R311.7.5 limits used in most US jurisdictions: maximum riser 7.75 in (19.7 cm), minimum tread 10 in (25.4 cm), and minimum headroom 6 ft 8 in (203 cm) when a ceiling clearance is supplied. Code issues appear in red and comfort warnings in amber, so a deck plan that needs one more step or a slightly deeper tread is flagged before any wood is cut. Quick presets load common scenarios: a low backyard deck, a high above-grade deck, a residential floor-to-floor of 8 ft 6 in, a steep basement stair near code limits, and a 2.7 m metric floor-to-floor. Inputs accept inches as a plain number, feet-inch forms such as 8 ft 6 in, simple fractions like 1/2, mixed numbers like 1 1/2, comma decimals for European keyboards, and metric centimeters when the unit toggle is set to metric. Useful for deck builders sizing a stringer before buying lumber, homeowners planning a porch or basement stair, contractors laying out an interior staircase, DIYers checking that a new step run will pass inspection, and anyone double-checking a finished stair against the residential code. Everything runs in your browser; the numbers you enter here are never uploaded.

Free to use. Works in your browser. No signup, no login.