Free Stair Calculator – Riser, Tread & Blondel Compliance

Stair Calculator Guide

The Stair Calculator turns a single total-rise measurement into a complete, comfortable stair layout: riser height, tread depth, step count, and total run. Measure the total rise as the exact vertical distance from the finished lower floor to the finished upper floor — include the thickness of both finished surfaces, because a measurement taken to the subfloor will throw off every riser. Pick a style — indoor, outdoor, or deck — so the tool applies the right comfort and code targets, since exterior and deck stairs tolerate shallower risers and deeper treads than a tight interior flight. The calculator divides total rise into equal risers (risers must be uniform; building codes limit the variation between the tallest and shortest riser in a flight) and pairs each riser with a tread depth that keeps the stair comfortable to climb. It then flags any geometry that falls outside accepted riser/tread limits so you can adjust the step count before cutting a single stringer.

Step count = round(total rise ÷ target riser). Riser height = total rise ÷ step count. Tread depth follows a comfort rule — Blondel's relation, 2 × riser + tread ≈ 25 inches (about 63–64 cm), so tread ≈ 25 − 2 × riser. Total run = (step count − 1) × tread depth (the top riser lands on the floor, so there is one fewer tread than riser). Worked example: total rise 109 inches. At a 7.5-inch target, 109 ÷ 7.5 = 14.5 → 15 risers. Riser = 109 ÷ 15 = 7.27 inches. Tread = 25 − 2 × 7.27 = 10.46 inches, comfortably above the 10-inch minimum. Total run = (15 − 1) × 10.46 = 146.4 inches ≈ 12.2 ft. Check: 2 × 7.27 + 10.46 = 25.0, ideal. Stringer length ≈ √(rise² + run²) per flight = √(109² + 146.4²) ≈ √(11,881 + 21,433) ≈ 182.5 inches of sloped material before cuts and waste.

The dominant error is uneven risers from mismeasuring total rise or ignoring finished-floor thickness; codes are strict here because a single odd step is a trip hazard. The IRC for residential work caps riser height at 7-3/4 inches and requires at least a 10-inch tread depth, with maximum riser-to-riser and tread-to-tread variation of 3/8 inch within a flight; the IBC for commercial stairs is tighter (commonly 7-inch max riser, 11-inch min tread). Always confirm local amendments, which can override the model code. Provide required headroom (IRC minimum 6 ft 8 inches measured vertically from the tread nosing) and a graspable handrail at 34–38 inches. For cut stringers, lay out rise and run with a framing square on dimensional lumber, keep the throat (remaining material behind the notch) at least about 5 inches for strength, and add an intermediate stringer for stairs wider than 36 inches. Round toward more steps if a riser exceeds the limit rather than stretching tread depth.