Staircase calculator

The engine picks the smallest riser count whose equal risers all sit at or under the maximum, keeps every riser identical, and treats the upper floor as the last tread, so treads are one fewer than risers. The comfort figure 2 × riser + going lands between 550 and 700 mm on a stair people actually want to walk; Approved Document K requires that band on private stairs, IRC does not use it. Code limits here are the published headline numbers (IRC R311.7 and AD K, private stairs), not a substitute for the documents or for local amendments: landings, guards, handrails, winders and site conditions are outside this tool.