What Is the Curved Calculator?

Design elegant curved and helical staircases with our free calculator. A curved stair sweeps through a continuous arc instead of turning at landings, which makes it the showpiece of an entry but also one of the hardest stairs to build right. The geometry that matters is the walk line, the path a person actually takes, which runs about 12 inches in from the narrow edge of the treads. Code treats curved treads like winders: the tread depth at the walk line has to meet the minimum (10 inches under IRC winder rules), and the tread gets progressively wider toward the outer edge and narrower toward the inner edge. This calculator takes the inner and outer radii, total rise, and tread count, then works out arc lengths along the inner edge, the walk line, and the outer edge, plus the tread depths at each so you can see before fabrication whether the inner steps are usable. Two things make curved stairs special beyond the math: the handrail has to follow the same radius as the stair, which means a curved rail is shop-bent to match, and the whole stair is typically shop-fabricated as a unit, so a measurement error discovered on site is expensive. Measure the space, run the numbers here, and hand the fabricator a spec sheet that already works.

1. Measure and enter the inner radius and outer radius of the planned stair. The inner radius sets how tight the curve feels; the outer radius minus the inner radius is the tread width.
2. Enter the total rise and the number of treads. The calculator divides rise into equal risers, each within the 7-3/4 inch IRC maximum.
3. Review the arc lengths at three lines: inner edge, walk line, and outer edge. The walk-line arc length divided by the number of treads gives the tread depth where people actually step.
4. Check the walk-line tread depth against the 10-inch minimum. If it fails, the fix is a larger inner radius, more treads, or both, not wishful measuring.
5. Check headroom along the inner curve, where the arc is shortest and ceilings close in fastest. The IRC minimum is 6 feet 8 inches of clear headroom.