Designing a spiral staircase is mostly arithmetic. With four measurements — floor-to-floor height, opening (hole) size, diameter and desired riser — you can work out everything else. Here is the whole process, and the exact formulas our free calculator uses.
Step 1 — Measure the total height
Measure from the finished floor of the lower level to the finished floor above (floor-to-floor height, H). This is the total vertical distance the stairs must climb.
Step 2 — Choose a target riser
A comfortable riser (vertical height of one step) is usually between 15 and 19 cm. Pick a starting value, for example 18 cm.
Step 3 — Find the number of steps
Example: for a 280 cm floor-to-floor height and a 18 cm target riser, 280 ÷ 18 ≈ 15.6, so you need 16 steps and the real riser is 280 ÷ 16 = 17.5 cm — right in the comfortable range.
Step 4 — Choose the diameter and useful tread
Pick a diameter that fits your opening with room to spare. The useful tread is the step depth measured along the walking line, not at the outer edge. For spiral stairs a walking-line depth between 26 and 30 cm feels comfortable.
Step 5 — Check the angle per step and headroom
Divide the full turn by the number of steps to get the angle per step (the calculator shows this instantly). Finally, check headroom: the clearance above your head as you climb, which must stay above roughly 200 cm at every point of the turn.
Total angle ÷ steps per turn = your step angle. Keep 9–20 steps per full 360° turn for a comfortable climb.
The safest way to do all this is to let the geometry do the maths for you: adjust the sliders in the live calculator below and watch the riser, tread, opening, headroom and safety status update in real time. Then read Blondel's rule for comfortable proportions, the safety checklist and our guide to compact stairs for small spaces.
