Stair Calculator
Free stair calculator. Calculate riser height, tread depth, stringer length, and angle from floor-to-floor height. Validates against building code.
👆Enter floor-to-floor height to calculate riser, tread, and stringer dimensions
How to Use the Stair Calculator
Enter the total rise — the vertical distance between finished floor levels. The calculator finds all valid step counts satisfying building code, then recommends the one closest to the ideal 175mm riser. If you have a fixed horizontal space, enable the total run option and enter that measurement. The calculator derives tread depth from your run ÷ number of treads, and flags if dimensions meet code.
About This Tool
A comprehensive stair calculator that finds optimal riser height, tread depth, stringer length, and angle from a single floor-to-floor measurement. Auto mode uses Blondel’s rule (2R + T = 630mm) to derive ideal tread depth. Manual mode accepts a fixed horizontal run and derives tread from available space. Up to 5 valid solutions are shown, sorted by proximity to the ideal 175mm riser height.
Building code compliance is checked live against UK BS 5395 residential limits: risers 150–220mm, treads 220–300mm, with the comfort guideline R+T = 430–450mm. Each check shows pass, warn, or fail badges. A headroom warning triggers when the staircase angle exceeds 42°.
The stringer length is calculated as the hypotenuse of the rise/run triangle: √(rise² + run²). For a 2.7m rise with 15 risers at 180mm and 270mm treads, the stringer is 4.645m at 35.5°. Pair with the Concrete Calculator for step foundation volumes.
Quick Reference Table
| Total Rise | Typical Risers | Riser Height | Tread Depth |
|---|---|---|---|
| 2.4m | 13–14 | 171–185mm | 260–288mm |
| 2.6m | 14–15 | 173–186mm | 258–284mm |
| 2.7m | 15–16 | 169–180mm | 270–293mm |
| 3.0m | 16–17 | 176–188mm | 255–279mm |
Frequently Asked Questions
How many steps do I need for a 2.7m rise?
For a 2.7m floor-to-floor height, the most common solutions are 15 risers at 180mm each (giving a comfortable 270mm tread) or 16 risers at 168.75mm (giving 292.5mm treads). Both meet UK building code. 15 risers is usually preferred because 180mm is closer to the ideal 175mm and R+T = 450mm sits at the comfort limit.
What is the standard riser height for stairs?
The most comfortable riser height is 170–180mm. UK regulations permit 150–220mm. US IRC permits a maximum of 7¾ inches (≈197mm). Risers under 150mm waste space; risers over 220mm increase tripping risk, especially when descending.
Why does the number of risers equal the number of steps?
Each step has one riser. The number of treads (horizontal surfaces) is one fewer than risers, because the top tread is the upper floor itself. A staircase with 15 risers has 14 intermediate treads plus the landing.
What angle should a staircase be?
Most residential staircases are 30–40°. UK Building Regulations recommend a maximum of 42° for private stairs. 35–38° is the most comfortable range. Above 42°, verify headroom clearance — minimum 2.0m measured perpendicular to the stringer.
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