Building stairs follows strict rules based on years of carpentry experience and human ergonomics. Here is a quick overview of the most important terms you should know before cutting:
Ideal Step (Blondel's Rule)
The basic rule for comfortable stairs is: 2 × riser height + stringer going = approximately 630 mm, or about 600–650 mm. The calculator uses Blondel’s rule to calculate the ergonomic layout.
Riser Height
For comfortable interior stairs, a height of 170–180 mm (approx. 7") is recommended. Above 200 mm, stairs become very steep and less safe.
Tread Depth
The net horizontal area for the foot. The practical ideal range is 270–300 mm (approx. 10.6–11.8") to safely accommodate a whole foot.
Two Tread Design Modes
Automatic mode designs the depth from the rise, ergonomics, and available space. Fixed Depth mode preserves the physical tread board size you already have or need to use.
Tread Board vs. Stringer Going
The overall physical board depth includes the nosing. The actual horizontal stringer going therefore equals the overall board depth minus the nosing.
Available Stair Run
In Fixed Depth mode, the calculator checks whether all steps fit within the entered space. If they do not, it keeps your board depth unchanged and shows the actual required run.
Stringer Throat
The narrowest part left after cutting notches. Carpenters recommend keeping at least 115–120 mm (4.5") of solid wood for standard loads.
First Step Cut (Dropping)
The bottom cut of the stringer must be shortened by exactly the tread thickness. Otherwise, the first step would be higher than the rest.
Tread Nosing
Increases the total stepping area. Usually 20–30 mm. Helps significantly when walking down by providing more heel space.
Sawn vs. Housed Stringers
Calculator dimensions are universal. Use them for cutting teeth on sawn stringers or as precise guides for routing housed stringer grooves.
Headroom
Don't forget your head! The vertical distance between the step edge and the ceiling above should be at least 2100 mm (83").
Top Mounting
Stringers can end "dropped" (floor above acts as the top step) or "flush" (stringer is level with the floor and a tread is mounted on top).
⚠️ Disclaimer: This calculator is a geometric tool for carpenters and DIYers. It does NOT perform structural engineering. Always consult a professional engineer regarding load-bearing capacity.
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