Stair calculator
Plan a comfortable deck staircase. Calculate step count, step dimensions, and stringer length.
Note: These results are a planning aid to help you get started. Step dimensions like rise, run, and headroom vary by local building code, so please double-check requirements in your area before you buy materials or start construction.
How to calculate stairs
- Enter your stair height: the vertical distance from the ground to the top of the deck surface. This is the key measurement.
- Set the stair width, and check whether your stairs need to fit into a fixed distance between two points.
- Stair calculations have several settings under Advanced options worth looking at, including tread board dimensions, waste factor, and whether the stairs need closed risers.
Advanced options
Number of stringers sets how many diagonal supports carry the steps across the stair width. A good starting point is one stringer every 400 to 500 mm. Wider stairs or heavier loads need more.
Board width and board length let you pick the tread board dimensions, either from the common sizes dropdown or as custom values.
Board thickness is included in the stringer length calculation, so the stair top lines up with your deck framing. Standard 5/4 decking is 25 mm (1 inch).
Board gap is the spacing between tread boards on each step. Drainage gaps work the same way here as on the main deck surface.
Waste factor adds extra boards and screws to the estimate. 10% is a sensible starting point.
Closed risers adds riser boards behind each step, using the same board dimensions as the treads. Open risers leave that gap empty.
Step height and step depth
Each step has two dimensions. The step height (rise) is how far you lift your foot vertically. The step depth (tread) is how much horizontal space you have to land on. A useful rule of thumb: step height plus step depth should add up to roughly 430 to 460 mm (17 to 18 inches). That ratio produces a natural walking rhythm. Most residential codes set a minimum step depth of 250 mm (10 inches), but always verify with your local requirements before you build.
Stringer length
The stringers are the diagonal supports that carry the weight of the steps. The calculator measures stringer length as the diagonal from the top of the stair height to the far end of the total horizontal run, which is the length you see on a drawing. Buy slightly longer stock to account for the plumb cut at the top and the level cut at the bottom.
The calculation includes your tread board thickness in the rise, since the stringer needs to reach from the ground up to the deck framing, not just to the top of the last tread.
Stair angle
A staircase that feels natural to climb usually falls between 30 and 38 degrees. Steeper than 40 starts to feel like a ladder, and shallower than 25 eats up a lot of yard space. If your angle is coming out high, try increasing the step depth or splitting the run with a landing.
Frequently asked questions
What is a comfortable step height?
Most builders aim for 165 to 190 mm (6.5 to 7.5 inches). Much higher and the stairs become tiring to climb. Much lower and the shallow steps can trip people up because they break the natural walking rhythm.
How many stringers do I need?
A good starting point is one stringer every 400 to 500 mm across the stair width. Use the advanced options to set the number that fits your project, and check spacing requirements with your local building code.
Does tread board thickness matter for the stringer length?
Yes. The calculator adds your tread board thickness to the stair height before calculating stringer length, so the stringer reaches from the ground to the deck framing at the correct length. When cutting, trim the bottom of the stringer by one tread thickness so all steps end up the same height.
Can I build platform stairs along the full side of my deck?
Platform stairs are wide, full-length steps that run along an entire side of the deck, rather than a narrow stair run with stringers. For that style, treat each step like its own small deck and use the Deck Board Calculator to work out tread board count and waste per platform. The standard tread calculation in this calculator does not apply to that build.