Stair calculator

How it is worked out

• It uses the fewest risers that reach the rise at 7¾ in or less each, and makes them all the same height.
• There is one tread fewer than there are risers, since the floor above is the last step.
• Stringers are cut from 2×12s and stand at most 18 in apart.
• Treads are 1 in thick and nose 1 in past the ¾ in riser under them.
• The floor above is framed with a well over the stair, long enough to leave 6 ft 8 in of headroom over the steps. The well has a doubled trimmer along each side and a doubled header across each end.
• It frames one straight flight. A stair whose stringers would be longer than a 20 ft board needs a landing, so it is turned down, and we say why.

We lay out and count the sizes you pick, close enough to plan a weekend and price the lumber. Span tables, your local code and, on a bigger job, an engineer settle what size each board has to be.

Questions

How many steps do I need?

Divide the total rise by 7¾ in and round up to get the number of risers. The rise is then divided evenly among them.

What is the total run of the stair?

The number of treads times the tread depth, plus the thickness of the top riser. It is given above the list, so you can see whether you have the room.

Does it include a landing or a handrail?

It frames one straight flight, with its stringers, treads and risers and the opening in the floor above. Landings, guards and handrails are yours to add.