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How do you calculate the Tread to Riser ratio when designing a set of stairs?
Use the following formula:

(2 x Riser) + (1 x Tread) = 630 to 660mm (25 to 26")

2R+T = 630 to 660

There are variations, of course, and indoor stairs are usually smaller than outdoor stairs. But this formula will give you the ideal length of a stair tread for a given stair riser height, or vise versa.

Wikipedia has a large article on Stairs here, and this links to a list of common tread and riser combinations. This formula will give you relationships of stair steepness that are suitable to the typical human stride. It's also desirable to have steps in odd numbers, which help people keep their pace.

And remember, no single steps in the landscape please.

Find this formula and other helpful details in these valuable reference books: