Here are some of the rules that govern stairs, including the riser heights and the tread depths. As usual though, there are plenty of disclaimers and things to keep in mind.
These rules come from the 2021 version of the International Residential Code. Some states and local jurisdictions enforce different codes and so these numbers might vary slightly where you are. These rules don’t apply to stairs leading to nonhabitable attics and to crawl spaces. These rules don’t apply to spiral stairways – they have their own rules that I won’t touch on here.
Also keep in mind that lots of very old, semi-old, and kind-of-old houses were built before these rules were standardized. It’s very hard to know if old stairs were built according to the codes and rules in place (if any) at the time they were built. But you’re giving up a little bit of safety by living with stairs that don’t comply with these rules. That might be a reasonable trade-off, but you should know what you’re dealing with.
The maximum riser height is 7.75 inches. Within any flight of stairs the difference between the tallest and shortest riser can be no more than 3/8 inch. As you’re walking up and down stairs you’ll expect that all the risers are the same, so if there’s a difference it’s a trip hazard.
The tread depth should be at least 10 inches, with a nosing extending out between ¾ inch and 1-1/4 inch. With the standard nosing this gives you at least 11 inches for your foot to land on the tread. As before, within any flight of stairs the difference between the greatest and smallest tread depth should be no more than 3/8 inch. Consistency is important in building stairs.
This gives you a total of 11 inches for your foot to fit on the tread, including the area under the nosing.
If you don’t want to use a nosing on the treads then the minimum tread depth is 11 inches.
This still gives you 11 inches for your foot to fit on the tread.
The treads should be flat. They can be sloped a maximum of 1 vertical unit in 48 horizontal units.