Figuring stairs comes down to three measurements: total rise (the vertical height between floors), riser height (the height of each step), and tread depth (how deep each step is). Once you have those numbers, the rest of the layout follows a straightforward sequence of division and a bit of geometry. Here’s how to work through the entire process.
Measure the Total Rise
Total rise is the vertical distance from one finished floor surface to the next. This single measurement drives every other calculation, so getting it right matters more than anything else in the process.
If both floors already have their finished flooring installed (hardwood, tile, carpet), measure straight from one floor surface to the other. If you’re working during construction and only subfloors are in place, you need to account for the difference in finish materials. The formula is: total rise equals the subfloor-to-subfloor measurement, plus the thickness of the upper floor’s finish material, minus the thickness of the lower floor’s finish material. For example, if the subfloor-to-subfloor distance is 106 inches, the upstairs flooring is 3/4-inch hardwood, and the downstairs is 3/8-inch tile, your total rise is 106 + 0.75 – 0.375 = 106.375 inches.
Measure in at least two spots to make sure the upper floor is level relative to the lower one. If it’s not, use the larger measurement and plan to shim or scribe at the top.
Calculate the Number of Risers
Divide your total rise by a target riser height to find how many steps you need. A comfortable riser height for most people falls around 7 to 7.5 inches. The residential building code (IRC) sets a maximum of 7 3/4 inches per riser, so you can’t exceed that.
Take a total rise of 106 inches as an example. Divide 106 by 7 (a comfortable target) and you get 15.14. Since you can’t build a fraction of a step, round to the nearest whole number: 15 risers. Now divide the total rise by that number to get the actual riser height: 106 divided by 15 equals 7.07 inches per riser. That’s well within code and comfortable underfoot.
If rounding down gives you a riser height over 7 3/4 inches, round up to the next whole number instead. Every riser in the staircase must be the same height. Code allows no more than 3/8-inch variation between the tallest and shortest riser in a flight, and even small inconsistencies create a trip hazard.
Determine Tread Depth and Total Run
Tread depth is how far each step extends from front to back. The code minimum is 10 inches, measured from the nose of one tread to the nose of the next. Most builders aim for 10 to 11 inches, which feels natural for an average stride.
A classic rule of thumb ties risers and treads together: riser height plus tread depth should equal roughly 17 to 18 inches. So a 7-inch riser pairs well with a 10- or 11-inch tread. This ratio keeps the stairs comfortable to walk up and down without feeling too steep or too shallow.
Once you’ve chosen a tread depth, multiply it by the number of treads to find the total run, which is the horizontal distance your staircase covers. One important detail: the number of treads is always one less than the number of risers, because the upper floor itself acts as the final tread. With 15 risers and 10-inch treads, you have 14 treads, giving a total run of 140 inches (11 feet 8 inches). Make sure you have enough floor space to accommodate that distance.
Figure the Stringer Length
Stringers are the diagonal boards that support the treads and risers. They’re typically cut from 2×12 lumber. To know how long your stringer boards need to be, use the Pythagorean theorem: stringer length equals the square root of (total rise squared plus total run squared).
Using the example above with a total rise of 106 inches and a total run of 140 inches: square 106 to get 11,236, square 140 to get 19,600, add them for 30,836, and take the square root. That gives you roughly 175.6 inches, or about 14 feet 8 inches. Always buy boards longer than your calculation to account for end cuts and any knots or defects you’ll want to avoid. For a stringer this long, a 16-foot 2×12 gives you comfortable working room.
Drop the Stringer for the First Step
This is the step most beginners miss, and it’s the difference between a staircase that feels right and one where the bottom step is noticeably off. When you set your cut stringer on the floor and add a tread board on top, the first step becomes taller than all the others by the thickness of that tread.
The fix is called “dropping the stringer.” You cut the thickness of one tread off the bottom of the stringer. If your tread material is 1 inch thick, you remove 1 inch from the bottom. This makes the first riser shorter on the stringer itself, but once the tread sits on top, that step matches every other step in the flight.
There’s one more wrinkle: if the finished floor at the bottom of the stairs isn’t installed yet when you mount the stringers, add its thickness back to the bottom riser height (or shim the stringers up by that amount). Otherwise, once that flooring goes in, your first step will be too short.
Nosing and Overhang Rules
If your treads are less than 11 inches deep and you’re using solid risers (the vertical boards between steps), code requires a nosing projection. The tread nose must overhang the riser by 3/4 inch to 1 1/4 inches. This gives your foot a little extra surface when descending. If your treads are 11 inches or deeper, a nosing overhang isn’t required, though many builders include one anyway for a finished look.
Keep in mind that tread depth is measured from nosing to nosing, not from the back of one tread to the front. So a 10-inch tread with a 1-inch nosing overhang actually has a flat surface of about 9 inches, with the nose extending over the riser below.
Landings and Direction Changes
If your staircase changes direction or needs to break up a long run, you’ll need a landing. Landings at the top and bottom are required at door openings because stepping through a doorway directly onto a stair slope is a safety hazard. Intermediate landings serve as rest points and catch zones if someone trips.
A landing must be at least as deep as the stairway is wide, measured in the direction of travel. For a straight-run stair, landings don’t need to exceed 48 inches in depth. If your stairway is 36 inches wide, the landing should be at least 36 inches deep. The width of the landing can’t be narrower than the stairs feeding into it.
Handrail Height and Size
Handrails are required on at least one side of any staircase with four or more risers. The top of the handrail must sit between 34 and 38 inches above the stair nosing, measured vertically from the nose of each tread.
The rail itself needs to be graspable. For a round handrail, that means a diameter between 1 1/4 and 2 inches. Non-circular profiles (oval, rectangular with rounded edges) have their own size limits based on perimeter, but the basic idea is the same: your hand needs to wrap around it comfortably enough to catch yourself. A 2×4 laid flat fails this test. A turned wooden dowel or standard metal rail passes easily.
Putting the Numbers Together
Here’s the full sequence applied to a real example. Say your total rise is 96 inches (a common 8-foot ceiling height with floor framing).
- Number of risers: 96 divided by 7 equals 13.7, so round to 14 risers. Actual riser height: 96 / 14 = 6.86 inches.
- Number of treads: 14 risers minus 1 = 13 treads.
- Tread depth: Using the 17-18 rule, 17.5 minus 6.86 = about 10.6 inches. Round to 10.5 inches.
- Total run: 13 treads times 10.5 inches = 136.5 inches (11 feet 4.5 inches).
- Stringer length: Square root of (96² + 136.5²) = square root of (9,216 + 18,632) = square root of 27,848 = about 166.9 inches (13 feet 11 inches). A 14-foot 2×12 works.
- Stringer drop: Subtract one tread thickness (typically 1 inch for dimensional lumber treads, 1.5 inches for a full 2x board) from the bottom of each stringer.
Write all your numbers down before you pick up a saw. Mark the stringer using a framing square with stair gauges clamped at your riser height and tread depth. Cut one stringer first, test-fit it in place, and verify the top and bottom riser heights with a level and tape measure. Once that first stringer checks out, use it as a template for the rest.