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Stair Calculator

Enter the total rise from one finished floor to the next. You get the riser height, the number of treads, the total run, the stringer length, and the cut height of the top and bottom risers when the finished floors are not installed yet.

  • Riser height and tread count
  • Stringer length and what to buy
  • End risers adjusted for the finished floors

Project measurements

Where are the stairs going? This sets the starting numbers

A full flight inside the house, from one finished floor to the next. Both floors usually go down after the stair is framed.

Measurements Total rise, then the tread and the width

ft
in
in

The floors at each end Finish floor thickness changes the end risers

Floor at the top of the stair

Floor at the bottom of the stair

The stair is framed before either floor is down, so the first riser is cut taller by the bottom thickness and the top riser shorter by the top one. The risers between them are the plain division, so every finished step comes out the same height. Enter the floors that will be there when the job is finished, not the ones that are there today.

How this number is built

This calculator works out a straight flight of stairs, inside a house or off a deck. From the total rise, measured from one finished floor to the next, it gives the riser height, tread count, total run, and stringer length, so it works as a stair stringer calculator too. It also counts the lumber and checks the result against the stairway limits in the code.

Round the riser count up, never down

The code sets a maximum riser height of 7¾ in and no minimum. Divide the rise by 7.75 and round up to get the number of risers. A rise of 104 in gives 13.4, so the flight takes 14 risers at 7.43 in each. Rounding down to 13 makes every riser 8 in, which is a quarter inch over the limit and fails inspection.

The stair runs between finished floors

Stair math runs from the finished floor at the top to the finished floor at the bottom, but the framing goes in before either floor does. So the thickness of the floor at each end is part of the calculation, and the two are separate numbers: the floor at the top raises where the stair arrives, and the floor at the bottom raises the first step. Carpet is not entered here, because the code measures stairs “exclusive of carpets, rugs or runners,” so an inspector measures the stair without it.

Two thicknesses, one adjustment

Once both floors are down, every riser should measure the same height, which is the plain division (7.43 in in the example above). To get there, the bottom riser is cut as the division plus the bottom floor thickness, the top riser is cut as the division minus the top floor thickness, and the risers in between are the plain division. These heights are to the top of the finished tread, so the notch seat, and the riser board under it, sit one tread thickness lower. This adjustment is carpentry practice, not a code rule. The code states the 3/8 in uniformity limit, and this arithmetic keeps the stair inside it.

A flight has one fewer tread than risers

The upper floor serves as the top step, so it takes no tread of its own. A flight of 14 risers has 13 treads, and the run is 13 tread depths: 13 × 10 in is 10 ft 10 in. A closed stair has a riser board under each tread. An open stair leaves them out, and deck steps are often built that way.

Headroom is not checked here

The code requires 6 ft 8 in of clearance, measured up from the sloped line that touches the tread nosings. This calculator cannot check it, because it depends on the size of the floor opening above, which the stair’s dimensions do not give. Measure it on site. Headroom is tightest at the top of the flight, so that is where the opening may need to be cut back.

Stringer length and notching

Stringers are usually cut from 2 × 12 stock. The length is the diagonal of the rise and the run: the square root of the rise squared plus the run squared. Lumber comes in 8, 10, 12, 14, and 16 ft lengths, so the diagonal is rounded up to the next one before ordering. The notches remove wood where the stringer is under the most stress, and span tables apply to the material left at the notch, not to the full board. A long or steep flight needs a mid-span support or an engineered stringer, not a deeper notch. Those figures come from a span table, so this page does not give a remaining depth.

The code values this page uses

Requirement Value What it covers
Riser height, maximum 7¾ in A maximum only. The code states no minimum riser height at all.
Tread depth, minimum 10 in Measured from one nosing to the next, not from board to board.
Uniformity 3/8 in The largest riser, tread, or nosing may not exceed the smallest by more than this. All three are covered.
Headroom 6 ft 8 in Measured from the sloped line that touches the nosings up to the ceiling or the floor opening above.
Stairway width 36 in Clear width. A handrail on one side drops it to 31½ in, and handrails on both sides drop it to 27 in.
Flight rise, maximum 151 in The vertical rise of one flight, between floor levels or landings. More than that needs a landing partway up.
Walking surface 2% Treads and landings may be no steeper than 2 percent out of level.
Handrails 4 risers Required on at least one side of a flight with four or more risers.
Handrail height 34–38 in Measured above the tread nosings.
Nosing ¾–1¼ in The leading edge radius is 9/16 in at most. A nosing is not required once the tread itself is 11 in.
Open risers 4 in sphere Where the opening sits more than 30 in above the floor or grade below, a 4 in sphere may not pass through it.
Landings 36 in Deep in the direction of travel, at the top and at the bottom of a straight run.

These are the limits for a straight flight in the 2018 International Residential Code, taken from the 2019 ICC Public Comment Agenda, which reproduces the section word for word. Later editions renumber the sections, so the page names the edition and does not cite section numbers. The code your building department has adopted is the one that applies. A stair with winders, a spiral, or alternating tread devices is measured differently.

Deck steps usually land on a footing or a concrete pad, and the concrete calculator sizes one. The 4 in sphere that limits open risers is the same test the fencing page applies to pool barriers. Railing and baluster spacing are not part of this tool.

Where these numbers come from

  • The stairway limits above come from the 2018 International Residential Code, read from the 2019 ICC Public Comment Agenda, which reproduces the section verbatim and is published at cdn-web.iccsafe.org.
  • Residential stair and egress checklists published by Wilson County TN, Placer County CA, Teller County CO, and White Bear Lake MN, and the SafeBuilt Colorado 2018 residential stair guide, were used to cross-check those values, not as their source.
  • The finished-floor adjustment, the stringer stock, and the rule about three stringers on a wide stair come from trade guidance on stair layout (Fine Homebuilding and general carpentry sources), not from the code, and the page labels them as practice for that reason. The figures behind stair notching and remaining depth need a span table, so no number for them appears here.
  • The three finish thicknesses are typical installed buildups: 3/4 in is the thickness of nail-down hardwood, tile over a 1/4 in backer board usually comes to about 3/4 in, and 5/4 or composite decking measures 1 in. Carpet has no option because the code measures stairs without it. Measure the actual materials for the job rather than trusting the round numbers.
  • Demand for this page was measured with DataForSEO keyword data (US, location 2840, English) pulled on 2026-10-15. No volume figure is used in the calculation.