How many stairs do I need?

Divide the total rise, floor to floor, by about 7.5 inches and round to a whole number of risers, then divide the rise back by that count for the exact riser height. A 9 foot (108 inch) rise works out to 14 steps of 7-11/16 inches each, with 13 treads. The number of treads is always one less than the number of risers. This calculator does it for any rise.

How do I calculate stringer length?

The stringer length is the diagonal of the stair, the square root of the total run squared plus the total rise squared. For a 130 inch run and a 108 inch rise, that is the square root of 28,564, or about 169 inches (14 feet 1 inch), so you cut it from a 16 foot 2x12. It is the same right-triangle formula, with the run and the rise as the two legs.

What is the maximum riser height?

7-3/4 inches for residential stairs under the IRC, section R311.7.5.1. Every riser in a flight must also be within 3/8 inch of the others. Commercial stairs under the IBC are stricter at 7 inches. This calculator splits the rise into equal risers and adds a step if any riser would exceed the 7-3/4 inch cap.

What is the minimum tread depth?

10 inches for residential stairs under the IRC, section R311.7.5.2, measured nosing to nosing. Commercial stairs need 11 inches. Deeper treads walk easier, so most comfortable stairs use 11 inches. The nosing overhang is not part of the tread depth and does not change the total run.

What is the 7-11 rule for stairs?

The 7-11 rule is a comfort target of a 7 inch riser and an 11 inch tread, the shape most people find easiest to climb. It matches the stricter commercial IBC limits (a 7 inch max riser and an 11 inch min tread), not residential code: a home stair can legally use a 7-3/4 inch riser and a 10 inch tread. Treat 7-11 as the comfortable ideal, not a hard requirement for a house or a deck.

Why are my stairs uneven?

Almost always because the bottom step was not adjusted. The bottom notch of the stringer must be cut shorter by one tread thickness, called dropping the stringer, so that once the treads are installed every finished riser is equal. Skip that and the first step ends up a tread thickness taller or shorter than the rest. Code allows no more than 3/8 inch of variation between risers.

How much horizontal space do stairs need?

The total run is the number of treads times the tread depth, and the treads are one fewer than the risers. A 9 foot rise at 14 risers and a 10 inch tread needs 13 times 10, or 130 inches, about 10 feet 10 inches of run. Deeper treads or a taller allowed riser change it. Add landing space at the top and the bottom.

Can I use this for deck stairs?

Yes. Deck and exterior stairs use the same geometry and the same IRC limits as interior stairs; there is no looser 8 inch riser allowance. Measure the total rise to the finished landing surface, including the pad or pavers and the deck boards. Deck stairs are often built with open risers and a wider 48 inch width. Set the stair width and open risers in the cut list section.

What size lumber do I need for stair stringers?

A 2x12. A cut stringer must keep at least 3-1/2 inches of solid wood behind the deepest notch, and 5 inches for a deck. For a typical 7-3/4 inch riser and 10 inch tread the notch cuts about 6.1 inches into the board, so a 2x12 (11-1/4 inches actual) leaves about 5.1 inches and passes, while a 2x10 (9-1/4 inches) leaves only about 3.1 inches and fails.

How many stringers do I need?

Enough that they sit no more than about 16 inches apart, and at least three for a 36 inch wide stair. At a tight 12 to 16 inch spacing a 36 inch stair works out to four stringers; at 18 inches it is three. Wider or composite-tread stairs need them closer. The cut list here sizes the count from your stair width and spacing.

How do I measure the total rise?

Measure straight down from the finished upper floor to the finished lower floor, the true vertical height, not along a slope. Include the finish flooring top and bottom if it is not installed yet. For a deck, measure from the top of the deck boards to the finished landing pad. The total rise is the single most important number; every riser, the run, and the stringer come from it.

What is the difference between rise and run?

The rise is vertical and the run is horizontal. Total rise is the full floor-to-floor height; unit rise is one riser. Total run is the full horizontal distance the stair covers; unit run is one tread depth. Stairs are described as rise over run, and the stringer is the diagonal that connects them.

How many treads are in a set of stairs?

One fewer than the number of risers. On a standard flight the upper floor acts as the top tread, so a stair with 14 risers has 13 treads. Using the riser count for the treads, and the run, is the most common stair mistake and over-states the run by one full tread.

Do open riser stairs need anything special?

Open riser stairs have no riser board, but where the total rise is more than 30 inches the gap between treads must be small enough to stop a 4 inch sphere from passing through, so a child cannot slip through. The geometry, the riser count, tread depth, run, and stringer, is identical to a closed stair. Toggle open risers in the cut list section to drop the riser boards from the count.

Is this stair calculator free?

Yes. It is free, needs no signup, and runs entirely in your browser. Nothing you enter is uploaded anywhere.

Does this replace a full deck or addition takeoff?

No. This lays out one straight flight of stairs. When you bid a whole deck or addition you also need the framing, decking, railing, and footings measured to scale off the plan. Easy Takeoffs is the construction takeoff software that measures all of it straight from a PDF and exports a bid-ready list. 14-day trial, no card.

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Stairs

Stair Calculator

Enter the total rise floor to floor and get the number of steps, the exact riser height, the tread depth, the total run, the stringer length, and the angle, with the risers split evenly so they are all equal. It checks each number against the IRC code, adds a stringer cut list, and works for interior and deck stairs. Free to use with no signup.

Lay out by

Pick a comfortable riser height and it solves the number of steps.

Total rise

Floor to floor, finished surface to finished surface. This is the one number the whole layout hinges on.

Target riser height (in)

Comfort target is 7 to 7.5″. The code cap is 7-3/4″; the real riser is the rise split evenly, so every step is equal.

Tread depth (in)

The run of each step, nosing to nosing. Code minimum is 10″; 11″ or more walks easier. The nosing overhang does not change the run.

Illustration, not to scale

Your stair layout

14steps7 11/16″ each

108″ of rise split into 14 equal risers of 7 11/16″ (7.7″).

Steps (risers)1413 treads

Riser height7 11/16″first step 6 11/16″ after the tread goes on

Tread depth10″

Total run10 ft 10 in130″ front to back

Stringer length14 ft 1 in169″; cut from a 16 ft 2x12

Stair angle39.7°37.6° per-step slope

IRC code check

Riser within the 7-3/4″ max7 11/16″

Tread meets the 10″ min10″

Every riser equal (within 3/8″)0″ spread

Handrail required (4+ risers)34 to 38″

Comfort check (guideline, not code)

25.4

2R + T
aim 24–25

17.7

R + T
aim 17–18

R × T
aim 71–74

Cut one stringer first and test-fit it before cutting the rest. The bottom step is cut shorter by one tread thickness so every finished riser ends up equal. This lays out one straight flight; a rise over 12 ft needs a landing, and headroom, guards, and footings are separate checks.

Building the deck or addition around these stairs?

When you bid the whole job, Easy Takeoffs measures the framing, decking, and footings off your plan PDF to scale and exports a bid-ready list. 14-day trial, no card.

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Code

What does code require for stairs?

The IRC sets six main limits for residential stairs: a maximum riser height of 7-3/4 inches, a minimum tread depth of 10 inches, no more than 3/8 inch of variation between the tallest and shortest riser or tread in a flight, at least 6 feet 8 inches of headroom, a minimum width of 36 inches, and a handrail 34 to 38 inches high once a flight has four or more risers. A guard 36 inches high is required where the drop is more than 30 inches. Splitting the rise into equal risers meets the uniformity rule on its own, which is why this calculator always divides evenly.

Stair code at a glance (IRC residential)

Requirement	Value	Code
Max riser height	7-3/4 in	R311.7.5.1
Min tread depth	10 in	R311.7.5.2
Riser and tread uniformity	Within 3/8 in	R311.7.5.1
Min headroom	6 ft 8 in (80 in)	R311.7.2
Min stair width	36 in	R311.7.1
Handrail height (4+ risers)	34 to 38 in	R311.7.8
Guard height (over a 30 in drop)	36 in	R312.1
Intermediate landing	Over 12 ft of rise	R311.7.3

2021 IRC residential values, unchanged from the 2018 and 2024 editions. Local amendments vary, so confirm with your building department. Headroom is exactly 80 inches, not 6.8 feet.

Residential vs commercial

Dimension	Residential (IRC)	Commercial (IBC)
Max riser	7-3/4 in	7 in
Min tread	10 in	11 in
Guard height	36 in	42 in

This calculator checks against the residential IRC by default. The commercial IBC numbers are the "famous" 7 and 11, but they are not residential code, so a home or deck stair does not have to hit them.

Comfort

What makes a comfortable stair? The 7-11 rule and the stride rule

A comfortable stair keeps the riser near 7 to 7.5 inches and the tread at 11 inches or more, the classic 7-11 shape. Three rules of thumb describe the feel: two risers plus a tread should total about 25 inches (the stride rule), a riser plus a tread should total 17 to 18 inches, and a riser times a tread should land between 71 and 74 square inches. They do not all agree at once, so they are guidance, not code. A code-compliant stair can sit just outside a band and still be fine; steeper risers with shallow treads simply walk harder.

Stair comfort rules (advisory)

Rule	Target	What it means
2 × riser + tread	About 25 in	The stride rule (Blondel); matches an average step
Riser + tread	17 to 18 in	A quick single-step comfort check
Riser × tread	71 to 74 sq in	Balances step height against depth
Riser height	7 to 7.5 in	Lower is easier; 7-3/4 in is the code max
Tread depth	10 to 11 in	11 in fits a foot; 10 in is the code min

Advisory ergonomics from Architectural Graphic Standards and the Blondel stride formula, not building code. A comfortable stair slopes about 30 to 37 degrees per step; the overall angle across the whole flight reads a couple degrees steeper, because the run spans one fewer tread than there are risers.

Measure

How do you measure and lay out a stair?

You lay out a stair from the total rise. Measure the vertical height floor to floor, divide it into equal risers near 7.5 inches, set a tread depth of at least 10 inches, and the run and the stringer follow. Cut one stringer, test-fit it, then use it as the template for the rest.

1
Measure the total rise straight down, from the finished upper floor to the finished lower floor. Use the true vertical height, not a slope, and include any finish flooring top and bottom.
2
Divide the total rise by about 7.5 inches and round to a whole number of risers, then divide the rise back by that count so every riser is exactly equal and stays under the 7-3/4 inch code max.
3
Pick a tread depth of at least 10 inches. The number of treads is one less than the number of risers, and the total run is the tread count times the tread depth.
4
The stringer length is the diagonal, the square root of the run squared plus the rise squared. Mark it with a framing square, cut the bottom shorter by one tread thickness, and test-fit before cutting the rest.

Measuring a whole deck or addition by hand, the stairs plus the framing, decking, and footings, is slow and error prone. For a bid-ready quantity, take the measurements off the plan to scale, which is what construction takeoff software does.

Formula

How do you calculate stairs?

Divide the total rise by about 7.5 inches and round to a whole number of steps, then divide the rise back by that count so every riser is equal and under the 7-3/4 inch code cap. The treads are one fewer than the risers, the run is the treads times the tread depth, and the stringer is the diagonal. Here is the exact math, with a worked example.

Number of steps: total rise ÷ target riser (about 7.5 in), rounded to a whole number of risers
Riser height: total rise ÷ number of steps, so every riser is equal (7-3/4 in code max)
Number of treads: number of steps − 1, because the upper floor is the top tread
Total run: number of treads × tread depth (nosing excluded, measured nosing to nosing)
Stringer length: the square root of (total run² + total rise²), the diagonal
Stair angle: the arctangent of total rise ÷ total run, in degrees

Ordering adjustments

Two details separate a real stair layout from a rough guess. First, the treads are always one fewer than the risers, because the upper floor is the top tread, so a 14-riser stair has 13 treads and its run uses 13, not 14. Second, the nosing overhang does not change the run: the tread depth is measured nosing to nosing, so a 1 inch nosing on a 10 inch tread still lays out on a 10 inch run. When you cut the stringer, drop the bottom notch by one tread thickness so that, once the treads are on, every finished riser ends up equal. Code sets the limits (a 7-3/4 inch riser, a 10 inch tread); the comfort rules are guidance on top.

Worked example

A 9 ft (108 in) floor-to-floor rise, a 7.5 in target riser, and a 10 in tread

Steps: 108 ÷ 7.5 = 14.4, rounded to 14 risers
Riser height: 108 ÷ 14 = 7.71 in (7-11/16 in), under the 7-3/4 in code max, and every riser is equal
Treads: 14 − 1 = 13, because the upper floor is the top tread
Total run: 13 × 10 = 130 in (10 ft 10 in), using the tread count, not the riser count
Stringer length: the square root of (130² + 108²) = the square root of 28,564 = 169 in (14 ft 1 in), cut from a 16 ft 2x12
Stair angle: the arctangent of 108 ÷ 130 = 39.7 degrees

Result: About 14 steps of 7-11/16 in, 13 treads of 10 in, a 130 in (10 ft 10 in) run, and a 169 in (14 ft 1 in) stringer at 39.7 degrees. Using 14 for the run instead of 13 would over-state it by a full 10 in tread, the most common stair mistake.

Reference

Stairs reference tables

How many steps by total rise

Total rise	Steps	Riser height	Total run
2 ft (24 in)	4	6 in	2 ft 6 in
3 ft (36 in)	5	7-3/16 in	3 ft 4 in
4 ft (48 in)	7	6-7/8 in	5 ft
5 ft (60 in)	8	7-1/2 in	5 ft 10 in
6 ft (72 in)	10	7-3/16 in	7 ft 6 in
7 ft (84 in)	11	7-5/8 in	8 ft 4 in
8 ft (96 in)	13	7-3/8 in	10 ft
9 ft (108 in)	14	7-11/16 in	10 ft 10 in
10 ft (120 in)	16	7-1/2 in	12 ft 6 in

Using a 7.5 inch target riser and a 10 inch tread. The riser is the rise split evenly, so it lands under the 7-3/4 inch code max; a 2 ft or 4 ft rise rounds to shorter risers to stay under the cap. Change the target riser or tread depth for your own numbers.

Standard stair dimensions

Part	Typical	Notes
Riser height	7 to 7-3/4 in	Lower walks easier; 7-3/4 in is the code max
Tread depth	10 to 11 in	Nosing to nosing; the run of one step
Nosing	3/4 to 1-1/4 in	Overhang on solid risers; does not change the run
Stair width	36 in min	Decks are often 48 in
Stringer stock	2x12	Leaves enough wood behind the notch
Handrail height	34 to 38 in	Above the nosing line, 4+ risers

Residential figures. A single flight over 12 feet of rise needs an intermediate landing.

Stringer stock: 2x12 vs 2x10

Stock	Actual width	Wood behind notch	Passes
2x12	11-1/4 in	About 5.1 in	Yes
2x10	9-1/4 in	About 3.1 in	No

For a 7-3/4 inch riser and a 10 inch tread, the notch cuts about 6.1 inches into the board. A cut stringer needs at least 3-1/2 inches left behind the notch (5 inches for a deck), so it is cut from a 2x12, not a 2x10. Solid-mount stringers that are not notched can be smaller.

FAQ