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CalcTheBuild

Stair Calculator

Building basement stairs, a staircase to a new loft or a set of deck steps? Enter the total rise from floor to floor and your preferred step size, and the calculator splits the rise into equal risers, works out the treads, total run, stair angle and stringer length, sizes the stairwell opening for headroom and checks everything against the IRC, IBC, Canadian, UK or Australian rules. It also prices the stringers, treads, riser boards and handrail.

in

Measure from the finished lower floor to the finished upper floor (or deck surface).

in

The calculator splits the rise into equal steps close to this, never above the code maximum.

in

Horizontal depth of each step from riser face to riser face, not counting the nosing.

in

¾–1¼″ (20–30 mm) is typical; 0 for square-edged treads.

in
in

Joists plus subfloor and ceiling. Used to size the stairwell opening for headroom.

$

For example a 2 × 12 or 47 × 225 mm board.

$
$
$/ft

Your estimate

Number of risers
15 risers
Riser height
7.2 in
Number of treads
14 treads
Estimated material cost
$679.27
Tread run (going)
10.5 in
Tread board depth (run + nosing)
11.5 in
Total run (horizontal length)
12.25 ft
Stair angle
34.4 degrees
2 × riser + run
24.9 in
Stringer length
15.2 ft
Stringer board to buy
16 ft
Stringers
3 stringers
Riser boards
15 boards
Handrail length (each)
14.85 ft
Required headroom
80 in
Minimum stairwell opening length
11 ft
Stringers cost
$105.00
Treads cost
$350.00
Riser boards cost
$150.00
Handrail cost
$74.27

Riser 7 3/16″: within the IRC limit of 7 3/4″ max.

Tread run 10 1/2″: meets the IRC minimum of 10″.

2 × riser + run = 24 7/8″: in the comfortable range of about 24″–26″.

Stringer layout: set the framing square to 7 3/16″ on the tongue and 10 1/2″ on the blade, and mark 14 treads. Trim the bottom of the stringer by one tread thickness so the first step comes out the same height as the rest.

The stairwell opening is measured back from the top nosing so the line of nosings stays at least the required headroom below the ceiling edge. Handrail length runs from the bottom nosing to the top nosing; add any extensions and returns.

Notched stringers in deck stairs are usually limited to about 6 ft (1.8 m) of horizontal span without a middle support. Open risers must not let a 4″ (100 mm) ball pass through, and most codes require a handrail once a stair has 4 or more risers (US) or rises more than 600 mm (UK).

These are planning figures from the IRC dimensional rules for straight stairs. Codes are amended locally, so confirm risers, treads, headroom, handrails and guards with your building department, and get a stair designer or engineer for anything unusual.

How to use this calculator

  1. Measure the total rise from the finished lower floor to the finished upper floor or deck surface, and enter it in inches or millimeters.
  2. Choose the building code that applies to you. It sets the maximum riser height, the minimum tread run and the headroom.
  3. Enter your preferred riser height (7–7½″ or 170–180 mm feels comfortable) and the tread run you want, plus the nosing overhang.
  4. Enter the stair width and stringer spacing, choose open or closed risers, and give the thickness of the upper floor so the stairwell opening can be sized.
  5. Add local prices for a stringer board, a tread, a riser board and handrail per foot or meter to see the material cost. Use the step layout table and code check notes to mark out your stringers.

How the stair calculation works

The total rise is divided by your preferred riser height and rounded up, so every step is the same height and none is taller than you asked for. If that would still break the code maximum, more risers are added. A 108″ rise with a preferred 7½″ riser gives 108 ÷ 7.5 = 14.4, so 15 risers of exactly 7.2″ (just over 7 3/16″). There is always one tread fewer than risers, because the upper floor acts as the top step: 14 treads.

With a 10½″ tread run, the stair runs 14 × 10.5 = 147″, or 12 ft 3 in, horizontally. The stair angle is the arctangent of 7.2 ÷ 10.5, about 34.4°, and the comfort check 2 × riser + run comes to 24.9″, right in the 24–26″ sweet spot. The stringer spans the full rise and run, so its length is √(147² + 108²) = 182.4″, about 15.2 ft, and you would cut it from a 16 ft 2 × 12.

A 36″ wide stair with stringers no more than 18″ apart needs three stringers. For headroom, the stairwell opening has to reach back far enough that the line of nosings stays 6 ft 8 in below the ceiling edge: (80″ + 10.5″ floor) × 10.5 ÷ 7.2 = 132″, about 11 ft. With three stringers at $35, 14 treads at $25, 15 riser boards at $10 and a 14.9 ft handrail at $5 per foot, the materials come to about $679.

In metric, a 2,700 mm rise with a preferred 180 mm riser gives 15 risers of 180 mm and 14 goings of 250 mm: a total going of 3.5 m, a pitch of 35.8° and 2R + G of 610 mm, all within UK Approved Document K. The stringer is 4.42 m long, so buy 4.5 m boards.

Pro tips

  • Measure the rise at the exact spot where the stair will land, using a straightedge and level from the upper floor. Floors are rarely flat, and an error of half an inch ends up in the top or bottom step.
  • Allow for finished flooring. If the lower floor is getting tile or the treads are thicker than the upper floor finish, adjust the total rise and the first riser so every step comes out the same height. Codes allow only about 3/8″ (9.5 mm) of variation between steps.
  • Cut one stringer, test it in place with a level on every tread, then use it as the pattern for the others.
  • For deck stairs, keep notched 2 × 12 stringers to about 6 ft (1.8 m) of horizontal span, or add a support post or landing in the middle.
  • Plan the stairwell early. The headroom opening in the floor above often decides whether a straight stair fits at all, and moving joists later is expensive.

Frequently asked questions

How many steps do I need for a 9 ft (108 inch) rise?

With a preferred 7½″ riser, a 108″ rise needs 15 risers of 7.2″ each and 14 treads, because the upper floor serves as the last step. With 10½″ treads, the stair runs 12 ft 3 in horizontally.

How do you calculate stair stringer length?

Use the Pythagorean theorem on the total run and total rise: stringer length = √(run² + rise²). For 14 treads of 10½″ (147″ run) and a 108″ rise, that is √(147² + 108²) = 182.4″, about 15.2 ft, so you would buy a 16 ft board.

What is the maximum riser height for stairs?

In US homes the IRC allows 7¾″ and requires treads of at least 10″. Commercial buildings under the IBC are limited to 7″ risers and 11″ treads. Canadian homes allow 200 mm, UK homes 220 mm (with a 42° maximum pitch) and Australian homes 190 mm. Local amendments can be stricter, so check with your building department.

How many steps for a 30 inch high deck?

A 30″ rise with a 7½″ riser needs 4 risers and 3 treads. With 11″ treads, the stairs run 33″ (2.75 ft) out from the deck and each stringer is about 3.7 ft long.

How many stringers do I need for 36 inch wide stairs?

With 2× wood treads and stringers no more than 18″ apart, a 36″ stair needs 3 stringers and a 48″ stair needs 4. Thinner 5/4 or composite treads usually call for 12″ spacing, which means 4 stringers for a 36″ stair.

What is a good angle for stairs?

Most comfortable house stairs sit between about 30° and 37°. A 7.2″ riser with a 10½″ tread works out at 34.4°, and a 180 mm riser with a 250 mm going at 35.8°. The UK limits private stairs to 42°; anything steeper starts to feel like a ladder.

How many steps for 2.7 m floor to floor?

With a preferred riser of 180 mm, a 2,700 mm rise divides into exactly 15 risers of 180 mm and 14 treads. With a 250 mm going, the stair is 3.5 m long, the pitch is 35.8° and 2R + G is 610 mm, which meets the UK and Australian rules.

These results are estimates for planning and budgeting. Actual quantities depend on site conditions, products and workmanship. Check structural and code requirements with a licensed professional before you build.