Stair Calculator: Risers, Treads, Stringers and Landings

How to measure total rise

Every stair starts from one number: the total rise, measured from the finished floor at the bottom to the finished floor at the top. If the flooring is not down yet, add its thickness now. A stair laid out on bare subfloor ends up with a short top or bottom step once 3/4 in of hardwood or 20 mm of tile goes down, and that one odd step is what trips people.

Measure at the point where the stair will actually land, not at a corner of the room. Floors are rarely level over a long run, so on a deck or a garage with a sloping slab, hold a straightedge level from the top floor out over the landing spot and measure down from it.

How many stairs do I need?

Divide the total rise by a comfortable riser height and round to the nearest whole number. That is your number of risers. Then divide the total rise by that number to get the exact riser height, which will almost never be a round number, and that is fine.

• 105 in total rise at a 7.5 in target: 105 / 7.5 = 14 risers of exactly 7.5 in.
• 107 in total rise: 107 / 7.5 = 14.3, so 14 risers of 7.643 in (about 7-5/8 in).
• 2,600 mm in the UK at a 190 mm target: 2,600 / 190 = 13.7, so 14 risers of 185.7 mm.

The number of treads is one fewer than the number of risers, because the upper floor itself is the last step. The calculator never lets rounding push a riser over the code maximum: if it would, it adds a riser.

Rise and run rules: US IRC and UK Part K

Use the switch at the top of the tool to check against the rules where you build. Each line turns green or red, and failing items turn red on the drawing too.

Local amendments exist on both sides of the Atlantic, so your building department or Building Control has the final word. Commercial and public stairs follow stricter rules (IBC 1011 in the US, the general access tables of Part K in the UK) that this calculator does not check.

What makes a stair comfortable

A stair can pass code and still feel wrong. The test most carpenters use is two risers plus one tread: between about 24 and 25-1/2 in in the US, or 600 to 640 mm in the UK (the middle of Part K's 550 to 700 mm band). A 7.5 in riser with a 10.5 in tread gives 25.5 in, the classic house stair. Shorter risers need deeper treads to keep the same stride: 7 in risers pair with 11 in treads.

Steeper than about 40 degrees starts to feel like a ladder going down, which is why Part K caps private stairs at 42 degrees. Shallower than about 30 degrees eats floor space fast without feeling much easier.

How long does a stair stringer need to be?

The stringer is the diagonal of the rise and the run: the square root of rise squared plus run squared. For a 105 in rise over a 136.5 in run that is about 172 in, or 14 ft 4 in. Buy stringer stock at least a foot longer to allow for the plumb cut at the top and the level cut at the bottom. In the US that usually means a 16 ft 2x12.

Leave enough solid wood below the notches. A cut stringer is only as strong as the wood left under the deepest notch, and AWC's DCA 6 deck guide calls for at least 5 in of it on deck stairs.

Cutting the stringer: the cut list

1. Set the framing square. Put stair gauges at the riser height on the tongue and the tread depth on the blade.
2. Mark from the top. Trace one rise and one tread, slide the square along, and repeat for every tread in the cut list.
3. Drop the first step. Cut the tread thickness off the bottom of the stringer. Without that, the first step comes out taller than the rest by the thickness of the tread.
4. Use one template. Cut the first stringer, check it in place, then trace every other stringer from it so all of them match.

Stairs with a landing

Choose "With a landing" and the calculator splits the rise into two flights with a flat landing between them, and shows two stringer lengths. In the US a landing is required once a single flight would rise more than 151 in, and it must be at least 36 in deep in the direction of travel. In the UK every flight needs a landing at the top and bottom, at least as long as the stair is wide. Landings also let a stair turn 90 degrees (an L stair) when the straight run will not fit.

Deck stairs

Outdoor stairs usually use two deck boards per tread, which is why the deck setting starts with an 11-1/4 in tread. Stringers should sit no more than 16 in apart for 2x or 5/4 decking, so a 36 in stair needs four. Set the bottom of the stringers on a concrete pad or compacted gravel, never on bare soil, and fasten the top with hangers or a ledger rated for stairs. The joist span calculator counts the nails and screws for the deck frame by weight.

Concrete steps

For solid concrete steps, the calculator adds up the concrete in each step: the lowest step is one riser tall, the next two, and so on, across the full width, plus any top platform. It gives the volume and the number of 80 lb or 60 lb bags (25 kg or 20 kg in the UK) with 10 percent extra. Three 7 in risers with two 11 in treads, 48 in wide, come to about 6.4 cu ft. Anything much bigger is usually cheaper as ready-mix; the concrete bag calculator compares the two.

Stairwell opening and headroom

The opening in the upper floor has to be long enough that nobody walking up hits their head on its edge. The calculator works it out from the headroom (6 ft 8 in, or 2.0 m) plus the depth of the floor structure, divided by the riser height, times the tread depth. With 7.5 in risers, 10.5 in treads and a 10 in deep floor, the opening needs to be about 126 in long. The dashed blue line on the drawing is the headroom line: anything above the stair and below that line is too low.

In a UK loft conversion the headroom can drop to 1.9 m at the centre of the stair and 1.8 m at the side, which is often what makes a loft stair fit at all. The loft head height calculator checks the room it lands in.

Worked example: stairs computation in metric

Floor to floor height 2,800 mm, with a target riser of 175 mm:

1. Risers: 2,800 / 175 = 16 risers, each exactly 175 mm.
2. Treads: 16 - 1 = 15 treads. At 270 mm each, the total run is 15 x 270 = 4,050 mm.
3. Check: 2R + G = 2 x 175 + 270 = 620 mm, inside both the Part K band (550 to 700 mm) and the comfortable range (600 to 640 mm). Pitch = atan(175 / 270) = 33 degrees, under 42.
4. Stringer: the square root of 2,800² + 4,050² = 4,924 mm, so buy 5.2 m lengths.
5. Opening: (2,000 + 250) / 175 = 12.9, so 13 goings x 270 mm = 3,510 mm of opening for full 2.0 m headroom under a 250 mm floor.

Related tools

Adding a stair to a new floor? Size the joists with the joist span calculator, price a platform with the mezzanine floor calculator or a whole loft with the loft conversion cost calculator, and work out a roof's angle with the roof pitch calculator. To price the job, use the job costing calculator.