Stair Calculator

How to use this calculator

1. Choose the stair type. Wood deck adds the American Wood Council DCA 6 deck-stair checks: stringer size, span, throat, landing height and open risers. Interior checks the IRC geometry only.
2. Measure the total rise. This is the vertical distance from the finished lower floor to the finished upper floor (see below).
3. Set the max riser height. 7 3/4 inches is the IRC limit. A lower value gives more, shorter risers and a longer stair.
4. Enter the tread depth, the run per step measured nosing to nosing. 10 inches is the IRC minimum. If you have a fixed amount of floor space instead, switch to Fit an available run. The calculator then picks the riser count whose stride is closest to the 2R + T rule of thumb, with treads of at least 10 inches.
5. Enter the stair width, stringer lumber, nosing, riser type, tread boards (2x or 5/4) and riser-board thickness. These set the number of stringers, the throat check, the tread board size, the open-riser gap and how much to drop the stringer.
6. Measure the ground below. Guards and open risers depend on how far the stair sits above the floor or grade below and beside it, not on the total rise alone. Until you enter that height, those two checks ask for it instead of passing.

Results update as you type. Each check says whether it comes from the IRC, from DCA 6 (deck stairs) or from the 2R + T rule of thumb. The layout marks give the cumulative height of every step and its distance along the stringer.

How to measure total rise

Measure from finished floor to finished floor, the surfaces people actually walk on. Measuring from a bare subfloor or concrete slab is the most common reason a finished stair ends up with an odd first or last step.

• Add flooring that isn’t down yet. 3/4-inch hardwood upstairs and 1/2-inch tile downstairs change the rise by 1/4 inch, and any error you ignore lands on the first or last step.
• Don’t trust the lower floor to be level. Hold a long level or straight board level off the upper floor edge, then measure plumb down to where the bottom step will land.
• For deck stairs, measure from the top of the deck boards to where the bottom step will land, not to the grade a few feet away.
• Find the lowest point below. Separately, measure to the lowest point of the floor or grade under the stair and within 36 inches horizontally of each open side. The IRC guard and open-riser rules measure to this point, and on sloping ground it can be well below the bottom landing. A stair with a 28-inch total rise can still have open sides more than 30 inches above grade.

The formulas

Risers = total rise ÷ max riser height, rounded up

Riser height = total rise ÷ risers

Treads = risers - 1 (the upper floor is the top step)

Total run = treads × tread depth

Stringer length = √(total rise² + total run²)

Stair angle = arctan(riser ÷ tread)

Rounding the riser count up means no riser exceeds your maximum. The exact riser height is shown as a decimal and rounded to the nearest 1/16 inch, which is what you can read on a tape. Don’t just repeat that rounded number, though. The layout section explains why.

Worked example: 108-inch total rise

A basement stair with 108 inches from finished floor to finished floor, a 7 3/4-inch maximum riser and 10-inch treads:

• Risers: 108 ÷ 7.75 = 13.94 → 14 risers
• Riser height: 108 ÷ 14 = 7.714 in → 7 11/16 in to the nearest 1/16
• Treads: 14 - 1 = 13 treads
• Total run: 13 × 10 = 130 in (10 ft 10 in)
• Stringer length: √(108² + 130²) = 169 in (14 ft 1 in), plus 2 inches for squaring the ends = 171 in → buy 16-foot boards
• Stair angle: arctan(7.714 ÷ 10) = 37.6°
• 2R + T: 2 × 7.714 + 10 = 25.4 in

The rounding trap. 7 11/16 inches is slightly less than the true 7.714 inches. Step off 7 11/16 for 13 risers and you reach 99 15/16 inches. The top riser then has to make up the remaining 8 1/16 inches, which is over the 7 3/4-inch limit and 3/8 inch taller than the rest. Lay out from cumulative heights instead (7 11/16, 15 7/16, 23 1/8, 30 7/8 …). The calculator lists them under Cumulative layout marks.

Stride. 25.4 inches is just above the 24-25 inch range of the 2R + T rule of thumb, so the stride is slightly long. Deeper treads would make it longer. To shorten it, use more, lower risers. With a 7 1/4-inch max riser you get 15 risers of 7.2 inches, and with 10 1/2-inch treads 2 × 7.2 + 10.5 = 24.9 inches. That stair runs 14 × 10.5 = 147 inches (12 ft 3 in) and is less steep, at 34.4°.

2R + T describes the stride, not the steepness. Two stairs can both score 24.5 inches at different angles, so check the angle separately. Only the riser, tread and other IRC limits are code.

IRC residential stair requirements

The IRC stair rules apply to stairways inside or serving a house, porch or deck. The 2024 IRC (R318.7) excludes stairways not within or serving a building, porch or deck, such as garden steps, and stairs to unfinished attics and crawl spaces. We checked the 2024 values below against the ICC 2024 IRC text on October 9, 2026.

The 2024 IRC moved the stair rules from R311.7 to R318.7, and the handrail and guard rules to R320 and R321. As far as we could verify, the dimensions above are the same in both editions. The 2024 edition changed details such as the nosing-uniformity scope and some landing exceptions. Both nosing exceptions in the table are in the 2024 text (R318.7.5.3). Many jurisdictions still enforce the 2021 or an older edition, and some states, such as California, publish their own residential code. Ask your building department which one applies. Dimensions are measured without carpet, rugs or runners.

Laying out a stringer with cumulative marks

1. Don’t repeat a rounded riser. If you set stair gauges on a framing square to the rounded riser and step it off again and again, the rounding error adds up on every step. In the example above, the top riser ends up 8 1/16 inches.
2. Use the cumulative marks. For every step, the calculator lists its height above the lower finished floor and its distance along the stringer’s top edge from the first notch corner. Measure each distance with a tape from that one starting point, never from the previous mark. Use the framing square only to draw the rise and run lines at each mark.
3. Check with a story pole. Mark the cumulative heights on a straight board standing at the stair location. The tread lines on the stringer and the marks on the pole should agree.
4. Mark the end cuts: the top plumb cut where the stringer meets the header or rim, and the level cut at the bottom where it sits.
5. Drop the stringer. Trim the bottom so the first riser on the stringer is one tread thickness shorter than the rest: 1 1/2 inches for 2x treads, 1 inch for 5/4 boards. The shopping list gives the figure for your choice. Otherwise the first step ends up one tread thickness taller once the treads go on. Adjust for any flooring still to be installed.
6. Cut the notches with a circular saw, stop at the lines and finish the corners with a handsaw. Overcutting weakens the stringer.

Check the first stringer against the cumulative marks before you use it as a template for the others.

Deck stairs: what DCA 6 requires

The American Wood Council’s Prescriptive Residential Wood Deck Construction Guide (DCA 6, current edition based on the 2015 IRC) gives prescriptive rules for deck stairs built with No. 2 grade, preservative-treated or naturally durable lumber. These come from the deck guide, not the IRC, and they are not written for interior stairs.

• Stringers: 2x12 minimum. A cut stringer may span at most 6 ft 0 in horizontally with at least 5 in of wood left below the notches. A solid stringer may span up to 13 ft 3 in. A longer cut stringer needs a 4x4 post on a footing under it, or an intermediate landing. 2x10 stringers are outside DCA 6 and need a separate design or approval.
• Spacing: cut stringers no more than 18 in apart, and at least three when only cut stringers are used.
• Treads on cut stringers: Figure 29 says “treads: 2x_ or 5/4 board” with cut stringers no more than 18 in apart, and Table 6 lists 2x4 or 5/4 as the minimum cut-stringer tread. The calculator offers only those two materials.
• Risers, where used, are “1x material, minimum” (Figure 27), which is 3/4 in actual. In deck mode the calculator flags a thinner riser board as outside the prescriptive DCA 6 design, and the shopping list leaves it out with a note.
• Open risers: Figure 27 says a “riser may be open, but shall not allow the passage of a 4” diameter sphere”. Unlike the IRC rule, there is no 30-inch threshold, so in deck mode this applies to every riser.
• Height: you need an intermediate landing when the total height of the stairway exceeds 12 ft 0 in, which is stricter than the IRC’s 12 ft 7 in. The landing must be built as a non-ledger deck using the DCA 6 details. In DCA 6, a non-ledger deck carries its own vertical load but still relies on the house for lateral support. A free-standing deck does not rely on the house at all.
• Footing at grade: the stringers bear on 2x4 bearing blocks fastened to posts, and the posts bear on footings at least 12 in below undisturbed ground or below the frost line, whichever is deeper. Stringers must not bear on new or existing concrete pads or patios that are not founded at that depth.

For comparison, the 2024 IRC requires deck footings at least 12 in below the undisturbed ground surface (R507.3.2). Where the deck is attached to a frost-protected structure, the footings also need frost protection, for example by extending below the local frost line in Table R301.2 (R507.3.3). DCA 6 asks for the deeper of 12 in or the frost line in all cases, which is the more conservative rule.

That footing rule is often missed. A concrete pad at the bottom of the stair can serve as the landing the IRC requires, but under DCA 6 it can’t support the stringers unless it is founded at footing depth. If you pour a pad, our concrete slab calculator sizes the concrete. The stair footings are a separate item. For a deck stair of three or fewer risers that is not the required route from the egress door, the 2024 IRC allows a bottom landing just 36 in wide, whatever the stair width.

Tread boards and actual lumber sizes

A tread board must be at least as deep as the run per step plus the nosing. With closed risers, it depends on how they go together. If each riser board sits on the back of the tread below it, the tread also has to reach under that riser board, so add the riser thickness. The calculator sizes treads this way, the deeper of the two, and notes it in the shopping list. If your treads butt against the riser faces instead, run + nosing is enough.

Lumber is sold by nominal size. Under the American Softwood Lumber Standard (PS 20-25, Table 3), the actual dry sizes are:

• 2x6: 5 1/2 in wide. 2x8: 7 1/4 in. 2x10: 9 1/4 in. 2x12: 11 1/4 in.
• 2x lumber is 1 1/2 in thick, a 1x board 3/4 in and a 5/4 board 1 in (a 5/4 x 6 is 1 × 5 1/2 in).

These are minimum dry sizes, so check the boards you buy.

In the worked example, a 10-inch run with a 1-inch nosing and 3/4-inch riser boards needs 10 + 1 + 3/4 = 11 3/4 inches. That is more than one 2x12 (11 1/4 inches) or two 2x6s (11 inches), so use a 2x6 and a 2x8 side by side (12 3/4 inches), ripped to fit. With open risers, or treads that butt against the riser faces, 11 inches is enough, and one 2x12 ripped to 11 inches covers it. An 11-inch run with a 1-inch nosing and open risers needs 12 inches, again a 2x6 plus a 2x8. With 5/4 x 6 boards, 11 inches takes two boards and 11 3/4 inches takes three, ripped to fit. The calculator picks the board combination for your numbers and uses that board thickness for the open-riser gap and the stringer drop.

Open risers

The opening between open-riser treads is the riser height minus the tread thickness. With 2x treads (1 1/2 inches thick), that gap is more than 4 inches once risers are taller than 5 1/2 inches. With 1-inch (5/4) treads, it happens once risers are taller than 5 inches. A gap of 4 inches or less can’t pass a 4-inch sphere, and the calculator doesn’t flag it.

• IRC: openings more than 30 inches, measured vertically, above the floor or grade below must not pass a 4-inch sphere. Take the 108-inch example with open risers on a floor level with the bottom landing. The gap is 7 11/16 - 1 1/2 ≈ 6 3/16 inches, so it needs riser boards or filler strips from riser 4 up (4 × 7.714 = 30.9 inches). If the floor or grade under the stair is lower than the bottom landing, more risers count. That is why the calculator gives a firm answer only once you enter the ground below.
• DCA 6 (deck stairs): a riser “may be open, but shall not allow the passage of a 4” diameter sphere”, at any height. In deck mode, close every riser with a gap over 4 inches.

Tips and safety

• Check headroom before you cut. A beam or duct over the stair is the most common reason a layout has to change.
• Keep every step identical. Small differences in riser height are what make people trip. The 3/8-inch limit is a maximum, not a target.
• Add guards on open sides more than 30 inches above the floor or grade below, and check your code’s guard height and opening rules for stairs.
• Local codes vary. Stairs are a life-safety item. Confirm the requirements, and whether you need a permit, with your building department before you build.

Last reviewed Oct 9, 2026. See how we calculate. Results are planning estimates - confirm with your supplier and local code.