Deck Stair Calculator
This deck stair calculator finds the number of risers and treads, the riser height, the total run, the stringer length and angle, the number of stringers and the board to buy. It draws your stairs from the side, checks them against common limits and estimates the cost. It works as a stair stringer calculator and a stair rise and run calculator for any deck height. Tap the i button beside any field for an explanation.
Quick answer: risers = total rise ÷ about 7.25 in, rounded to a whole number. Treads = risers − 1. A deck 42 in high needs 6 risers of 7.00 in and 5 treads. With 11 in treads, the stringer is 69.2 in long.
Deck stair calculator: enter your deck height
Example values are loaded so you can see a result straight away. Change any field and the numbers and the diagram update.
1. Choose your units
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2. Your stairs
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3. Stringers and treads
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4. Prices (optional)
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Side view of your stairs
The diagram redraws as you change the numbers. The dashed orange line is the stringer length.
Checks against common limits
Riser height by number of risers
Total run by tread depth
Estimates for planning only. Prices shown are examples you can edit, not quotes. Stair rules vary by location, so confirm them with your local building department before you build. See our Disclaimer.
How the deck stair calculator works
Deck stairs look simple until you stand at the bottom with a tape measure and a framing square. The deck is a fixed height above the ground, every step has to be the same size, and the stringer has to be cut so that the last step lands exactly on the deck. A small error at the start adds up over the whole flight, and you only find out after the boards are cut.
This deck stair calculator does that arithmetic for you. Enter the total rise, which is the height from the ground to the top of the deck surface, along with the width, the tread depth and the riser height you would like. It works out how many risers and treads you need, the exact riser height, the total run, the length of the stringer and the board to buy. It also shows the stair angle, the shorter cut for the bottom step, how many stringers you need, and a rough material cost. A side-view drawing updates as you type, and a list of checks tells you when a number goes past a common limit.
The limits in the calculator are the common ones. Your building department has the final say.
How to calculate stairs: the step-by-step method
Every stair calculation comes from one fact: the total rise has to be split into equal steps. The same four steps work for a deck, a porch or an indoor flight, and the calculator follows them in this order.
- Measure the total rise. Measure straight down from the top of the deck surface to the ground, or to the top of the landing, where the stairs will end. Measure at the spot where the stairs will land, because ground often slopes.
- Divide by a target riser height. A riser is the vertical part of one step. Divide the total rise by about 7 1/4 inches and round to the nearest whole number. That is the number of risers.
- Find the exact riser. Divide the total rise by the whole number of risers. This is the real height of every step, and it is rarely the same as your target.
- Count the treads and find the run. A deck stair has one fewer tread than risers, because the deck surface is the top step. Multiply the number of treads by the tread depth to get the total run.
Worked example: a deck 42 inches high
Start with a total rise of 42 inches. Divided by 7.25, that is about 5.8, which rounds to 6 risers. Dividing 42 by 6 gives a riser of 7.00 inches. There are 5 treads, and at 11 inches each the total run is 55 inches. These are the numbers the calculator loads when you open the page.
Rise and run: how risers and treads fit together
The rise is how far the stairs go up and the run is how far they go out. A stair rise and run calculator links the two, because making the stairs steeper shortens the run and making them gentler lengthens it.
Two limits shape the choice. The International Residential Code, as summarised by Viewrail and the Knox County stair guide, allows a maximum riser of 7 3/4 inches and a minimum tread of 10 inches. Riser and tread sizes within one flight should not vary by more than 3/8 inch. When the calculator finds that your target riser would go over 7 3/4 inches, it adds a riser so the result stays within the limit.
Comfort matters as much as the limit. Trex notes that 7 to 7 1/2 inches is typical for a deck riser, and the North American Deck and Railing Association suggests 7 inches. A shallow riser adds steps and run, so it needs more space. The first bar chart shows how the riser height changes as you add or remove a step.
Tread depth and nosing
The tread is the part you step on. An 11 inch tread is common because the code only asks for a nosing, a small overhang at the front edge, when the tread is under 11 inches. In that case the nosing should be between 3/4 and 1 1/4 inches. Trex gives the example of two boards with a 3/16 inch gap, which makes a tread of about 11 3/16 inches. The second chart in the calculator shows how the total run changes as the tread gets deeper.
How many steps for each deck height
If you only want a quick answer, this table shows the results for common deck heights with 11 inch treads and a target riser of 7 1/4 inches. The calculator uses the same method.
| Deck height (total rise) | Risers | Treads | Riser height | Total run (11 in treads) | Stringer length | Angle |
|---|---|---|---|---|---|---|
| 24 in | 4 | 3 | 6.00 in | 33 in | 40.8 in | 28.6° |
| 30 in | 4 | 3 | 7.50 in | 33 in | 44.6 in | 34.3° |
| 36 in | 5 | 4 | 7.20 in | 44 in | 56.9 in | 33.2° |
| 42 in | 6 | 5 | 7.00 in | 55 in | 69.2 in | 32.5° |
| 48 in | 7 | 6 | 6.86 in | 66 in | 81.6 in | 31.9° |
| 54 in | 7 | 6 | 7.71 in | 66 in | 85.3 in | 35.0° |
| 60 in | 8 | 7 | 7.50 in | 77 in | 97.6 in | 34.3° |
| 72 in | 10 | 9 | 7.20 in | 99 in | 122.4 in | 33.2° |
The table stops at 72 inches. Professional Deck Builder gives 147 inches as the most rise in one flight, and the code checks below explain what happens after that.
A taller deck needs a longer run across your yard, so check that you have the space before you settle the deck height.
Stringer length: the hypotenuse of your stairs
The stringer is the sloped board that carries the steps. Its length comes from the Pythagorean theorem. The total rise is one side of a right triangle, the total run is the other, and the stringer is the long side:
Stringer length = square root of (total rise² + total run²)
In the 42 inch example, the rise is 42 inches and the run is 55 inches, so the stringer measures 69.2 inches, or about 5.77 feet, along its long edge. You cannot buy a board that exact length, so the calculator adds 6 inches for the end cuts and picks the shortest stock size that covers it. That is my own allowance and not a code figure, so give yourself more if you want room to adjust. For this example the board to buy is 8 feet long. The calculator offers 6, 8, 10, 12, 14 and 16 foot lengths. If your stairs need a longer board than that, the result says so.
The angle of the stairs
The stair angle comes from the riser and tread: angle = arctangent of (riser ÷ tread). With a 7.00 inch riser and an 11 inch tread it is 32.5 degrees. Trex describes 35 to 37 degrees as comfortable. Your result may be a little lower, which means a gentler climb that needs more run.
How to cut stair stringers
A good layout makes the cutting easy. Sources such as Trex and the Decks.com guides use the same basic method.
- Set a framing square. Put the riser measurement on one leg and the tread measurement on the other. Stair gauges clamped to those marks let you repeat the layout without measuring each time.
- Trace the steps. Start at the top of the board and trace along the square, sliding it down the board for each step. Mark all the notches, one for each tread.
- Cut the bottom shorter. The tread sits on top of the stringer, so the bottom riser must be cut shorter by the tread thickness. If you skip this, the first step will be too tall by exactly that thickness. In the example the bottom riser is cut to 6.00 inches.
- Cut the notches. Use a circular saw for the lines and finish the corners with a handsaw.
- Use the first stringer as a template. Trace it onto the other boards so they match.
Check the uncut depth
Cutting notches removes wood. The part under the notch is what carries the load. Professional Deck Builder recommends at least 5 inches of uncut depth. The calculator finds it for your stairs using the lumber you pick. In the 42 inch example, a 2 × 12, which is 11 1/4 inches deep, leaves about 5.34 inches. A 2 × 10 leaves less, so the check list tells you when you should choose the larger board.
How many stringers do you need?
The stringers hold up the treads, so their spacing depends on the decking you use. DCA 6, the Deck Construction Guide, allows up to 18 inches between stringers, and NADRA recommends 12 inches. Check the instructions for your decking, because some makers ask for tighter spacing.
To find the number of stringers, divide the stair width by the maximum spacing, round up, and add one for the last board. The calculator then spaces them evenly. A 36 inch wide stair with a 16 inch maximum needs 4 stringers about 12 inches apart. A wider stair needs more.
The span matters too. A cut 2 × 12 stringer should not span more than about 6 feet horizontally without support. Past that, add a mid-span post or change the design. The calculator warns you when your total run is longer than that.
How much do deck stairs cost to build?
The calculator estimates the lumber for the stringers, the tread boards and a connector at the top of each stringer. It uses prices that you can edit, so the figures are only examples and not a quote. Here is what the example comes to:
| Item | Quantity | Example price | Cost |
|---|---|---|---|
| Stringers (2 × 12 boards) | 4 | $22 each | $88 |
| Tread boards, 2 per tread | 30 ft | $2.50 per ft | $75 |
| Stringer connectors | 4 | $6 each | $24 |
| Total | $187 |
These totals leave out railings, footings, fasteners and the landing pad. Replace the example prices with a local quote.
Code checks the calculator makes
The results panel lists each check in green or red. These are the limits behind it:
| Item | Limit | Where it comes from |
|---|---|---|
| Maximum riser height | 7 3/4 in | IRC R311.7.5.1 and DCA 6 |
| Minimum tread depth | 10 in | IRC R311.7.5.2 and DCA 6 |
| Variation between risers or treads | 3/8 in | IRC |
| Nosing, when the tread is under 11 in | 3/4 to 1 1/4 in | IRC R311.7.5.3 |
| Stringer spacing | 18 in maximum, 12 in recommended | DCA 6 and NADRA |
| Span of a cut 2 × 12 stringer | 6 ft horizontal | Professional Deck Builder, Simpson Strong-Tie |
| Uncut depth left under the notch | 5 in minimum | Professional Deck Builder |
| Landing at the bottom | 36 in solid surface | Simpson Strong-Tie, Professional Deck Builder |
A flight with a total rise of more than 147 inches needs a landing. Simpson Strong-Tie also says the bottom of the stairs should land on a solid surface such as concrete or pavers, extending about 36 inches from the bottom tread, so the stringers do not sit on soil. If your stairs need a pad, the concrete slab cost calculator and the paver base calculator can help.
Codes differ from place to place. Handrails, guardrails and permit rules are not part of this calculator, so check them locally.
Common mistakes with deck stairs
- Measuring the rise from the wrong place. Measure where the stairs will land, because ground often slopes.
- Forgetting the bottom riser adjustment. This is the most common error. Without the shorter bottom cut, the first step is taller than the rest, which is a trip hazard.
- Letting risers differ. Keep every riser within 3/8 inch of the others.
- Ignoring the landing surface. Simpson Strong-Tie calls for a solid surface under the bottom of the stairs.
- Cutting into the stringer too deeply. JLC’s guide to common deck stair defects highlights notches that leave too little wood. Check the uncut depth.
- Skipping the permit check. Ask your building department before you buy materials.
Deck stair calculator FAQ
How many steps do I need for a 42-inch deck?
Divide 42 by a riser of about 7.25 inches and round to a whole number. That gives 6 risers of 7.00 inches, with 5 treads, because the deck surface is the top step.
What riser height is best for deck stairs?
The IRC allows a maximum of 7 3/4 inches. Trex notes that 7 to 7 1/2 inches is typical, and the North American Deck and Railing Association (NADRA) suggests 7 inches. A lower riser feels easier to climb, but it needs more steps and more run.
How wide should deck stair treads be?
The minimum is 10 inches from nose to nose. A tread of 11 inches or more does not need a nosing. Trex gives the example of two 5 1/2 inch boards with a 3/16 inch gap, which makes about 11 3/16 inches.
How many stringers do I need for deck stairs?
Divide the stair width by your maximum spacing, round up, and add one. DCA 6 allows up to 18 inches between stringers, while NADRA recommends 12 inches. A 36 inch wide stair at 16 inch spacing needs 4 stringers.
Why is the bottom step on a stringer cut shorter?
The first tread sits on the stringer, and the bottom of the stringer rests on the ground. If the bottom riser were cut at full height, adding the tread would make the first step too tall. So the bottom riser is cut shorter by the tread thickness, and every finished riser comes out equal.
What size lumber should I use for stair stringers?
Trex describes pressure-treated 2 × 12 lumber. The check that matters is the depth of uncut wood left under the notch, which Professional Deck Builder says should be at least 5 inches.
How far can a stair stringer span?
A cut 2 × 12 stringer is limited to about 6 feet of horizontal span between supports, according to Simpson Strong-Tie and Professional Deck Builder. A longer run needs a support, such as a mid-span post, or a different design.
How many treads are there compared with risers?
There is one fewer tread than risers on a deck stair, because the deck surface works as the top tread. A stair with 6 risers has 5 treads.
When do deck stairs need a landing?
Professional Deck Builder gives 147 inches as the most rise in one flight before a landing, and says a landing should be at least 36 inches deep. Simpson Strong-Tie also calls for a solid surface, such as concrete or pavers, 36 inches out from the bottom tread.
What angle should stairs be?
It is set by the riser and tread, so you choose it by choosing those two numbers. The calculator shows the angle for your stairs, and the article above gives a comfortable range.
Do I need a permit to build deck stairs?
Rules differ by city and county, and stairs are often reviewed together with the deck. Ask your local building department before you buy materials, and ask which code edition applies.
Sources and further reading
The geometry is standard right-triangle maths. The limits come from the published guides below, which I read when writing this page. Where they differ from your local code, follow your local code.
- Trex, how to calculate and cut stair stringers (riser and tread examples, bottom riser adjustment, angle)
- Viewrail, tread depth and risers (IRC riser and tread limits, nosing)
- Simpson Strong-Tie, building safe and sturdy deck stairs (stringer span, landing surface)
- JLC, common defects in deck stairs (stringer notches and uncut depth)
- Knox County Codes Administration, stairs guide (residential stair limits)
Lumber sizes, prices and local rules vary, so check them against your supplier and your building department.
Plan the rest of the project
Stairs are usually one part of a bigger build. The deck board calculator counts the boards for the deck itself, and the rafter calculator and roof pitch calculator help if you are adding a cover. Planning a border or fence? Try the fence cost calculator. For footings and landing pads, see the concrete calculators hub.