Stair Stringer Calculator
Lay out each stringer: riser height, throat, cut marks, and the board to buy.
Code reference
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Riser height
7 3/4 in maximum
IRC R311.7.5.1 (2024: R318.7.5.1)
Residential stair riser. The IRC does not set a minimum.
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Tread depth
10 in minimum
IRC R311.7.5.2
Measured horizontally from nosing to nosing.
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Nosing
3/4 to 1 1/4 in
IRC R311.7.5.3 (2024: R318.7.5.3)
Required when the tread is under 11 inches.
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Headroom
6 ft 8 in minimum
IRC R311.7.2
Checked only when you enter a headroom measurement.
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Stair width
36 in minimum
IRC R311.7.1
Clear width above the handrail height.
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Stringer throat
5 in minimum outdoors
AWC DCA 6 Figure 28
Deck stairs. Indoors the same number is a warning.
Code checks reference the 2021 International Residential Code (section numbers for the 2024 edition shown in parentheses). States and cities adopt different editions and often amend them, so limits in your area may differ.
Based on the IRC model code. Your local jurisdiction may adopt a different edition or amendments — confirm with your building department.
How to measure
Total rise is the vertical distance from the finished surface you step off to the finished surface you step onto. On an indoor stair that is finished floor to finished floor. On a deck it is the top of the finished decking down to the finished landing. Set a straightedge or a level on the upper surface and drop a tape to the lower surface, and read the inches. Measure both sides of the opening. If they differ, enter the taller one so the risers still fit the high side. Do not measure from a subfloor on one end and a finished floor on the other, and do not stop the tape on a threshold, a mat, or a temporary block. Record the number before you pick a board. The page turns that height into equal risers, then into the sloped length of the stringer and the throat that remains after the cuts. Write the inches down next to the stair, including the fraction, so the number you type matches the tape and not a rounded guess.
Formula
The number of risers is the total rise divided by the maximum riser, rounded up to a whole number. Riser height is the total rise divided by that count, so every riser is the same. With the top tread below the floor, the tread count is one less than the riser count. The total run is the tread count times the tread depth. Stringer length is the riser count times the diagonal of one riser and one tread. Throat is the actual board width minus the depth of that triangular cut, measured square to the slope. The page rounds the stringer length up to the next stock length from 8 feet to 20 feet. The first cut, where the stringer meets the lower floor, is the riser height minus the tread thickness.
A 40.5 inch rise with a 10.25 inch tread is 6 risers at 6.75 inches, a 73.64 inch stringer, and a 5.61 inch throat. The stock length to buy is 8 feet.
Common mistakes
- Measuring to the subfloor at one end and the finished floor at the other.
- Using the nominal width of a 2x12 instead of the actual 11 1/4 inches.
- Setting the riser count below the minimum so a riser exceeds 7 3/4 inches.
- Leaving the spacing at 18 inches after switching to a composite tread with a shorter maximum.
- Forgetting that flush with the floor adds one tread and a longer run.
FAQ
How do I measure total rise?
Measure from the finished surface you leave to the finished surface you step onto, and record inches. If the two sides differ, use the taller measurement so every riser still clears the high side. Do not mix a subfloor at one end with a finished floor at the other. A deck stair is the same measurement: finished decking down to the finished landing.
How many risers does the page use?
The page divides total rise by your maximum riser, usually 7 3/4 inches, and rounds up to a whole number of risers. Each riser is then total rise divided by that count, so the steps match. You can step the count up. You cannot step it below the minimum, because that would push a riser over the maximum. The stair calculator can pass a count in the link, and this page keeps that count.
What is stringer throat?
Throat is the wood left after the triangular steps are cut from the board. The page takes the actual width of a 2x10 or a 2x12 and subtracts the depth of the step cut, measured square to the slope. For an outdoor stair, AWC DCA 6 Figure 28 calls for at least 5 inches of throat. A thinner throat is a fail outdoors and a warning indoors. The drawing shows the throat as a dashed line.
What board do I buy?
The sloped length is the number of risers times the diagonal of one riser and one tread. The page rounds that length up to the next stock length from 8 feet through 20 feet. Three stringers is the minimum. A wider stair adds one stringer for each spacing of the tread material, and the spacing cannot exceed the material maximum. The shopping list shows the count and the length. A stringer longer than 20 feet has no stock length.
Does a passing check mean the stair is legal?
A passing row means the number meets the model-code limit printed beside it. Outdoor throat, span, and total rise follow AWC DCA 6 Figure 28, and the riser, tread, nosing, headroom, and width follow the 2021 IRC, with the 2024 section in parentheses. Your city may adopt another edition or amend a number. A pass is not a permit, a structural design, or a substitute for the local inspector. Confirm the adopted code before you cut.
What is the difference between the top tread settings?
Subflush, the default, leaves the top tread below the upper floor, so the floor itself is the last tread and the stringer has one fewer tread than risers. Flush puts a tread level with the upper floor, so the tread count equals the riser count and the run is one tread longer. Throat, stringer length, and the cut marks do not change between the two. Pick the one that matches how the stair lands.
Related tools
- Plan rise, run, and headroom first Plan rise, run, and headroom before you cut.
- Count outdoor stringers and tread boards Count stringers and tread boards for an outdoor stair.
- Space balusters on the open side Space balusters on an open stair or deck edge.
- Measure finished floor to finished floor Measure finished floor to finished floor before you lay out a stair.
- Mark the stringer before you cut Mark equal risers and treads, then check the throat before you cut.
By Lumber Calc Editorial Team · Verified against 2021/2024 IRC & AWC DCA 6 · Last updated Oct 24, 2026