Stair Stringer Calculator

How the Stair Stringer Calculator Sizes a Staircase

A stringer is cut from one number: the total rise, measured from the finished lower floor to the finished upper floor. Everything else follows from it. This stair stringer calculator divides the rise by your target riser to get a whole number of risers, then re-divides the rise by that count so every riser is exactly equal — which is what a building inspector checks first.

On a 110 in total rise with a 7 in target riser and an 11 in tread run, that gives 16 risers of 6⅞ in, 15 treads, a total run of 13 ft 9 in and a stringer line of 16 ft 6⁵⁄₁₆ in at 32.0°. Every figure on this page comes from those same formulas, so you can reproduce all of them in the calculator above.

Why Your Target Riser Is Almost Never the Riser You Get

The target riser is the most misread field on any stair calculator. It is not the riser height you will build — it is only a hint used to pick the number of risers. Once that count is fixed, the real riser is the rise divided by it, and the target is discarded. Because the count must be a whole number, wide bands of targets collapse onto the same answer:

All six rows are the same 110 in rise. Typing 6¾ or 7 gives an identical staircase, and so does 7¼ or 7½ — the target only matters when it crosses the boundary where the rounded count changes. If you want a specific riser height, work backwards: pick the riser count you want and check that rise ÷ count lands where you expect.

Setting the Target at the Code Maximum Backfires

The 2026 residential rule most people know is the 7¾ in maximum riser height in section R311.7.5.1 of the International Residential Code, alongside a 10 in minimum tread depth in R311.7.5.2. The instinct is to type 7¾ into the target riser to get the fewest, steepest steps allowed. That instinct produces a staircase that breaks the rule.

The same rise with a target of 8 in is worse: 14 risers of 7⅞ in again, but now 2 × riser + run = 26.7, well outside the 24–25 comfort band the calculator checks. That band is the old Blondel rule, and it is the difference between a staircase that feels normal and one people trip on.

Throat Depth: Why a 2×10 Stringer Comes Up Short

The throat is the timber left behind the notches — the narrow spine that actually carries the load once the triangles are cut out. It is not the board width. Cutting a step removes a wedge measured along the slope, so the throat is the board width minus (riser × run) ÷ √(riser² + run²). On the 110/7/11 staircase that wedge is 5.83 in of timber gone from every board:

The 2×10 is the row worth staring at. It misses the calculator’s 3½ in threshold by one sixteenth of an inch — close enough that it looks fine on a cut list and fails on the saw. For an 11 in run, a 2×12 is effectively the smallest board that works, and a 2×8 is not in the conversation at 1⁷⁄₁₆ in of remaining timber.

Standard vs Flush Mount: One Tread of Difference

The mount setting changes how the top of the stringer meets the landing, and it changes the tread count by exactly one. On a standard mount the upper floor itself is the last tread, so treads = risers − 1. On a flush mount the stringer hangs so its top tread sits level with the floor, so treads = risers. Same rise, same riser height, different footprint:

That is 11 more inches of floor consumed and 9¼ in more stringer, for the identical staircase. In a tight basement or against a deck’s frost line, one extra tread is often the difference between fitting and not — so set the mount before you argue with the rise.

Stringer Length, Slope Angle and the Board You Buy

The stringer length is the plain hypotenuse of the whole staircase: √(total rise² + total run²). It is the cut line along the back of the board, not the length of timber you should order — the notches need material beyond the line at both ends, and lumber is sold in even foot lengths. The calculator rounds up accordingly, which is why a 16 ft 6⁵⁄₁₆ in stringer asks for an 18 ft board rather than a 17 ft one that does not exist.

The slope angle comes from a single step, not the whole flight: arctan(riser ÷ run). At 6⅞ in over 11 in that is 32.0°, which sits in the comfortable domestic range. If your drawings or your site work in gradients rather than degrees, the degree to percentage calculator converts that angle to a percentage grade, and the slope percentage calculator goes the other way from a rise-over-run measurement.

The Bottom Riser Cut Everyone Forgets

Cut all sixteen notches identically and your finished staircase will be wrong at both ends. Once the treads go on, every step gains the thickness of a tread — except the bottom one, which sits on the slab with nothing under it. The fix is to trim the bottom of the stringer by exactly one tread thickness before it is set down.

How to Use the Stair Stringer Calculator

1. Measure the total rise, finished floor to finished floor

   Measure from the surface you will stand on at the bottom to the surface you will stand on at the top — including any tiles, screed or decking not yet laid. This one number drives everything else, so measure it twice. Use the picker beside the box to work in inches or centimetres.

2. Set a target riser and a tread run

   About 7 in is the comfort sweet spot for the riser. For the run, type the depth cut into the stringer — any nosing overhang is extra and does not belong in this field.

3. Choose Standard or Flush mount

   Standard treats the upper floor as the top tread; flush adds one more tread and about one run of extra floor space. Set this before you judge whether the staircase fits.

4. Enter your tread thickness and stringer board

   The tread thickness produces the bottom riser cut. The board width drives the throat check — pick the size you actually intend to buy, or choose “Skip throat check” if you are using a mono stringer with no notches.

5. Read the checklist, not just the headline

   The results panel lists the riser, tread run, comfort figure and throat side by side with the limits each is measured against. Open the step-by-step working underneath to see every division and square root that produced the numbers.

Worked Examples

Every row is reproducible in the calculator above, all with a 1 in tread thickness on a 2×12 board:

The basement row is the one to copy. A 105 in rise divides by 15 to give a riser of exactly 7 in with no remainder — the rare case where the target and the result agree, because the rise happens to be a clean multiple. When a rise divides evenly like that, the staircase is easier to cut and easier to check.

Stair Stringer Calculator: FAQ

Code figures on this page refer to the 2021 International Residential Code, section
R311.7.5, Stair treads and risers,
as published by the International Code Council. Many jurisdictions amend it — the
City of Boise treads and risers handout
is one example of how a local authority restates the same section. Always confirm the figures that apply to your job with your own building department before cutting.