STAIR STRINGER · REVIEWED MAY 2026 · BY BRENT

STAIR STRINGER

Q: What is the IRC code for stair rise and run?

Per IRC R311.7.5: maximum riser height is 7-3/4 inches and minimum tread depth is 10 inches measured nose to nose. The largest riser cannot vary more than 3/8 inch from the smallest within a flight. The "rule of comfort" target for a usable stair is 2R + T between 24 and 25 inches, where R is unit rise and T is unit run. This calculator flags violations on each side of those limits.

Q: How long of a 2x12 do I need for a stair stringer?

Take the diagonal stringer length the calculator returns, round up to the nearest standard length, and add at least 12 inches for the seat cut at the bottom and the plumb cut at the top. A 9-foot rise with a 14-tread layout typically needs a 16-foot 2x12 even though the diagonal is only ~14 ft. Always lay out the cuts on paper before going to the lumberyard. A framing square with stair gauges makes the layout repeatable across all stringers. This sizing is an estimate — confirm with your local building inspector before cutting.

Q: Should I use 2 or 3 stringers?

For typical 36-inch-wide residential stairs, 3 stringers are recommended (one per edge plus a middle stringer). For wider stairs (over 36"), add an extra stringer per 16 inches of width. 2x12 is the standard size; 2x10 is allowed on shorter runs but leaves only ~3-1/2" of throat after the cuts, which gets fragile. Always verify spacing and stringer size with your inspector and engineered drawings.

L = √(rise² + run²)

Total rise (floor to floor)?

Number of risers?

Tread depth (run)?

RESULT

—

FILL IN ABOVE

IRC R311.7: max riser 7-3/4", min tread 10". This is an estimate — verify with a licensed structural engineer or local building inspector. Not a substitute for engineered drawings.

RECOMMENDED TOOLS

Carpentry Tools we recommend for projects like this

→

About this calculator

Side profile of a staircase stringer with rise and run dimensions labeled on individual treads

This stair stringer calculator gives you the four numbers a framer or DIY builder needs before cutting a stringer: unit riser height, unit tread depth, total horizontal run, and the diagonal stringer length. Enter the total rise (finished floor to finished floor), the number of risers you want, and your tread depth, and the calculator returns each value plus an IRC R311.7 code check (max riser 7-3/4", min tread 10", target 2R + T = 24"–25" for comfortable stride). The stringer length is the Pythagorean diagonal — buy a 2x12 at least one foot longer to leave room for the seat cut at the bottom and the plumb cut at the top. ESTIMATE ONLY — verify with your local building department before cutting.

How to use this calculator

Measure the total rise from the finished surface of the lower floor to the finished surface of the upper floor (or deck) in inches. For a typical basement-to-main-floor run, this is around 100–110 inches. Decide on the number of risers — IRC R311.7 caps individual riser height at 7¾ inches, so total rise ÷ 7.75 gives you the minimum riser count. Most installs use 13–15 risers for residential floor-to-floor runs.

Enter your tread depth (run): IRC minimum is 10 inches; 10½ or 11 inches gives a more comfortable step. The calculator returns unit riser height, total horizontal run, the diagonal stringer length, the IRC code check (riser ≤ 7¾, tread ≥ 10), and the 2R+T stride check (target 24–25 for natural walking rhythm). Buy a 2x12 at least one foot longer than the calculated stringer length to leave room for the seat cut at the bottom and plumb cut at the top.

Worked example

For a 108-inch total rise, 14 risers, 10½-inch tread:

Unit riser height: 108 ÷ 14 = 7.71 inches. Treads: 14 − 1 = 13 (one less than risers since the top step is the upper floor). Total horizontal run: 13 × 10.5 = 136.5 inches (11.4 ft).

Stringer length (diagonal): √(108² + 136.5²) = √(11,664 + 18,632) = √30,296 = 174 inches = 14.5 ft.

Stride check: 2 × 7.71 + 10.5 = 25.92 — slightly above the comfortable 24–25 target, but within IRC. Code check: passes (riser 7.71 ≤ 7.75, tread 10.5 ≥ 10).

Buy: one 2x12 at least 16 ft long for each stringer. Standard residential stairs use 3 stringers (one per side, one center) for 36-inch-wide stairs. Total stock: 3 × 16-ft 2x12s, plus tread material (13 treads × 36 inches = 39 linear feet of 2x12 or 5/4 deck board).

Common mistakes & waste factors

Mixing riser heights. The first or last riser slightly off (¼ inch) is the #1 cause of stair-related falls. Cut all stringers identically and shim the bottom plate or top hanger if needed.

Skipping the bottom-step adjustment. The bottom of the stringer sits on the lower floor; the seat cut for the first tread needs to account for the tread thickness so the first riser height matches the others. Forget this and the bottom step is 1 inch tall (the top of the seat cut) when it should be 7.71 inches.

Using a single stringer in the center for wide stairs. Stairs over 36 inches wide need 3+ stringers; over 48 inches need 4. A single center stringer plus two outer stringers is the residential standard for 36-inch stairs.

Building stairs without checking IRC compliance. Riser > 7¾, tread < 10, or 2R+T outside 24–25 will fail inspection. The calculator checks all three; trust the warning if it fires.

Rules of thumb

IRC R311.7: max riser 7¾ inches, min tread 10 inches.

Stride formula: 2R + T = 24 to 25 inches (tested for natural walking gait).

Stringers: 2x12 #1 grade or better. One 2x12 per 12–16 inches of stair width. 36-inch-wide stairs need 3 stringers.

Total rise budget: typical floor-to-floor for an 8-foot ceiling + joist depth = 105–115 inches. Plan for 13–15 risers.

Always buy stringer stock at least 1 foot longer than the calculated diagonal — the seat and plumb cuts eat material.

Common questions

Tool and material links below are affiliate links — we may earn a small commission if you buy, at no extra cost to you.

What is the IRC code for stair rise and run?

Per IRC R311.7.5: maximum riser height is 7-3/4 inches and minimum tread depth is 10 inches measured nose to nose. The largest riser cannot vary more than 3/8 inch from the smallest within a flight. The "rule of comfort" target for a usable stair is 2R + T between 24 and 25 inches, where R is unit rise and T is unit run. This calculator flags violations on each side of those limits.

How long of a 2x12 do I need for a stair stringer?

Take the diagonal stringer length the calculator returns, round up to the nearest standard length, and add at least 12 inches for the seat cut at the bottom and the plumb cut at the top. A 9-foot rise with a 14-tread layout typically needs a 16-foot 2x12 even though the diagonal is only ~14 ft. Always lay out the cuts on paper before going to the lumberyard. A framing square with stair gauges makes the layout repeatable across all stringers. This sizing is an estimate — confirm with your local building inspector before cutting.

Should I use 2 or 3 stringers?

For typical 36-inch-wide residential stairs, 3 stringers are recommended (one per edge plus a middle stringer). For wider stairs (over 36"), add an extra stringer per 16 inches of width. 2x12 is the standard size; 2x10 is allowed on shorter runs but leaves only ~3-1/2" of throat after the cuts, which gets fragile. Always verify spacing and stringer size with your inspector and engineered drawings.