How to Calculate Stair Stringer Length
Quick answer: measure the total rise, divide by 7.75 in for the step count, multiply treads by 10 in for total run, then take the hypotenuse: √(rise² + run²). A 10-ft rise works out to 16 steps, 160 in of run, and a stringer board about 200 in (16 ft 8 in) long.
Step 1 — Measure the total rise
The total rise is the full vertical drop the stairs must climb: from the top of the finished upper floor down to the top of the landing surface. Measure to the finished surface, not the subfloor — half an inch of flooring changes every step.
Hold the tape at the upper finished floor, drop a plumb or laser to the landing, and read the distance. Write it down in inches (a 10-ft rise is 120 in). If the stair has a middle landing, treat each flight as its own calculation.
Step 2 — Count the steps
Divide the total rise by your target rise per step. The framing standard is 7.75 in per step. A 120-in rise ÷ 7.75 = 15.5, so you round up to 16 steps.
Now divide the total rise by that whole number to get the actual rise per step: 120 ÷ 16 = exactly 7.5 in. Never leave the odd leftover in one step — every riser must be equal. That single "comfort step" at the bottom is the classic inspection failure.
Step 3 — Find the total run
Multiply the number of treads by your run per tread. Most residential stairs use a 10-in run (the horizontal depth of each step, measured nose to nose). With 16 steps and a 10-in run: 16 × 10 = 160 in (13 ft 4 in) of total run.
The total run is the horizontal distance the stringer spans. If your opening is shorter, you need steeper rise (within code) or a different layout — you can't just squeeze the treads.
Step 4 — Calculate the stringer length
The stringer is the hypotenuse of a right triangle: total rise on one leg, total run on the other. Stringer length = √(rise² + run²).
For our example: √(120² + 160²) = √(14,400 + 25,600) = √40,000 = 200 in (16 ft 8 in). Add 6–12 in of overhang for the top bearing connection and buy a 17-ft 2×12 so you have layout room.
Code limits to know
- Rise: maximum 7.75 in per step in most jurisdictions (IRC). Some locals allow 8 in for older homes.
- Run: minimum 10 in of tread depth, measured nose to nose.
- Equal steps: all rises in a flight must match within 3/16 in.
- Headroom: at least 6 ft 8 in above the nosing.
- Stringers: one per 16 in of width, minimum three.
Common framing mistakes
- One odd bottom step — recalculate the whole flight so every rise matches.
- Measuring to the subfloor — your flooring thickness shifts every step by that amount.
- Forgetting the top bearing — the stringer hangs into the header; add overhang or you'll cut it short.
- Two stringers on a 36-in stair — treads flex and squeak. Use three.
- Buying the board exactly at the hypotenuse — you need extra for layout, cuts and bearing.
Ready to cut?
Run your numbers through the stair stringer calculator — it returns steps, rise, run and board length in seconds, then mark your stringer with a framing square set to your rise and run. Verify against local code before you cut the good lumber.