How to Calculate Stair Rise and Run

Formula

riser = total_rise / N. treads = N − 1. tread = total_run / (N − 1). Comfort check (inches): 2R + T ≈ 24–25.

Units

Work in inches for riser/tread. Convert feet of floor height carefully (9′ 1″ = 109″).

Step-by-step

1. Measure finished floor-to-floor rise accurately.
2. Divide by a target riser (~7.5″) to estimate N; adjust N until exact riser is in a comfortable/code range for your AHJ.
3. If total run is fixed, compute tread = run / (N − 1).
4. Check 2R + T and stair angle before cutting stringers.

Example — 109″ total rise (9′ 1″)

Target 7.5″ → 109/7.5 ≈ 14.5 → try 15 risers: 109/15 ≈ 7.27″. Treads = 14. If total run = 126″, tread = 126/14 = 9″. 2R+T = 14.54+9 = 23.54″ (a bit low — consider longer run if space allows).

Example — 8′ rise, 10′ run

Rise = 96″. 14 risers → 6.86″ each (often low). 13 risers → 7.38″. Treads = 12; tread = 120/12 = 10″. 2R+T = 14.77+10 = 24.77″ — comfortable for many residential plans.

Common mistakes

• Using rough framing height without finished floor thicknesses.
• Setting tread count equal to riser count (usually one less tread).

Practical notes

• Local code limits vary — treat ranges as planning aids.
• Next step: lay out stringers from the exact R and T (see layout guide).

Related calculator

Use the free Stair Stringer Calculator to run the same math with your field numbers.

Related tools

• Stair Stringer Calculator — Residential stringer layout from total rise and run.
• Baluster Calculator — Even baluster spacing for rails and decks.

Related guides

• How to Lay Out Stair Stringers — Translate rise/run math into a stringer cut layout.