Stair stringer calculator
Total rise, maximum riser and tread depth: you get the number of steps, exact riser height, total run, stair angle and stringer length.
Stringer length
4.76 m
- Risers15 unit
- Actual riser height18 cm
- Treads14 unit
- Total run3.92 m
- Stair angle34.56 °
- Comfort rule 2R + T64 cm
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How it's calculated (visible formula)
risers = ceil(total rise ÷ max riser); R = rise ÷ n; run = (n − 1) × T; stringer = √(rise² + run²)
- Net geometric measurement, no waste.
- Adjustable waste % for cuts and breakage.
- Purchase rounded up to sellable units (boxes, bags, buckets, boards).
How to calculate it, step by step
- Measure total rise between finished floors.
- Divide by maximum riser height and round up the number of risers.
- Find the actual riser: total rise ÷ risers.
- Multiply treads (risers − 1) by tread depth for total run.
- Get stringer length and check the 2R + T comfort rule.
Worked example: 106 in total rise, 7¾ in max riser, 10 in tread
- 106 ÷ 7.75 = 13.7 → 14 risers of 7.57 in
- 13 treads × 10 in = 130 in run
- √(106² + 130²) ≈ 168 in
→ Stringer about 14 ft long (cut from 16 ft stock), 39°.
Common mistakes
- Counting as many treads as risers.
- Measuring rise before finished flooring on both levels.
- Uneven first or last riser — a classic trip hazard.
Pro tips
- Drop the stringer by one tread thickness at the bottom when laying it out.
- Use stair gauges on a framing square so every step matches.
- Check headroom (at least 6 ft 8 in).
- Add a handrail on stairs with four or more risers.
From calculation to quote
Add labour, overhead and your margin, and generate the professional PDF.
FAQs
- What does the IRC require?
- Maximum riser 7¾ in, minimum tread 10 in, with no more than ⅜ in variation.
- What is the 2R + T rule?
- Two risers plus one tread ≈ 24–25 in for a comfortable stride.
- How many stringers do I need?
- Three for a standard 36 in stair; add one every 16 in of width.
- What riser and tread are comfortable?
- A 7–7½ in riser with a 10–11 in tread fits 2R + T ≈ 24–25 in.
- How big is the stair opening?
- Long enough to keep 6 ft 8 in of headroom along the run.
- What lumber for stringers?
- 2×12 (or larger) graded lumber, or LVL for long runs.
Sources and standards
Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.