How do you calculate the number of stairs?

Divide the total rise (floor-to-floor height) by your desired riser height — typically 7 inches. Round to the nearest whole number for your riser count. Treads = risers − 1. For example: 105" ÷ 7" = 15 risers, 14 treads.

What is the ideal riser height and tread depth?

IRC residential code allows risers between 4" and 7¾" and treads of at least 10". The classic comfort formula is: 2 × riser + tread = 24–25". A 7" riser pairs well with a 10.5"–11" tread for a comfortable, natural stride.

How long should a stair stringer be?

Stringer length = √(total rise² + total run²) — the Pythagorean theorem applied to the stair triangle. Example: 105" rise and 140" run gives √(105² + 140²) = 175". Always add a few inches for the top plumb cut and bottom seat cut.

What is the minimum headroom for stairs?

IRC R311.7.2 requires at least 6 ft 8 in (80 inches / 2032 mm) of vertical clearance measured from the stair nosing vertically to the ceiling or soffit above, along the entire stair path.

How many stringers do I need?

Two stringers are sufficient for stairs 36" (900 mm) or narrower. Use three stringers for wider stairs. Commercial applications and stairs with open sides may require additional stringers per IBC requirements.

What is the 7-11 rule for stairs?

The 7-11 rule is a common design guideline: riser height ≤ 7" and tread depth ≥ 11". These dimensions provide a comfortable stride for most adults and exceed IRC minimums.

Where must doors be placed relative to stairs?

The arc of a door swing must be completely on a landing or floor — it cannot swing over steps. The landing must be at least as wide as the door and at least 36 inches deep in the direction of travel.

When are handrails and guards required?

Guards are required when the total rise is more than 30 inches above the floor below. Handrails are required on at least one side for any stairway with four or more risers. Handrail height must be 34–38 inches measured from the top of the treads.

Back to Construction tools Construction

Stair Calculator

The Stair Calculator is an online tool for calculating various parameters involved in the construction of stairs. Enter your total rise and run to instantly get riser count, riser height, tread depth, stringer length, headroom clearance, and a complete lumber materials list — all verified against IRC residential building code.

Stair Dimensions

Total rise (in)

Total run (in)

Headroom (in)

Stringers

Nosing depth (in)

Stair width (in)

Materials & Cost

Include risers

Tread thickness (in)

Riser thickness (in)

Price per board foot ($)

Stair width assumed 36" / 900 mm. Adjust price per BF to match your lumber species and grade.

Results

risers / 14 treads

Riser height: 7.000"
Tread depth: 10.000"
Stringer length: 14.58 ft (175.00")
Total run: 140.00 in
Stairwell opening: 33.33 in

IRC Code Check

Riser: 7.000" ✓ within code range

Tread: 10.000" ✓ meets minimum

Headroom: 80in ✓ meets 6′8″

Materials

Stringers: 3 pcs
Stringer lumber: 61.52 BF
Tread lumber: 52.50 BF
Riser lumber: 19.69 BF
Total board feet: 133.71 BF
Lumber cost est.: $601.70

Nosing Adjustments

Tread face-to-face: 9 ¼"
Adj. total run: 130.25 in
Nosing depth: ¾"

Landing

Min. width: 36.00 in
Min. depth: 36.00 in
Area: 1,296.0 sq in

Handrail & Balusters

Handrail length: 199.00 in
Post count: 5 posts
Baluster count: 44 balusters
Baluster spacing: 3.977 in
Handrail height: 34–38 in
Guard height: ≥ 36 in

Estimates for planning only. Verify with local code before building.

Reference

Stair code requirements by type

Stair Type	Riser	Tread	Headroom	Code
Interior residential	6¾"–7¾"	≥ 10"	≥ 6′8"	IRC R311.7 — standard homes
Exterior deck stairs	4"–7¾"	≥ 10"	Open	IRC R507.12 — deck/patio
Commercial (IBC)	4"–7"	≥ 11"	≥ 7′	IBC 1011 — offices, retail
Spiral stairs	≤ 9½"	≥ 7½" at 12"	≥ 6′6"	IRC R311.7.10

Terminology

Stair terminology and building codes

Stairs come in many forms, and while building a basic staircase may seem straightforward, there are numerous parameters, calculations, and building codes to consider. Building codes can differ at a local level — always refer to the codes specific to your location.

Run / Tread

Min. tread depth: 10 in (25.4 cm)

The run or tread is the part of the stairway that a person steps on. Its length is measured from the outer edge of the step (including any nosing) to the vertical riser below. When measuring total run, the length of the tread above the last riser is not included. When nosing is present, the total run is not simply the sum of tread lengths — the nosing overhang must be subtracted.

Rise / Riser

Max. riser height: 7¾ in (19.7 cm)

The rise, or height of a step, is measured from the top of one tread to the top of the next tread. It is not the physical height of the riser board itself, because that excludes the thickness of the tread above it. The number of risers — not treads — is used to determine the total number of steps in a staircase.

Nosing

Min. 0.75 in (1.9 cm) · Max. 1.25 in (3.2 cm)

The nosing is the protrusion at the edge of a tread that hangs over the riser below. Not all steps have nosing, but when present it is included in the tread length measurement. The main purpose of nosing is safety — it provides extra foot space when descending.

Headroom

Min. 6 ft 8 in (203.2 cm)

Headroom is the vertical clearance measured from the top of a tread to the ceiling directly above. Building codes require far more than the average person's height to allow for moving furniture and other large objects up and down the stairway.

Stair Width

Min. 36 in (91.44 cm)

Stair width is measured from edge to edge across the tread, perpendicular to its length. Unlike most rectangles where length > width, stair width is usually the longer dimension. Handrails are not included in this measurement.

Handrails & Guards

Guards req. when total rise > 30 in. Handrail height: 34–38 in. Guard height: ≥ 34 in.

A handrail is a railing running up the stair incline for users to hold while ascending or descending. A guard (guardrail) is a building component near the open side of an elevated surface that prevents falls. Guards can be rails, walls, half-walls, or benches.

Stringer

Typically 2×12 lumber; 3 stringers for stairs wider than 36 in

A stair stringer is a structural member that supports the treads and risers. Typically there are three stringers — one on each side and one in the middle. Stringers are not always visible; on open-sided stairs they can be seen clearly. They can be cut to the step shape or left uncut to conceal tread edges.

How it works

How to calculate stair dimensions

This stair calculator helps you design code-compliant stairs for any floor-to-floor height. Enter your total rise (floor-to-floor vertical distance) and total run (horizontal footprint), and the calculator instantly outputs every dimension you need for layout and materials.

For structural framing costs on your stair platform, see our Lumber Calculator. To check the concrete pad or landing, use the Concrete Calculator. Browse all Construction tools.

Step 1 — Measure total rise

Measure the vertical distance from the finished floor at the bottom to the finished floor at the top. This is your total rise.

Step 2 — Calculate risers

Divide total rise by 7" (178 mm) and round to the nearest whole number. Divide total rise by that number to get the exact riser height per step.

Step 3 — Find tread & stringer

Tread depth = total run ÷ (risers − 1). Stringer length = √(rise² + run²). Check all dimensions against IRC R311.7.

Risers = round(total_rise / 7)

Riser height = total_rise / risers

Tread depth = total_run / (risers − 1)

Stringer = √(rise² + run²)

Comfort check = 2 × riser + tread ≈ 24–25"

Reference

Fraction to decimal conversion

Common fractions used in stair measurements and their decimal equivalents.

16th	8th	4th	2nd	Decimal
1/16				0.0625
2/16	1/8			0.125
3/16				0.1875
4/16	2/8	1/4		0.25
5/16				0.3125
6/16	3/8			0.375
7/16				0.4375
8/16	4/8	2/4	1/2	0.5
9/16				0.5625
10/16	5/8			0.625
11/16				0.6875
12/16	6/8	3/4		0.75
13/16				0.8125
14/16	7/8			0.875
15/16				0.9375
16/16	8/8	4/4	2/2	1

FAQ

Frequently asked questions

Accuracy & Review

Reviewed by: Liam Santos. Liam reviews our construction calculators to confirm accurate dimensional calculations and practical building code defaults.

Last updated: May 2026

Important disclaimer: These results are based on IRC R311.7 residential stair requirements. Commercial, industrial, and specialty applications follow different codes (IBC, OSHA). Always verify with your local authority having jurisdiction (AHJ) and a licensed contractor before construction.

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