ConstructionApril 12, 2026

Stair Calculator Guide: Building Code, Rise & Run Dimensions

Q: What is the standard stair rise and run?

The IRC (International Residential Code) allows a maximum rise of 7¾ inches and minimum run (tread depth) of 10 inches. The ideal comfortable stair has a 7-inch rise and 11-inch run, known as the 7-11 rule. All risers in a staircase must be within 3/8 inch of each other in height.

Q: How do I calculate the number of stairs I need?

Divide the total rise (floor-to-floor height) by your target riser height. For a 108-inch (9-foot) floor-to-floor height with 7-inch risers: 108 ÷ 7 = 15.4, so you need either 15 risers at 7.2 inches each or 16 risers at 6.75 inches each. The number of treads is always one less than the number of risers.

Q: What is the minimum headroom for stairs?

The IRC requires a minimum headroom of 6 feet 8 inches (80 inches) measured vertically from the stair nosing to the ceiling or any obstruction above. This measurement must be maintained along the entire length of the staircase and at the landings.

Q: How wide do stairs need to be?

Residential stairs must be at least 36 inches wide per the IRC. This is measured between the walls or between the wall and the edge of the handrail. The actual clear walking width must be at least 31.5 inches (accounting for handrail projection). Commercial stairs typically require 44 inches minimum.

Q: What is the 18-inch rule for stairs?

The comfort rule states that rise + run should equal approximately 17-18 inches. A 7-inch rise + 11-inch run = 18 inches. Another formula: 2× rise + run should equal 24-25 inches. These rules of thumb produce stairs that feel natural to climb. Stairs outside this range feel either too steep or too shallow.

By The hakaru Team·Last updated March 2026

Quick Answer

*Max rise: 7¾”. Min tread depth: 10” (IRC residential code).
*Number of risers = total rise ÷ target riser height. Treads = risers − 1.
*Comfort rule: rise + run ≈ 17–18 inches (the 7-11 rule is ideal).
*Minimum headroom: 6’8”. Minimum width: 36”.

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How to Calculate Stairs

Every stair calculation starts with one measurement: the total rise. That’s the vertical distance from the finished floor at the bottom to the finished floor at the top. For a standard 9-foot ceiling with floor framing, this is typically 105–110 inches.

Divide total rise by your target riser height (7 inches is ideal). Round to the nearest whole number. Then divide total rise by that number to get the exact riser height. For 108 inches: 108 ÷ 7 = 15.4, round to 15. Exact riser: 108 ÷ 15 = 7.2 inches.

The number of treads is always one less than risers (the top floor serves as the last “tread”). So 15 risers means 14 treads. Total horizontal run: 14 treads × 11 inches = 154 inches, or about 12 feet 10 inches.

IRC Building Code Requirements

Dimension	Minimum	Maximum	Ideal
Riser height	4”	7¾”	7”–7½”
Tread depth (run)	10”	—	11”
Nosing projection	¾”	1¼”	1”
Stair width	36”	—	36”–42”
Headroom	80” (6’8”)	—	84”+
Handrail height	34”	38”	36”

Critical rule: all risers must be within 3/8 inch of each other. Uneven risers are a major trip hazard and the most common code violation in stair construction. This is why precise calculation matters more than carpentry skill.

The Comfort Formulas

Over centuries of stair building, several formulas have emerged for comfortable stairs:

Rise + Run = 17–18” (most common rule of thumb)
2 × Rise + Run = 24–25” (European formula)
Rise × Run = 70–75 (the product rule)

A 7” rise with 11” run satisfies all three: 7 + 11 = 18, 2(7) + 11 = 25, 7 × 11 = 77 (slightly above range but still comfortable). This is why the 7-11 combination is considered the gold standard.

Stringer Calculation

Stringers are the diagonal boards that support the treads and risers. You need the stringer length, which is the hypotenuse of a triangle formed by total rise and total run. Use the Pythagorean theorem: stringer length = √(rise² + run²).

For 108” total rise and 154” total run: √(108² + 154²) = √(11,664 + 23,716) = √35,380 = 188 inches, or about 15 feet 8 inches. Stringers are typically cut from 2×12 lumber. You need a minimum of two, but three is recommended for widths over 30 inches.

Common Mistakes

Forgetting finished floor thickness: If you measure total rise before flooring is installed, add the thickness of both the upper and lower floor finishes. Hardwood flooring adds ¾ inch; tile over cement board adds about 1 inch. Miss this and your first or last riser will be off.

Not checking headroom early: A staircase that passes under a floor above needs 80 inches of clearance at every point. If headroom is tight, you may need to increase tread depth (extending the stair opening) or steepen the stairs (within code limits).

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Frequently Asked Questions

What is the standard stair rise and run?

IRC allows max 7¾” rise and min 10” run. The ideal is 7” rise and 11” run (the 7-11 rule). All risers must be within 3/8” of each other.

How do I calculate the number of stairs I need?

Divide total rise by target riser height. For 108” with 7” risers: 108 ÷ 7 = ~15 risers at 7.2” each. Number of treads = risers minus 1.

What is the minimum headroom for stairs?

The IRC requires 6 feet 8 inches (80 inches) minimum, measured from stair nosing to ceiling, maintained along the entire staircase.

How wide do stairs need to be?

Residential stairs must be 36 inches minimum. Clear walking width must be at least 31.5 inches after handrail projection. Commercial stairs need 44 inches.

What is the 18-inch rule for stairs?

Rise + run should equal about 17–18 inches for comfort. A 7” rise + 11” run = 18 inches. Stairs outside this range feel too steep or too shallow.