What is the maximum riser height allowed by building code?

The 2024 IRC allows a maximum riser height of 7-3/4 inches for residential stairs. The 2024 IBC allows a maximum of 7 inches for commercial occupancies. Local jurisdictions may adopt earlier code editions or apply amendments that differ from the model code. Always verify with your local building department.

What is the Blondel formula?

The Blondel formula (named after the 17th-century French architect Francois Blondel) states that twice the riser height plus the tread depth should equal 24 to 25 inches. This ratio produces stairs that feel natural and comfortable to climb. For example: a 7-inch riser with an 11-inch tread gives 2(7) + 11 = 25 inches. The formula is a comfort guideline, not a code requirement, but stairs that satisfy it are noticeably more comfortable to use.

How wide do stairs need to be?

The IRC requires a minimum clear width of 36 inches for residential stairs. The IBC requires a minimum of 44 inches for most commercial occupancies. ADA-accessible stairs in public buildings are typically 60 inches or wider. Handrails may project up to 4.5 inches into the required width on each side.

How do I calculate stringer length?

Stringer length is the hypotenuse of a right triangle with legs equal to total rise (vertical) and total run (horizontal). Use the Pythagorean theorem: stringer length = sqrt(total rise^2 + total run^2). Add 6 to 12 inches for the top and bottom bearing cuts. For a stair with a 108-inch rise and 154-inch run, the stringer is sqrt(11,664 + 23,716) = 188.1 inches, so a 16-foot board provides adequate length with bearing allowance.

Construction & EngineeringPopular

Stair Riser and Tread Calculator

Calculate IBC and IRC compliant riser height, tread depth, total run, stringer length, and stair angle from floor-to-floor height.

Total Rise / Floor-to-Floor Height (inches)

Measure from finished floor to finished floor. Standard story height is 96 to 120 inches.

Building Code

Preferred Riser Height (optional)

Leave blank for automatic calculation.

Tread Depth (optional)

Leave blank to calculate from riser using comfort rule.

Your Results

Enter total rise and click calculate.

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Introduction

A staircase with inconsistent riser heights is not just uncomfortable, it is a code violation and a liability. The 2024 International Residential Code (IRC) requires that no riser in a stairway vary from any other by more than 3/8 inch. That limit exists because the human body adapts to a rhythm when climbing stairs, and a single unexpected riser height breaks that rhythm and causes falls. According to the U.S. Consumer Product Safety Commission (CPSC), stairs account for over one million emergency room visits annually in the United States, with riser height inconsistency cited as a contributing factor in many stair-related falls. The calculation itself is a division problem with a code constraint: total rise divided by a target riser height, rounded to a whole number of risers, then back-calculated to the exact uniform riser height. This stair riser and tread calculator handles that math, checks compliance with both the IRC and IBC, and outputs total run and stringer length.

What This Calculator Does

This stair calculator determines the number of risers and treads, exact riser height, tread depth, total horizontal run, stringer length, and stair angle for any floor-to-floor height. It checks compliance with the 2024 IRC (residential) and IBC (commercial) limits for maximum riser height, minimum tread depth, and the Blondel comfort formula. Enter total rise (floor-to-floor height in inches), target riser height, and desired tread depth to get a code-checked layout.

The Formula

Risers = round(Total Rise / Target Riser Height) | Actual Riser Height = Total Rise / Number of Risers | Total Run = (Risers - 1) x Tread Depth | Stringer = sqrt(Total Rise^2 + Total Run^2)

The number of risers equals the total rise divided by the target riser height, rounded to the nearest whole number. The actual riser height is then back-calculated by dividing total rise by the riser count to ensure uniform risers. The number of treads always equals risers minus one (the top floor serves as the final tread). Total run is tread count times tread depth. Stringer length is derived from the Pythagorean theorem using total rise and total run. The Blondel formula (2R + T = 24 to 25 inches) is checked to verify ergonomic comfort.

Step-by-Step Example

Measure total rise precisely

Measure from finished floor to finished floor. Example: concrete slab to finished wood floor above = 108.5 inches. If finish flooring has not been installed, add its thickness to your rough framing measurement.

Determine target riser height and calculate riser count

Target: 7 inches. Risers: 108.5 / 7 = 15.5, round to 16 risers. Actual riser height: 108.5 / 16 = 6.78 inches. Check IRC limit: max 7.75 inches. Pass. Check IBC limit: max 7.0 inches. 6.78 passes IBC as well.

Set tread depth and check Blondel formula

Tread depth: 10.5 inches. Blondel check: 2(6.78) + 10.5 = 24.06 inches. This falls in the 24 to 25 inch comfort range. IRC minimum tread: 10 inches. Pass. Tread count: 16 - 1 = 15 treads.

Calculate total run and stringer length

Total run: 15 x 10.5 = 157.5 inches = 13.125 feet. Stringer length: sqrt(108.5^2 + 157.5^2) = sqrt(11,772 + 24,806) = sqrt(36,578) = 191.3 inches = 15.9 feet. Order 16-foot stringer stock.

Real-World Use Cases

New Home Interior Stair

A framing contractor building a main stair from first floor to second floor at 109 inches total rise calculates 15 risers at 7.27 inches each. With 11-inch treads: Blondel = 2(7.27) + 11 = 25.54. Slightly above the 25-inch comfort range. Adjusting to 10.5-inch treads: 2(7.27) + 10.5 = 25.04. Code compliant, comfortable, and fits a 12-foot horizontal run in the floor plan.

Exterior Deck Stairs

A deck builder designing stairs from a 52-inch high deck to grade calculates: 52 / 7 = 7.4, round to 7 risers at 7.43 inches. Tread depth: 11 inches. Blondel: 2(7.43) + 11 = 25.86. Slightly above preferred range. Adjust to 10-inch treads: 25.86 becomes 24.86. Total run: 6 x 10 = 60 inches = 5 ft. Stringer: sqrt(52^2 + 60^2) = 79.4 inches. One 8-foot 2x12 stringer works.

Commercial Building Renovation

A contractor replacing stairs in a commercial office building (IBC applies) at 130-inch total rise: 130 / 6.5 = 20 risers at 6.5 inches each. IBC maximum 7.0 inches: pass. IBC minimum tread 11 inches. Blondel: 2(6.5) + 11 = 24. Pass. Total run: 19 x 11 = 209 inches = 17.4 feet. Verify headroom clearance of 80 inches minimum along the full stair path.

Comparison

Code	Max Riser Height	Min Tread Depth	Min Width	Application
IRC 2024 (Residential)	7-3/4 inches	10 inches	36 inches	Homes, townhouses
IBC 2024 (Commercial)	7 inches	11 inches	44 inches	Commercial, multi-family
ADA Accessible	7 inches	11 inches	60+ inches (handrail span)	Public accessibility
Blondel Comfort Formula	N/A	N/A	2R + T = 24 to 25 in	Ergonomic standard

Common Mistakes to Avoid

Measuring floor-to-floor height before finished flooring is installed. If a 3/4-inch hardwood floor will be added to the upper level, the effective total rise increases by 3/4 inch. Divide the adjusted total rise by your riser count to see how much each riser changes, then verify it stays within the 3/8-inch uniformity tolerance.
Assuming the number of treads equals the number of risers. There is always one fewer tread than risers because the upper landing is the top tread. A stair with 14 risers has 13 treads. Cutting 14 treads leaves one extra.
Ignoring headroom clearance. Both IRC and IBC require 80 inches (6 ft 8 in) of clearance measured vertically from each tread nosing to any overhead obstruction. On stairs running under a floor above, confirm the headroom is maintained across the full stair run.
Not accounting for nosing overhang when specifying tread depth. IRC allows up to 1.25 inches of nosing overhang. Nosing does not count toward tread depth. A 10-inch tread with 1.25-inch nosing has a total tread width of 11.25 inches but only 10 inches of walking surface depth.

Frequently Asked Questions

Accuracy and Disclaimer

Stair dimensions must comply with your locally adopted building code, which may differ from the model IRC or IBC. Local jurisdictions adopt code editions on varying schedules and may apply amendments that change riser and tread requirements. Always verify stair design with your local building department before construction. This calculator is a planning tool and does not substitute for permit review or professional inspection.

Conclusion

Before cutting stringers, confirm that the total rise accounts for finished floor materials at both the top and bottom landings. A 3/4-inch hardwood floor added after the stair is framed changes every riser height by a fraction that may push the assembly out of the 3/8-inch uniformity tolerance. For exterior stairs and deck stairs with concrete or composite landing surfaces, use the Concrete Volume Calculator to size the landing footing. For complete stair framing lumber quantities including stringers, treads, and risers, add the dimensions to the Lumber Board Foot Calculator.

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