Stair Calculator — Free Online Staircase Design Tool

How to Use the Stair Calculator

1. Measure the total rise: Measure the vertical distance from the surface of the finished floor at the bottom of the staircase to the surface of the finished floor at the top. Use a tape measure or laser level for accuracy. This measurement must include the thickness of the upper floor construction, including joists and subfloor. For a standard 9-foot ceiling, the total rise is typically 108 to 118 inches depending on floor framing depth. Enter this value in inches.
2. Set your preferred riser height: Enter the riser height you prefer in inches. The default of 7.5 inches is a comfortable middle ground for most residential stairs. The calculator uses this value as a target and determines the closest whole number of risers that produces a uniform riser height close to your preference. The IRC maximum is 7.75 inches, and the practical minimum is about 4 inches.
3. Set the tread depth: Enter the tread depth in inches. The IRC requires a minimum of 10 inches measured from nosing to nosing. Standard residential treads are 10 to 11 inches. Deeper treads provide more comfortable footing but increase the horizontal space the staircase requires. Adjust this value based on your available floor space and comfort preferences.
4. Review results and code compliance: The results panel shows the calculated number of risers, number of treads, actual riser height, total horizontal run, stringer length, and code compliance status. Green indicates the design meets IRC requirements. Red indicates code violations with specific notes explaining what needs adjustment. Modify your inputs until you achieve a compliant, comfortable design.

All calculations update instantly as you adjust values. Try different riser heights and tread depths to find the combination that best fits your space while maintaining code compliance and comfort.

Stair Calculation Formulas

Variables Explained

• Total Rise: The vertical distance from the bottom finished floor to the top finished floor. This includes the ceiling height plus the thickness of the upper floor construction (joists, subfloor, and finished floor). Accurate measurement is critical because even small errors are multiplied across all risers.
• Riser Height: The vertical height of each step. The IRC maximum is 7.75 inches. All risers in a single flight must be uniform, with no more than 3/8-inch variation between the tallest and shortest riser. The calculator determines the actual riser height by dividing the total rise by the number of risers.
• Tread Depth: The horizontal depth of each step surface, measured from the front edge (nosing) of one tread to the front edge of the next tread above. The IRC minimum is 10 inches. This dimension determines how much of your foot fits on each step and directly affects comfort and safety.
• Total Run: The total horizontal distance the staircase covers from the face of the bottom riser to the face of the top riser. This is critical for determining how much floor space the staircase requires. Landing areas at the top and bottom add to the total floor space needed.
• Stringer Length: The diagonal length of the stringer board that supports the stairs, calculated using the Pythagorean theorem. This determines the lumber length needed for stringers, which are typically cut from 2x12 boards.

Step-by-Step Example

Calculate the stair design for a standard 9-foot ceiling with 10-inch floor framing (total rise of 118 inches), using a 7.5-inch target riser and 10-inch tread depth:

1. Calculate number of risers: 118 / 7.5 = 15.73, rounded to 16 risers
2. Calculate actual riser height: 118 / 16 = 7.375 inches (within code limit of 7.75)
3. Calculate number of treads: 16 - 1 = 15 treads
4. Calculate total run: 15 x 10 = 150 inches (12.5 feet)
5. Calculate stringer length: sqrt(118² + 150²) = sqrt(13,924 + 22,500) = 190.9 inches (15.9 feet)
6. Check comfort rule: 7.375 + 10 = 17.375 (within the 17-18 range)

This design is code-compliant and comfortable. The staircase needs about 12.5 feet of horizontal floor space plus landing areas. You will need stringer boards at least 16 feet long, typically cut from 2x12 lumber.

Practical Examples

Example 1: Alex's Basement Staircase

Alex is finishing his basement and needs to replace the temporary construction stairs with a permanent code-compliant staircase. The total rise from the basement finished floor to the first floor surface is 104 inches. He wants comfortable stairs with a 7.5-inch target riser and 10.5-inch treads:

• Number of risers: 104 / 7.5 = 13.87, rounded to 14 risers
• Actual riser height: 104 / 14 = 7.43 inches (under 7.75 max)
• Number of treads: 14 - 1 = 13 treads
• Total run: 13 x 10.5 = 136.5 inches (11 feet 4.5 inches)
• Stringer length: sqrt(104² + 136.5²) = 171.6 inches (14.3 feet)
• Comfort check: 7.43 + 10.5 = 17.93 (within 17-18 range)

Alex confirms his basement has enough room for the 11-foot 5-inch horizontal run plus a 36-inch landing at the bottom. He purchases two 16-foot 2x12 boards for stringers and thirteen 10.5-inch-deep treads. The design passes IRC code requirements, ensuring it will pass the building inspection for his basement permit.

Example 2: Beth's Split-Level Entry

Beth has a split-level home entry where guests step down 36 inches from the front door to the living room. She wants to replace the worn carpeted steps with hardwood. With only 36 inches of total rise, she uses a 7-inch target riser:

• Number of risers: 36 / 7 = 5.14, rounded to 5 risers
• Actual riser height: 36 / 5 = 7.2 inches
• Number of treads: 5 - 1 = 4 treads
• Total run: 4 x 10 = 40 inches (3 feet 4 inches)
• Stringer length: sqrt(36² + 40²) = 53.81 inches (4.5 feet)

Beth's short staircase requires only 40 inches of horizontal space. She selects 10-inch-deep oak treads and paints the risers white for a classic look. The 7.2-inch risers are comfortable and well within code limits. She orders five risers and four treads plus finishing materials.

Example 3: Carlos's Deck Steps

Carlos is building exterior stairs from his elevated deck down to the backyard patio. The deck surface is 48 inches above the patio. He prefers slightly shorter risers of 6 inches for outdoor comfort since the steps will be used in all weather conditions:

• Number of risers: 48 / 6 = 8 risers
• Actual riser height: 48 / 8 = 6 inches exactly
• Number of treads: 8 - 1 = 7 treads
• Total run: 7 x 11 = 77 inches (6 feet 5 inches)
• Stringer length: sqrt(48² + 77²) = 90.74 inches (7.6 feet)
• Comfort check: 6 + 11 = 17 (at the lower bound of the comfort range)

Carlos's outdoor staircase with 6-inch risers and 11-inch treads provides a gentle, safe ascent, which is ideal for carrying food and drinks from the kitchen to the patio. The wider treads also allow for better traction in wet conditions. He uses pressure-treated lumber and adds non-slip strips to each tread for safety.

Stair Dimensions Reference Table

All calculations assume 10-inch tread depth and target riser height of 7.5 inches. IRC max riser: 7.75 inches.

Tips and Complete Guide

Planning Your Staircase Layout

Before building stairs, consider the available floor space on both levels. A straight-run staircase is the simplest to build but requires the most horizontal space. An L-shaped staircase with a landing turn reduces the total run but requires more width. A U-shaped staircase with two landings fits in even less horizontal space but needs a wider stairwell opening. For tight spaces, a winding or spiral staircase may work, though spiral stairs have different code requirements and are generally less comfortable for daily use. Always verify that the stairwell opening in the upper floor provides at least 6 feet 8 inches of headroom clearance measured vertically from the stair nosing line.

Material Selection for Stair Construction

Interior stairs commonly use dimensional lumber for the structural stringers (2x12 boards), with pre-fabricated treads and risers of oak, maple, poplar, or engineered wood. Stringer boards must be wide enough to maintain at least 3.5 inches of material below the tread cut, which is why 2x12 lumber is standard. For outdoor stairs, use pressure-treated lumber rated for ground contact. Composite decking material can serve as treads for weather resistance and low maintenance. Steel stair stringers are available for industrial-style interiors and offer precise, consistent geometry. Calculate your total materials by multiplying the number of treads by the stair width plus waste for the tread material, and the stringer length times the number of stringers needed.

Building Code Essentials for Residential Stairs

The International Residential Code (IRC) establishes minimum safety standards for residential staircases. Key requirements include maximum riser height of 7-3/4 inches, minimum tread depth of 10 inches, minimum stair width of 36 inches, minimum headroom of 6 feet 8 inches, handrails on at least one side between 34 and 38 inches high, guardrails at least 36 inches high on open sides, and baluster spacing no more than 4 inches apart. All risers and treads within a flight must be uniform, with no more than 3/8-inch variation. Nosings must project between 3/4 inch and 1-1/4 inches beyond the riser face. Use our construction area calculator to determine the floor area needed for your stairwell opening.

Common Mistakes to Avoid

• Measuring total rise incorrectly: The most common stair-building error is measuring from subfloor to subfloor instead of finished floor to finished floor. If the upper floor will have tile (3/4 inch) and the lower floor will have carpet (1/2 inch), the difference in finish floor thickness changes your total rise. Measure after finished floors are installed whenever possible, or add the planned finish floor thicknesses to your subfloor measurement.
• Not accounting for floor framing depth: When measuring total rise from below, you must include the thickness of the floor joists, subfloor, and finished floor of the upper level. Forgetting the 10-to-12-inch floor framing depth results in stairs that do not reach the upper floor level, requiring a dangerous last step.
• Ignoring headroom clearance: As you walk up stairs, the underside of the floor above gets closer. If the stairwell opening is too short, you will bump your head before reaching the top. Verify at least 6 feet 8 inches of clearance from every tread nosing to the ceiling or header above. Increasing the stairwell opening size is much easier during planning than after framing.
• Uneven riser heights: Varying riser heights within a flight are a leading cause of stairway falls. The human body expects uniform step heights and compensates poorly for variations. Cut all stringers from the same template and verify measurements at every riser position. The IRC permits no more than 3/8-inch variation within a flight.
• Skipping code compliance checks: Building stairs that violate building codes can result in failed inspections, required demolition and rebuilding, liability issues, and problems when selling the home. Always verify compliance with local building codes before construction begins, as some jurisdictions have requirements stricter than the IRC.

Frequently Asked Questions