Stair Calculator

The stair calculator is a specialized construction tool that determines all key stair dimensions based on your input values. It calculates the number of steps, rise and run of each step, total run, stringer length, and stringer angle — ensuring your stairs are safe, comfortable, and built to standard proportions.

What Is a Stair Calculator?

A stair calculator is an online tool used to determine precise measurements for building stairs, including riser height, tread depth, total run, and stringer length. It takes your known step dimensions as input and calculates everything else automatically — helping you build stairs that are structurally sound, comfortable to use, and compliant with building regulations.

The Key Stair Formulas

Number of Risers

Total Rise ÷ Ideal Riser Height = Number of Risers (rounded up)

Actual Riser Height

Total Rise ÷ Number of Risers = Actual Riser Height

Number of Treads (Steps)

Number of Risers − 1 = Number of Treads (for standard stairs)

Total Run

Number of Treads × Tread Depth = Total Run

Stringer Length (Pythagorean Theorem)

√(Total Rise² + Total Run²) = Stringer Length

Blondel Rule (Comfort Check)

2R + G = 620–660 mm Where R = riser height and G = tread depth. Ideal values: R = 175 mm (7 in), G = 275 mm (11 in). Check: 2(175) + 275 = 625 mm — comfortably within the safe range.

How the Stair Calculator Works

Calculation Options (Image 4)

The calculator starts by asking what you already know:

• "I know..." dropdown — default: the run of one step

• "and..." dropdown — default: the rise of one step

Enter any known values and the calculator derives all remaining dimensions.

• Mount type — default: Standard mount (also Flush mount available)

• Optional: Enable headroom restriction details for low-clearance stairways

Your Stairs Outputs (Image 3)

• Run of one step — horizontal depth of each tread (cm)

• Total rise — total vertical height of the staircase (m)

• Rise of one step — vertical height of each riser (cm)

• Number of steps — total steps in the staircase

• Total run — total horizontal distance covered (m)

• Rise of the bottom step — may differ from other risers depending on mount type (cm)

Stringer Outputs (Image 2)

• Stringer height — vertical height of the stringer board (m)

• Stringer length — diagonal length of the stringer (m)

• Angle — stringer angle from horizontal (degrees)

How to Calculate Stairs — Step by Step

Example 1: Standard Deck Staircase

Total rise = 0.9 m (approximately 36 inches). Ideal riser height = 17.5 cm (175 mm).

Step 1: Number of risers = 90 cm ÷ 17.5 cm = 5.14 → rounded up to 6 risers. Step 2: Actual riser height = 90 ÷ 6 = 15 cm per step. Step 3: Number of treads = 6 − 1 = 5 treads. Step 4: Total run = 5 × 27.5 cm = 137.5 cm = 1.375 m. Step 5: Stringer length = √(0.9² + 1.375²) = √(0.81 + 1.891) = √2.701 = 1.64 m.

Example 2: Interior Staircase (10 ft Rise)

Total rise = 10 ft ≈ 3.05 m. Ideal riser = 17.5 cm.

Step 1: Risers = 305 cm ÷ 17.5 = 17.4 → 18 risers. Step 2: Actual riser height = 305 ÷ 18 = 16.94 cm. Step 3: Treads = 18 − 1 = 17 treads. Step 4: Total run = 17 × 27.5 = 467.5 cm = 4.675 m. Step 5: Stringer = √(3.05² + 4.675²) = √(9.30 + 21.86) = √31.16 = 5.58 m.

Stair Dimensions Reference Table

Note: Values use ideal riser height of 17.5 cm and tread depth of 27.5 cm (Blondel Rule). Actual values may vary based on your specific run and rise inputs.

Frequently Asked Questions

How many steps do I need for a 36 inch deck?

A 36-inch (0.9 m) rise typically requires 6 steps at a standard riser height of 17.5 cm. Enter your exact run and rise values into the stair calculator to get precise step count and stringer length.

How many stairs for a 12 foot ceiling?

A 12-foot ceiling has a rise of approximately 3.66 m, which requires around 21 risers at the standard 17.5 cm riser height. That gives you 20 treads and a total run of roughly 5.5 m.

How many steps are in a 10 ft staircase?

A 10-foot (3.05 m) staircase needs approximately 18 risers, giving you 17 treads. Use the stair calculator to adjust for your specific tread depth and get the exact stringer length.