Australian Stair Calculation Formula
Number of Rises = Total Rise ÷ Ideal Rise Height (rounded up)
Unit Rise = Total Rise ÷ Number of Rises
Number of Treads = Number of Rises - 1
Total Run = Number of Treads × Tread Run
Stringer Length = √(Total Rise² + Total Run²)
2R + G = (2 × Unit Rise) + Tread Run
Australian Compliance = 2R + G between 550mm and 700mm
Australian Standards:
• Riser Height: 115mm to 190mm (175mm ideal)
• Tread Depth (Going): Minimum 240mm
• 2R + G Formula: Must be between 550mm and 700mm
• Refer to National Construction Code (NCC) for specific requirements
Quick Answer: The 2R+G Stair Formula (Australian Standard)
The Australian stair formula is 2R + G = 550–700mm, where R is the riser height and G is the going (tread depth). Under the NCC, residential risers must be 115–190mm and goings 240–355mm. Mandatory: 2R + G must fall within 550–700mm for the stair to be code-compliant and comfortable to climb.
Common compliant combinations:
- Rise 175mm + Going 250mm = 600mm (typical residential)
- Rise 165mm + Going 280mm = 610mm (gentler stair)
- Rise 190mm + Going 240mm = 620mm (steepest legal in Class 1)
Use the calculator above for any total rise, and we’ll work out the exact dimensions.
Stair Calculator Australia — Rise, Run & 2R+G Formula
Calculate stair dimensions including rise, going (run), number of treads, and stringer length. Our free stair calculator follows Australian Building Code (NCC) requirements for residential and commercial stairs. Enter your floor-to-floor height to get compliant dimensions instantly.
Australian Stair Regulations (NCC Volume 2)
| Dimension | Residential | Commercial |
|---|---|---|
| Rise (R) | 115-190mm | 115-180mm |
| Going (G) | 240-355mm | 250-355mm |
| 2R + G formula | 550-700mm | 550-700mm |
| Minimum width | 600mm | 1,000mm |
| Headroom | 2,000mm min | 2,000mm min |
| Max flight (no landing) | 18 risers | 18 risers |
| Landing depth | ≥ stair width | ≥ stair width |
The 2R + G Stair Formula Explained
The standard comfort formula for stairs is 2R + G = 550 to 700mm, where R is the rise and G is the going. This formula has been used for centuries and represents the natural human stride adapted for climbing.
The ideal target is 2R + G = 620mm. This gives the most comfortable stair for most people.
For example: a 175mm rise with a 275mm going gives 2(175) + 275 = 625mm — right in the ideal range.
Step-by-Step: How to Calculate Stairs
- Measure total rise: Floor-to-floor height including all finishes (tiles, carpet, timber). Example: 2,700mm
- Choose a rise: Divide total rise by your target rise (e.g., 175mm). 2,700 / 175 = 15.4. Round to 15 or 16 risers
- Calculate actual rise: 2,700 / 16 = 168.75mm per riser. Check: between 115-190mm? Yes
- Calculate going: Use 2R + G = 620mm. Going = 620 – 2(168.75) = 282.5mm. Check: between 240-355mm? Yes
- Check 2R + G: 2(168.75) + 282.5 = 620mm. Between 550-700? Yes
- Calculate total going: 15 treads x 282.5mm = 4,237.5mm (this is your stair’s horizontal footprint)
- Calculate stringer length: sqrt(2700² + 4237.5²) = 5,024mm
Common Stair Configurations for Australian Homes
| Floor Height | Risers | Rise | Going | Total Going |
|---|---|---|---|---|
| 2,400mm | 14 | 171mm | 278mm | 3,614mm |
| 2,550mm | 15 | 170mm | 280mm | 3,920mm |
| 2,700mm | 16 | 169mm | 282mm | 4,230mm |
| 2,900mm | 17 | 171mm | 278mm | 4,448mm |
| 3,000mm | 17 | 176mm | 268mm | 4,288mm |
Winder Stairs and Landing Requirements
When a straight stair does not fit, winder stairs or landings are used to change direction:
- Quarter landing (L-shape): 90-degree turn with a flat landing. Most common in Australian two-storey homes. Landing must be at least as deep as the stair width
- Half landing (U-shape): 180-degree turn with a flat landing. Takes up less floor plan length but more width
- Winder treads: Triangular treads that turn without a landing. Minimum going of 240mm measured at the walk line (270mm from narrow end). Maximum 3 winder treads per quarter turn in residential
- Spiral stairs: Must comply with NCC minimum dimensions. Going measured at 270mm from the centre pole. Generally only permitted as secondary access in residential
Stringer Types and Construction
- Cut stringer (open): Treads sit on top of the stringer, which is cut in a sawtooth pattern. Most common for external decking stairs and budget internal stairs
- Housed stringer (closed): Treads fit into routed grooves in the stringer. Neater appearance, common for internal feature stairs. Requires precision routing
- Mono stringer: Single central stringer with treads cantilevered either side. Modern design feature. Requires engineering
“The number one mistake with stairs is not measuring floor-to-floor including finishes. If you calculate your rise based on structural floor height but the client adds 12mm tiles upstairs and 14mm timber downstairs, your first and last riser will be wrong by 26mm. That fails BCA.”
— Stair builder, 18+ years experience, Melbourne
Handrail and Balustrade Requirements for Stairs
- Handrail required on at least one side for residential stairs with 4+ risers
- Handrail height: 865-1,000mm measured from nosing line
- Handrail must be graspable: 30-65mm diameter circular, or equivalent profile
- Balustrade required where fall height exceeds 1 metre
- 125mm sphere test applies to all stair balusters
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