Stair Stringer Calculator Australia: How to Use

This stair stringer calculator works out the number of steps, the rise and run of each step, and the total length of the stringer you need to cut for a straight flight of stairs. You enter the total rise (floor-to-floor height), the total run (horizontal distance available), and the thickness of the tread material, and it returns the dimensions you need to mark and cut on a standard 2×12 or 290×45 mm timber stringer. The calculator uses the standard building formula that Australian carpenters apply to domestic stairs, giving you a compliant layout for a 10°–45° pitch.

Formula

The calculator uses the standard stair geometry formula based on the building code’s preferred rise and run. The number of steps is found by dividing the total rise by a target rise (usually 175 mm in Australia) and rounding to the nearest whole number. Then the actual rise per step is the total rise divided by the number of steps, and the actual run per step is the total run divided by the number of steps. The stringer length is the hypotenuse of a right triangle with the total rise and total run as the two legs:

Stringer length = √(R² + T²)

• R = total rise (mm) – vertical distance from the finished floor below to the finished floor above.
• T = total run (mm) – horizontal distance from the face of the first step to the face of the last step.
• Steps = number of individual risers (not the number of treads, which is one less).

For a typical domestic stair, the Australian standard (AS 1657) recommends a rise between 130 mm and 225 mm, and a run (going) between 215 mm and 355 mm. The formula here targets 175 mm rise, which is a common comfortable middle ground.

Worked Example

Using the default values from the calculator: total rise = 2800 mm, total run = 3600 mm, tread thickness = 32 mm, stringer thickness = 45 mm.

1. Number of steps = round(2800 / 175) = round(16) = 16 steps.
2. Actual rise per step = 2800 / 16 = 175 mm.
3. Actual run per step = 3600 / 16 = 225 mm.
4. Stringer length = √(2800² + 3600²) = √(7,840,000 + 12,960,000) = √20,800,000 ≈ 4560 mm.

The calculator shows 16 steps, a 175 mm rise, a 225 mm run, and a stringer length of 4560 mm. That matches the worked example exactly.

Why Australian Stair Rules Differ

Australian building codes are stricter than many other countries when it comes to stair geometry. The key rule is that the rise and run must be consistent throughout the flight – a variation of more than 5 mm between any two steps is not allowed. Also, the sum of two rises plus one run (2R + G) should be between 550 mm and 700 mm, with a preferred value around 630 mm. This calculator gives you a layout that meets the typical domestic requirement, but you should always check your local council or the National Construction Code (NCC) for the exact values that apply to your project, especially if it’s a public building or a rental property.

How to Use the Results on a Real Stringer

Once you have the rise and run per step, you mark them on a 290×45 mm (or 2×12 inch) timber stringer using a framing square. The key is to measure the run on the horizontal leg of the square and the rise on the vertical leg, then trace the outline onto the timber. Remember to subtract the tread thickness from the bottom cut – otherwise your first step will be too high. The calculator’s tread thickness input accounts for this adjustment. After cutting, you’ll typically have a stringer that sits on the floor at the bottom and attaches to the upper floor framing at the top. Always cut the bottom with a small notch (a “kick”) to allow for the tread overhang.

Common Mistakes When Cutting Stringers

• Forgetting to subtract tread thickness – this makes the bottom step taller than the rest.
• Using the wrong number of steps – count risers, not treads. A flight with 16 risers has 15 treads.
• Rounding too early – round the number of steps first, then calculate the rise and run; don’t round the rise and run independently.
• Ignoring the stringer thickness – the calculator includes it, but many DIY guides omit it, leading to a stringer that’s too short when you account for the angle.
• Not checking the 2R+G rule – even if your rise and run are within limits, the sum may be outside the comfortable range.

Costs and Materials in Australia

In Australia, a single 290×45 mm hardwood stringer typically costs between $30 and $60 at a timber yard, depending on the species (e.g., treated pine, hardwood, or LVL). For a standard 16-step flight, you’ll need two stringers (one on each side) for a 900 mm wide stair, or three if the stair is wider than 1200 mm. You’ll also need treads (usually 32 mm thick), riser boards if you’re closing them in, and fasteners. Total material cost for a simple straight stair is often $200–$500, but labour for a professional carpenter can add $500–$1500. The calculator helps you buy the right length of stringer – a 4.8 m length is common for a 4.5 m stringer, but you’ll need to account for waste and the bottom notch.

When to Call a Professional

If your total rise is more than 3 m, if the stair is curved or has landings, or if you’re building for a rental property or public access, you should have a licensed builder or structural engineer check your design. Stairs are a leading cause of household injuries, and a poorly cut stringer can fail under load. The calculator is a planning tool, not a substitute for engineering. Also, if your floor-to-floor height is unusual (e.g., 2700 mm with a 32 mm floor covering), you need to measure the finished floor height, not the structural height – a common error that throws off the rise.

Frequently Asked Questions

What is the maximum rise for a stair in Australia?

For domestic stairs, the maximum rise per step is typically 190 mm under the National Construction Code, with a preferred maximum of 175 mm for comfort. The minimum is 115 mm. This calculator defaults to 175 mm, which is the most comfortable for most people.

How do I measure the total rise for my stairs?

Measure the vertical distance from the finished floor level at the bottom to the finished floor level at the top. If you’re installing new flooring, add the thickness of the underlay and flooring to the structural height. Don’t measure from the subfloor unless you’re sure the finish will be the same at both levels.

Do I need a landing for a straight stair?

In Australia, a straight flight of more than 18 steps (or a rise of more than 3.6 m) generally requires a landing. For most domestic two-storey homes (around 2.8 m rise), you won’t need a landing. But if your rise is above 3 m, check your local code.

What is the 2R+G rule?

It’s a comfort formula: twice the rise plus the going (run) should be between 550 mm and 700 mm. For example, a 175 mm rise and a 225 mm run gives 2×175 + 225 = 575 mm, which is within range. This rule helps ensure the stair feels natural to climb.

Can I use this calculator for outdoor stairs?

Yes, but outdoor stairs often have a slightly lower rise (around 150 mm) to account for slippery conditions and the fact that people may carry items. You can adjust the total rise and run inputs to match your desired rise. The calculator doesn’t account for drainage or anti-slip surfaces, so add those separately.

What if my total run is too short for the rise?

If the run is too short, you’ll get a steep stair with a rise above 190 mm or a run below 215 mm, which is not compliant. You’ll need to either extend the run (build a landing or move a wall) or reduce the total rise (e.g., by adding a step at the top or bottom). The calculator will show you the resulting rise and run, so you can see if it falls outside the code.

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in rise | Total rise (floor to floor) | mm | 2800 | 0
in run_total | Total run (horizontal length) | mm | 3600 | 0
in tread_thick | Tread thickness | mm | 32 | 0
in stringer_thick | Stringer thickness | mm | 45 | 0
out rise_ideal | Ideal rise per step | mm | rise / round(rise / 175) | 0
out run_ideal | Ideal run per step | mm | run_total / round(rise / 175) | 0
out steps | Number of steps | - | round(rise / 175) | 0
out stringer_length | Stringer length (hypotenuse) | mm | sqrt((rise)^2 + (run_total)^2) | 0