Three everyday building calculations that each have an Australian twist: setting out a staircase to code, measuring floor area the way suppliers expect, and sizing an air conditioner in both kW and BTU for our climate.
A staircase is governed by two measurements: the riser (the vertical height of each step) and the going (the horizontal depth you tread on). Getting the count right starts from the total floor-to-floor height.
Take a floor-to-floor rise of 2,700mm. Divide by a comfortable riser height of around 180mm and you get 15 — but the real number of risers is chosen so each one is equal and within code. Seventeen risers gives about 159mm each, which is comfortable and compliant. The number of goings is always one fewer than the risers, so 17 risers means 16 goings. At a 250mm going, the staircase needs 16 × 250 = 4,000mm of horizontal run.
Our stair calculator takes the total rise and target dimensions and returns equal risers, the going, and the total run.
The code also governs headroom, balustrade height, and the maximum gap in a balustrade. Because these requirements change and are enforced at inspection, treat any calculator output as a starting point and confirm the current National Construction Code requirements — or have a licensed builder set out the stair — before cutting a stringer.
Floor area in square metres is the starting point for flooring, tiling, paint, and air conditioning, so it's worth measuring cleanly. For a rectangular room it is simply length × width — a 4m × 5m room is 20 m².
For an L-shaped or irregular room, split it into rectangles, calculate each, and add them. Don't try to average an odd shape. Our area calculator handles rectangles and combinations, and despite the name works in square metres for Australian use.
Air conditioner capacity is quoted two ways in Australia: kilowatts (kW) on the unit's label, and BTU per hour in a lot of online material. They measure the same thing, and one kilowatt is about 3,412 BTU/h.
A rough starting point for cooling is around 0.15 kW per square metre of floor for a standard, reasonably insulated room. Our 20 m² room therefore needs roughly 20 × 0.15 = 3 kW of cooling — about 10,200 BTU/h. Our BTU calculator gives both figures.
That 0.15 kW/m² figure is only a starting estimate. Several things push the real requirement up or down:
For anything beyond a rough estimate, an air conditioning installer will do a proper heat-load calculation that accounts for all of these.
For stairs, most of the cost is materials and skilled labour, and a compliant, well-finished staircase is not a place to cut corners. For air conditioning, the unit is only part of the outlay — installation, electrical work, and running costs over the unit's life all matter, and a correctly sized efficient unit is cheaper to run than an oversized one. Because prices vary, use the calculators to fix your dimensions and capacity, then get quotes against those figures.
How do I calculate the number of stairs I need?
Divide the total floor-to-floor height by a comfortable riser height of around 180mm to get an approximate riser count, then adjust so every riser is equal and within code. The number of goings is always one fewer than the number of risers.
What are the Australian rules for stair dimensions?
The National Construction Code sets minimum and maximum limits for riser and going dimensions and requires every step in a flight to be consistent, along with rules for headroom and balustrades. Requirements change and are checked at inspection, so confirm current figures or use a licensed builder.
How do I calculate the square metres of a room?
Multiply length by width for a rectangular room — a 4m by 5m room is 20 square metres. For an L-shaped room, divide it into rectangles, calculate each, and add them together.
What size air conditioner do I need?
As a rough starting point, allow around 0.15 kW of cooling per square metre for a standard insulated room, so a 20 square metre room needs about 3 kW, or roughly 10,200 BTU per hour. Ceiling height, windows, orientation, insulation and climate all adjust this.
What's the difference between kW and BTU for air conditioning?
They measure the same cooling capacity in different units. One kilowatt is approximately 3,412 BTU per hour. Australian units are labelled in kW, while a lot of online material uses BTU per hour, so it helps to know both.
Is a bigger air conditioner better?
No. An oversized unit cools the air fast but switches off before removing humidity, leaving the room cold and clammy while cycling on and off inefficiently. Correct sizing for the room gives better comfort and lower running costs.