Sink drains come in both sizes. A bathroom basin typically uses a 1 1/4-inch waste, commonly labelled 32 mm in Australia, while a kitchen sink usually uses a 1 1/2-inch waste, commonly labelled 40 mm. Check the existing fitting and product specifications because sizes and connections can vary.
| Fixture | Typical Australian size | Common inch label |
|---|---|---|
| Bathroom basin | 32 mm | 1 1/4 inch |
| Kitchen sink | 40 mm | 1 1/2 inch |
These are nominal product categories, not exact conversions. Mathematically, 1 1/4 inches is 31.75 mm and 1 1/2 inches is 38.1 mm. A component labelled 40 mm therefore does not necessarily measure exactly 40 mm across every part. The method plumbers use to measure and check pipe size accounts for which diameter or thread is being identified.
How to check the existing size
Check the markings and manufacturer’s specifications rather than relying on sink type or an approximate diameter measurement.
- Look for a moulded, stamped or printed size or product code on the accessible waste fitting, tailpiece or trap.
- Confirm what the measurement describes. Waste bodies, outlet threads, tailpieces, trap connections and nominal pipe sizes may have different dimensions.
- If you measure an accessible part, record whether it is the inside diameter, outside diameter or thread diameter, then compare it with the manufacturer’s technical drawing.
- Match the stated size, thread, seal and joint type. Do not force together components that appear close in size.
If the reason for checking is a slow or blocked sink, identify whether the accessible sink P-trap needs cleaning before assuming the waste size is wrong. Do not dismantle fixed or concealed wastewater pipework just to identify its size.
If markings and specifications do not confirm compatibility, contact our licensed plumbing team to assess the assembly before it is altered.