The easiest way to think about skip bin volume is in trailers. A standard 6x4 trailer holds just under a cubic metre when heaped up. So a 3m³ bin is roughly three trailer loads, a 6m³ is six, and so on. Most people have a reasonable feel for trailers even if cubic metres mean nothing to them.
The other trick: walk through the job before you book. Think about the bulkiest items you'll be loading - mattresses, old sofas, a kitchen worth of cabinetry. Bulky things eat volume fast even if they're not heavy. Then add a buffer. We'd rather you have a bit of space left over than stare at a pile of rubbish next to a full bin.
Not sure where you land? Our skip bin sizes page breaks down every bin in the fleet with dimensions and typical uses.
The Red Bin Estimate
If cubic metres still aren't clicking, try this. Most Sydney households have a 240L red bin for general waste (the wheelie bin that goes out every week). One of those is roughly 0.25m³, which means four red bins fill a 1m³ space.
As a rough guide: a 2m³ skip holds around 8 red bins worth of rubbish. A 4m³ holds about 16. A 6m³ holds around 25. If you have the smaller 120L red bins in your area, just double those numbers.
Walk through your job, count the red bins in your head, and pick the skip that comfortably covers it with a bit extra. If you're hovering between two sizes, size up. It's almost always cheaper than a second delivery.