Dad’s short answer: Big enough to finish your hard floors in one go. Most machines hold somewhere under a liter in each tank, and running out mid-clean means walking to the sink twice — dump the dirty, refill the clean — which is exactly the friction you bought the machine to avoid.

Match the tank to your floor, not to a number

The spec that matters isn't the liter figure on its own, it's whether you can do your hard-floor area on one fill. A galley kitchen and a bathroom, almost any machine manages. An open-plan main floor with kitchen, dining and a hallway, you want the bigger tanks.

The dirty tank is usually smaller than the clean tank, and that's the one that actually limits you — it fills with everything the clean tank put down plus the dirt. When people say they ran out mid-clean, it's normally the dirty tank that stopped them.

If you're between two machines and one has noticeably bigger tanks, that's worth more day to day than most of the smart features.

The dock tank on flagship machines

Some flagship units add tanks in the dock itself, so the dock refills the machine's clean tank and empties the dirty one for you. On the unit I tested that does this, the dock also heats the water before refilling.

It's a genuine convenience and it means your effective capacity is the dock's tanks rather than the machine's. It's also the kind of feature that pushes a machine into flagship pricing, so decide whether your floor area actually needs it.

Dad’s Take: Count on the dirty tank, not the clean one. That's the tank that ends your run.