Dad’s short answer: Big enough to finish a typical mess without refilling. Most spot cleaners hold well under a gallon, and the dirty tank is what actually stops you — it fills with everything the clean tank put down plus the mess itself.

Match the tank to your actual messes

For a single pet accident or a spilled drink, almost any tank size manages fine. For a dog that had a bad night in three different rooms, or a whole car interior, you'll be walking to the sink.

The dirty tank is the limiting one. It receives everything the clean tank dispensed plus the liquid already in the carpet, so it fills first. If you're comparing machines, that's the number to weigh.

The trade-off is weight. These are carried to the mess, often up stairs or out to a car, and a big full tank gets heavy. A mini unit with small tanks is genuinely more pleasant to carry for small jobs, which is why the compact models sell so well.

Two things that matter more than capacity

How easy the tanks are to fill and empty. A wide opening you can fill under a tap without spilling beats a bigger tank with an awkward neck. On one upright I tested, filling the tank let water sneak into the handle and spill when you set it back in — a small design flaw you notice every single time.

Whether the tanks come apart for rinsing. The dirty tank needs a real rinse after every use, and one that doesn't open properly ends up smelling.

If you're mostly doing small accidents, buy the lighter machine. If you're doing a car every weekend or dealing with an older pet, go bigger and accept the weight.

Dad’s Take: The dirty tank is what ends your session, not the clean one. And rinse it every time or it'll smell by next week.