We have a 12 litre bin for general garbage in our kitchen. Initially I bought 20 litre garbage bags, but they barely fitted and often tore when stretched over the top rim.
So I stepped up to 30 litre bags, which fit just fine … apparently, in my kitchen, 30 goes into 12 exactly once.
Do you fold the top over the edge? Maybe it’s called “30 litre” because that’s the volume of an unrolled bag?
Its your own fault for using metrics. If gallons were good enough for Jesus, they’re good enough for me.
Oh, a little, but this thing is allegedly more than double the capacity of the container it fits in fairly snugly!
It’s a fair cop - I’m am personally responsible for mandating the metric system in Australia, after all.
But at least we don’t have a system where if you buy a drink in a cup its in fluid ounces, but in a bottle it’s in metric!
I use 35 liter bags in my 20 liter container. They “fit” exactly the way I want.
I don’t understand the mathematical rationale either. I just do it.
I, OTOH, stockpiled plastic grocery bags for years before they were outlawed. They fit perfectly in the little kitchen bin. At my current rate of consumption I figure I have enough to last about 300 years. 
The bin size is the number of litres it takes to fill it up. The same applies to the liner, but you don’t want one that fits exactly - you want some fold-over and that can be a problem, especially if the bin is a tall, thin one.
The only solution is to try different sizes but start with one that is 50% oversize.
I’m convinced that garbage bag sizing is all just one big con-job, but I’m not sure what the point of it is.
Around here, we not only have the sizing in litres, as discussed, but some brands even print pictures of the standard designs of cans that they’re supposed to fit. Even going by the pictures, it’s all a goddamned lie. I’ve never had a bag fit the can it’s “supposed” to fit.
I just measured the bin and it is roughly 25 x 35 x 45 cm, nearly 40 l. The bags I use are 90 l.
Mine’s 35 x 16 x 21, so just under 12 l in fact. I’m certain I could not fit two of them in the 30 l bags I use!
So are they short when you put the bag in the bin, or does the bag just not fit around the rim of the bin?
When I used the 20 l bags, they barely fitted over the rim of the 12 l bin, and sometimes tore when putting them on.
The 30 l bags are rather looser and there’s no problem like that with them.