In UK it’s not uncommon to see price tags that say something like '50p per litre' (for bottled water) or '50p per 100 sheet' (for toilet paper).

You can see the prices per unit on their websites:

https://www.tesco.com/groceries/en-GB/search?query=bottled%2...

Not sure if this is required by regulations or it’s just the supermarkets being customer-friendly though.