But concrete is measured by volume at least in the US (cubic yards…sigh). I think this is because different mix designs can change the resulting weight. I’ve even heard of adding air to make for better pumping.

Air entrainment is critical for freeze-thaw protection, so any exposed concrete in places that have below freezing should have 5-6% air entrained.

The bags of ingredients at the hardware store are usually 20-25kgs here, and labelled that way.

It’s a silly way to buy a lot of concrete, but does help someone without a tow bar.