When should you NOT buy 50% diluted ready to use coolant?
If you're planning on flushing your cooling system and replacing the coolant, always]/i] buy undiluted coolant - here's the reason why.
The recommended procedure is usually to drain the existing coolant, then flush the cooling system with de-ionized water until it drains clear (colorless), and then refill with coolant - the problem with this procedure is that the flushing process leaves an unknown quantity of water trapped in the engine block. If you purchase undiluted coolant, you can lookup the manufacturer's spec for the cooling system capacity, divide that in half, measure that amount of undiluted coolant, add it to the cooling system and then top it up with de-ionized water - that will bring you a lot closer to the desired 50% coolant strength than if you simply fill with 50% diluted ready to use coolant.
You can now measure the remaining undiluted coolant, add an equal volume of de-ionized water - you now have 50% diluted coolant ready to use if you need to top up, and you know that it's compatible with what's already in the cooling system