Maths Question

[Palustris]

I have to mix some things together at the rate of 0.5kg to a cubic metre. Now 1 cubic meter is 1,000 litres. So the question is how much do I add to 1 litre?
TIA.

[Palustris]

The stuff is sold for professional use and they would mix half a kilo to 1000 litres and use the stuff straight away. The mix has to be used within a few days of being made. There is no way I can either mix that amount nor could I use that much before the use by date is reached.
So I need to be able to mix say about 20 litres with the additive and then use it straight away.
Thanks for the help. I hope the weather is kind enough to allow me to get on with it.
The additive has to be kept refrigerated until use and even then it only has a short shelf life.

[fiction-factory]

so you'll need 10g if you're going to mix with 20 litres