AS cmb said use a measuring stick I use my old faithful spoon measuring stick it is marked in 5 and 1 litre sections
its a 1 metre lenth of copper pipe with a spoon stuck in it and the markings are made visibe by quarter cutting the pipe with a pipe cutter to score it
Just tried me batch I brewed a few months ago tastes fantastic.
Brewed using 100 grams of cascade hops boiled in litre and half of water added to the mix with 500 grams spray malt light hopped and 500 grams of brew enhancer.