''How many litres?'' After a good deal of measuring, all current planters on the website now feature their approximate litre capacity.
Hopefully this will now prove very helpful when it comes to working out how many bags of compost each pot requires.
Now, this is of course subject to many variables > so do use the suggested amounts purely as a guide. For example, say we use a 40ltr planter as a case study... this 40ltr space will shrink after using crocks to line the base and, again, the rootball of your plant(s) will take up more space. What is listed as a 40ltr pot may end up requiring only 25ltrs of growing medium.
 In the case of large planters, there is almost a reverse effect. Compost in large quantities can see a decent amount of shrinkage, so you may find you require a little extra than stated... again, taking into consideration the size of your plants rootball.
A simple trick I always do at home is to fill a large pot with soil before planting and to then water the compost down before placing the plant inside. This way I've already slightly reduced the compost and I can make sure I'm planting at the golden zone of around an inch below the top of the rim. 
In extra large pots I'll often plant right up to the rim of the pot. Although this can look too high in the short term (and can be a bit of a nuisance when watering as water will easily spill over the rim of the pot), in the long term the growing medium will compact and I find it will end up at just about the right height.
All of my notes put aside, use these capacities as a guide and happy gardening!
Thomas Pearson