Scaling up water for cooking soup I'm cooking soup as a starter for Christmas dinner. I've made this soup before and I used 1.5 litres of water for 4 servings. I'm making 12 servings for Christmas day so that should be 4.5 litres of water required if I just put in 3 times as much of everything.

I'm not sure if I've correctly scaled up the water needed, 4.5 litres sounds like a lot of water. Maybe when cooking 4 servings 1.5 litres of water is used but 0.5 litres boils off, then with 12 servings 3.5 litres is used and still 0.5 litres boils off. Is this a smarter way to scale up the water needed?

Edit: To make 4 servings I add 6 tomatoes, a leek and 3 potatoes; I simmer it for 30 minutes with the lid on.
1.5 litres for 4 servings is 375ml per serving (plus some volume from the veg which I'll ignore) assuming no water boils off. That's a sensible portion. I reckon my soup bowls hold just a little less than that, but you'll leave some in the pan when serving . So I doubt you lose a lot of water when you normally cook it. 

That said, I'd err on the side of caution and (i)  measure the bowls (both the normal ones and the ones you plan to serve it in if they're not the same); and (ii)  start with a little less water than you think you need - adding water is quick, boiling it off less so and you don't need the hassle of your soup taking too long.