How do I work out the right amount of feed when I'm going to loan a horse?

The best place to start is the owner, who will already have a feed ration worked out for the horse. Once you know what it usually eats, I am happy to look over that ration with you if you like. If you would rather work it out yourself, there are good feed-ration calculators online, and there are also videos that walk you through how to body condition score. Hästsverige (a Swedish resource) is one place that offers both.

Score the body condition regularly. That way you can see whether the horse is gaining or losing condition and adjust the rations quickly as it changes.