Feeding a Town?

Are there any rules to determine how much farmland/how many farms are needed to support a town? Assume a town of about 1500 people with good access to land and sea travel routes, so it doesn't have to be 100% self-sustaining.

The go-to source is always Medieval demographics made easy.

There you can see that a square mile of settled land supports 180 people, so you'd need something like 8 square miles to support your town. That's pretty much all farmland.

Do, however, note that towns very seldom exist without a sizeable rural countryside. The whole point of a town is, after all, to stop self-sustainability and specialise in various professions. Therefore, I'd expect to find your 1500 people town as the center of a countryside with 15 000 or more people all in all.

I find that generally creating the impression that "these people have access to food and other things they would need" is enough to get the point across that "this is a place people can have lives." Getting it entirely realistic is probably less conducive to fun fantasy adventures, since you end up with situations where everybody is a farmer except for one priest and the one blacksmith/farrier who is also the mayor.

So if you've got a reasonably sized settlement make sure there are "Plenty of farms" to get the point across, but leave the specifics vague. If your players really do want to take a census, you can explain away any inconsistencies with historical medieval communities because the game takes place in a world where magic exists, and there are 0th level spells that will solve a lot of farming problems (e.g. "Create Water" is handy in a drought and "Purify Food and Drink" obviates most loss due to spoilage.)

You can also get really creative if you look outside of simple farming. For example, my players once visited the elemental plane of earth and stopped by a Pech village. They were hosted to a meal consisting of "meat of the stone" which was basically a giant kebab style tube of meat 3 ft. in diameter and 10 ft. long created by the Stone to Flesh spell. My calculations worked out this would be approximately 3,850 lbs of meat, enough to feed a large town if everyone eats 1 lb. per day. Note that small sized creatures by RAW eat only one quarter the weight of medium creatures, which we can assume applies to young medium humanoids as well.

Honestly though, I've never met a player who cared about more than "Which of these people can we kill so we can keep their stuff?" I'd expect the question of how these peasant villagers feed themselves is so low on the players' list of priorities it'll never really matter.

A 5th level druid or cleric with the plant domain reduces the needed land by a third if he is active in the community and visits each farmstead atleast once a year. (Plant Growth)