Useful Chart

whole milk: 3.5% butterfat
half-and-half: 11% butterfat
light cream: 18% butterfat, though the rules say between 18 and 30%.
[single cream, UK]: 20% butterfat
light whipping cream: 30-36% butterfat.
heavy cream: 36% butterfat (or more, in theory)
[double cream, UK]: 48% butterfat. Or more.
butter (US): 80% butterfat
butter (European style): 85-90% butterfat

Maybe this is why the heavy cream totally failed to make butter–not enough fat.  Not that I object to whipped cream, but it wasn’t what I was going for…