(1) Acre=43560 sf Why such an odd number?
The fundamental unit of land measurement in the US is the 'Township' (taken from the old English 'Parish'). A Township is 6 miles on a side, and is divided into sections, each one mile on a side. These are divided in half along each side to create quarter sections, then divided again the same way to create 4 parcels, each of 40 acres. So for each quarter section there are two "Front 40's" and two "Back 40's", which is where the term 'the back 40' came from. That makes an acre:
(5280 ft/mi /2/2) x (5280 /2/2) / 40=43560 sf. An odd-ball number, but there it is.
(1) cubic foot=7.48 gal. How can I remember this?
Picture a one-gallon milk jug. You can take four of them and set them together to make a square, if you turn them so the handles are sticking on the outside of the square. Now, if you put a piece of cardboard on them, you could stack on four more. That would be 8 one-gallon jugs in a space just one foot by one foot square, by one foot high; one cubic foot, in other words. But of course the handles are sticking out, so take off a half gallon for them and you get 7.5 gallons in a cubic foot. That's within 1/4 of one percent (0.25%). Not bad.