If you're just looking for a quick calculation you can do in your head at 3 AM after a long night of boiling, a simple rule is this: Your average sugar maple puts out 2% sap, and it takes around 40:1 to turn that into syrup.

So if a batch of syrup takes around 80 gallons of sap per gallon of syrup, your sap is running around 1%.

That make sense?