Not your fault, but that is the most annoying calculator I've ever encountered, as someone who uses the metric system.
I mean, what kind of maniac describes the amount of oxygen produced in pounds?
Also are those US gallons or UK gallons?
The increments used for the circumference of the tree is also incredibly weird, 7 and 3/4 inches? Really? Clearly converted metric to imperial. Why not include a slider to switch to metric, if that's what you've based your numbers on?