A hobby grower in Minnesota wants fresh tomatoes in March, which means holding a small glass box thirty degrees warmer than a frozen night. In plain words, a greenhouse's heating load grows with two things: its floor area and the temperature gap between inside and outside. Double the area and you double the heat escaping through the glazing; double the gap and you double the rate it escapes. A 120-square-foot hobby house held 30 degrees above a freezing night needs roughly twice the heater of the same house held only 15 degrees above. Backyard growers sizing propane or electric heaters, and homesteaders budgeting winter growing costs, start here. Undersize the heater and a cold snap wipes out the seedlings; oversize it and the electric bill eats the tomato savings.

This calculator estimates the relative heating load. Type the floor area in square feet into the Variable A box, say 120, and the inside-minus-outside temperature difference in Fahrenheit into the Variable B box, say 30. Read the proportional heat requirement from the Result box: 3600. Compare two sizes or two target temperatures to feel the trade-off before buying hardware.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Greenhouse Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.