Find the manufacturer rating plate attached to your natural gas appliance.
Read the line on the rating plate that describes your appliance's BTU input. For example, the third line from the top of the rating plate on your gas stove reads "INPUT BTU 74,000 TYPE GAS NATURAL."
Make an estimate of how many hours each month your natural gas appliance is operating. For example, you estimate you use your gas stove 10 minutes every day. Ten minutes of daily use for 30 days is 300 minutes (10 x 30 = 300) of use per month, or 5 hours (300/60 = 5) of use per month.
Multiply the BTU input of your natural gas appliance by your monthly usage in hours, using a calculator. For example, 74,000 x 5 = 370,000.
Divide your answer by 100,000. For example, 370,000/100,000 = 3.7. Your gas stove's therm usage in a month of 30 days is approximately 3.7 therms.