Calculate your gas bill in advance to avoid expensive surprises and spot potential mistakes on your bill. Checking the gas meter reveals exactly how much gas you've consumed, while adding up the amount used by each appliance can provide you with a useful estimate when the meter isn't accessible.
Things You'll Need
 Calculator
 Pencil
 Paper
 Appliance manuals
Reading the Meter

Check your gas meter on the first day of your billing cycle. Note the number you see. Gas meters provide usage figures in CCF, or 100 cubic feet.

Look at the gas meter again on the last day of your billing cycle. Note the value you see.

Subtract the starting value from the ending value to determine how much natural gas you used that month, measured in CCF.

Multiply the CCF value by 1.025 to convert to therms, the standard units of measure for natural gas billing. For example, 200 CCF of natural gas equals 205 therms.

Check a recent gas bill to find the cost per therm of gas in your area. Multiply the cost per therm times the number of therms consumed to calculate your bill for gas.

Add in extras. Check with your gas company to learn about delivery charges, storage, pressure charges or other extras to get an accurate estimate of your gas bill.
Calculating Consumption

Check the manuals, tags or metal plates on gas appliances to find out how much gas they consume, measured in British thermal units, or Btu. For example, the largest burner on a gas stove may be listed at 10,000 Btu. This means the burner uses 10,000 Btu for each hour of use. Estimate how many hours you use each gas appliance  including a gas water heater, furnace, and other major heating appliances  and use the rating on each appliance to calculate total monthly gas consumption in Btu.

Convert your Btu estimate to therms by dividing your total monthly Btu consumption by 100,000. For example, if you estimate you consume 5,000,000 Btu per month, your usage in therms is 50.

Multiply the number of therms by the local gas price per therm.

Add in extras. Check previous gas bills or contact your gas provider to ask about the costs of extras, including delivery, pressure charges and other fees. Add these costs to the value you calculated to determine your total gas bill.
References
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