The units on a gas bill
| Unit | What it measures | Equals |
|---|---|---|
| Btu | Heat energy | The base unit of heat energy |
| Therm | Heat energy | 100,000 Btu, or about 29.3 kWh |
| kWh | Electric energy | 3,412 Btu |
| CCF | Gas volume | 100 cubic feet, about 1.03 to 1.04 therms |
| Mcf | Gas volume | 1,000 cubic feet, about 10.3 to 10.4 therms |
The conversion from volume to heat depends on the heat content of the gas, which varies a little by region and month. Your bill shows the factor the utility used. It may be labeled therm factor or BTU factor.
How a gas bill uses therms
The meter counts cubic feet, usually shown in CCF. The utility multiplies by a heat content factor to get therms and then bills a price per therm, plus a fixed monthly charge and delivery charges. Some bills show the supply price and the delivery price separately, as electric bills do.
A worked gas bill, with made-up numbers
Say the meter shows 62 CCF used in a month, and the bill lists a heat factor of 1.037 therms per CCF. That is 64.3 therms. At an assumed all-in price of $1.45 per therm plus a $12 fixed charge, the bill is $93.23 plus $12, a total of $105.23.
The numbers are invented to show the steps. Your bill lists the factor, the price and the fixed charge, so you can repeat the arithmetic with your own figures.
Reading your own gas use
Look at your lowest summer month. In most homes that is water heating and cooking only, and it works as a baseline. Subtract the baseline from a winter month to estimate what heating used. If your baseline is high, check the water heater setting and look for leaks. If your winter number is high for your home size, an energy audit can find drafts and gaps in insulation.
Comparing gas with electricity
To compare, put both fuels on the same footing: the cost of one kWh-equivalent of heat delivered to the home. This example uses a gas price of $1.40 per therm, which is an assumed number for illustration. Use your own bill for the real figure.
| Heating option | Assumption | Cents per kWh of heat |
|---|---|---|
| Gas furnace | $1.40 per therm, 95 percent efficient | 5.03 |
| Electric resistance heat | 17.23 cents per kWh, 100 percent efficient | 17.23 |
| Electric heat pump | 17.23 cents per kWh, three units of heat per unit of electricity | 5.74 |
With these example prices, a heat pump and a gas furnace land in the same range, and electric resistance heat costs the most. Prices vary a lot by state and by year, and the heat pump figure depends on the outdoor temperature, so run your own numbers. The arithmetic is cost per therm divided by 29.3, divided by the efficiency.
Other fuels use other units
Propane and heating oil are sold by the gallon, not by the therm. A gallon of propane holds about 0.9 therm of heat and a gallon of heating oil about 1.4 therms, so you can convert a delivery receipt to therms and compare it with gas or electricity. Wood is sold by the cord, and electric bills use kWh. The conversion for any fuel is the same idea: find the heat in the unit you buy, divide the price by it, then adjust for how efficient your equipment is.
What a therm costs and what affects it
The price per therm changes with the cost of natural gas, the delivery charges and the season. Winter bills are larger mainly because you use more therms, not because the price moves much. To judge your own usage, divide the therms on a bill by the days in the period and compare winter with summer. Summer therms mostly cover water heating and cooking, so they show your baseline.
Homes that use both gas and electricity should compare their combined spend, not only the electric bill. Our bill checker covers the electric side.
Sources and further reading
Frequently asked questions
What is a therm?
A therm is 100,000 British thermal units of heat energy. It is the unit most natural gas utilities in the United States use for billing, along with the CCF.
How many therms are in a CCF?
One CCF of natural gas holds about 1.03 to 1.04 therms. Your bill shows the exact factor for your area and month.
How many kWh are in a therm?
One therm equals about 29.3 kWh of heat energy, because a kWh is 3,412 Btu and a therm is 100,000 Btu.
Is a therm cheaper than a kWh?
They measure different amounts of energy, so compare them per unit of heat. A therm holds about 29.3 times the energy of a kWh. Which is cheaper per unit of heat depends on local gas and electric prices and on the efficiency of your equipment.