What is a therm? Gas units explained — therms, CCF, MCF, BTU

Therm definition

BTU (British Thermal Unit) = energy to raise 1 pound of water 1F. One therm = 100,000 BTU = enough energy to heat a typical 50-gallon water heater from 60F to 130F about 2.5 times.

Natural gas is metered by volume (cubic feet or ccf) at the meter, then converted to therms via the gas's heat content (typically 1.020-1.040 BTU/ft3).

1 ccf (100 cubic feet) ≈ 1.025 therms. Some bills show ccf, some show therms. The how-to-read-your-natural-gas-bill guide covers both formats.

Typical therm use by appliance

Furnace (1,200 sq ft home, AFUE 0.85): 50-100 therms/month winter peak.

Furnace (2,500 sq ft home, AFUE 0.85): 100-180 therms/month winter peak.

Gas water heater (40-50 gal, family of 4): 15-25 therms/month.

Gas dryer: 1-3 therms/month.

Gas oven + range: 1-3 therms/month.

Gas fireplace (decorative use): 0.5-2 therms per hour of operation.

Shopping suppliers — price per therm

In deregulated gas states (Pennsylvania, Ohio, New York, Maryland, Illinois, Indiana, Georgia, parts of Massachusetts and New Jersey), the supply rate is quoted in dollars per therm. Utility default service is also priced per therm. Compare rates apples-to-apples: include all monthly charges (some suppliers charge a $5 to $15 monthly customer fee that only breaks even above 10 therms per month of use).

Lock a fixed rate ahead of winter. Index and variable supply rates can spike 30 to 100 percent during winter cold snaps because Henry Hub natural gas wholesale prices typically run 20 to 40 percent higher in December through February than in summer. The when-to-lock-in-electricity-rate guide covers timing windows; the same logic applies to gas.

For multi-fuel households (electricity plus gas in a deregulated state), shop both commodities independently first. Some suppliers offer dual-fuel discounts of 0.5 to 1.5 cents per kWh and 5 to 15 cents per therm; verify each side of the bundle is competitive on its own before signing.

For seasonal households (vacation homes, snowbirds), pay attention to the customer charge component. A summer-only home in a heating-dominated climate may use only 60 to 100 therms per year total, against which a $10 monthly customer charge ($120/year) plus a per-therm rate may be worse than a flat-rate utility default plan.

Therm conversions for cross-fuel comparison

1 therm equals 100,000 BTU equals 29.3 kWh-equivalent of heat content. So 1 therm of natural gas at $1.50 delivers the same heat as 29.3 kWh of electric resistance heat at $0.05 per kWh. Heat pumps invert the comparison because they deliver 2 to 4 kWh-equivalent of heat per 1 kWh consumed (the COP factor).

For electric resistance heat (baseboard, strip heater, boiler with electric backup), gas is almost always cheaper. At $1.50 per therm gas costs about $5.12 per million BTU of heat. At $0.13 per kWh electric resistance heat costs about $38 per million BTU. The 7x ratio is structural in most US markets.

For heat pumps the comparison is more nuanced. A heat pump with COP 3.0 effectively produces 1 kWh-equivalent of heat for $0.043 ($0.13 / 3.0). At $1.50 per therm gas costs $0.014 per kWh-equivalent. Gas still wins on direct cost but the gap narrows substantially. The cold-climate-heat-pump-vs-furnace and dual-fuel-heat-pump-furnace guides cover the math.

For propane and heating oil, the per-therm comparison is even more favorable to natural gas. Propane at $3.50 per gallon equals about $38 per million BTU. Heating oil at $4 per gallon equals about $29 per million BTU. Natural gas at $1.50 per therm equals $15 per million BTU — roughly half of either alternative.