How this instrument works
A therm is a fixed energy unit built for gas billing: the US therm equals exactly 100,000 British thermal units, where one BTU is the heat needed to raise one pound of water by one degree Fahrenheit. US utilities have historically metered gas by volume but billed customers by heat content, and the therm is the bridge between those two measurements -- the Energy Information Administration, the federal agency that tracks US energy statistics, uses the therm as the standard unit on residential gas bills for exactly this reason. Because the therm-to-BTU relationship is a fixed definition rather than a measured average, this conversion is exact, unlike converting a raw volume of gas (cubic feet) to energy, where the heat content per cubic foot varies slightly by region and season.
The BTU figure matters most when a gas bill's therms need to line up with an appliance's rated output. Furnaces, water heaters, and gas ranges are commonly rated in BTU or BTU per hour, so converting a monthly therm total to BTU makes it possible to compare a household's actual gas usage against the heat output of the equipment consuming it, or to check whether a proposed furnace size is proportionate to typical usage. A homeowner reading "75 therms" on a winter bill and "80,000 BTU/hr" on a furnace nameplate is looking at the same energy unit expressed two different ways, and this page converts between them without estimating anything -- the 100,000 figure per therm never changes.
- Enter your gas usage into Therms; it opens at 10, matching a small worked example.
- Read the converted energy from British thermal units (BTU) beneath it -- it updates as the Therms field changes.
- The page opens already showing 10 therms equal to 1,000,000 BTU, so you can check the fixed 100,000-per-therm relationship before entering your own bill's figure.
- To compare against an appliance's rating, look for its BTU or BTU/hr figure on its nameplate or spec sheet and convert your therm usage here to the same unit.
Worked example -- 10 therms to BTU
Enter 10 into Therms, the field's default. British thermal units (BTU) reads 1,000,000 -- ten therms at exactly 100,000 BTU each comes to an even one million BTU, with nothing left to round since the therm-to-BTU relationship is a fixed definition.
A US household's monthly winter gas usage often runs around 75 therms. Entering 75 into Therms returns 7,500,000 BTU -- useful for checking that figure against the combined BTU/hr ratings of a home's furnace and water heater running over the course of a billing month, rather than comparing a therm number to a BTU number directly.
Questions
What is a therm, exactly?
A therm is a fixed unit of heat energy equal to exactly 100,000 British thermal units. The US Energy Information Administration uses it as the standard billing unit on residential natural gas bills, since it lets a utility charge for the heat content delivered rather than a raw volume of gas.
Why 100,000 and not some other number?
The therm was defined as 100,000 BTU by convention, chosen because it was already a familiar round figure close to the heat content of roughly 100 cubic feet of typical natural gas -- close enough to be a convenient billing unit, though the underlying BTU definition is what makes the therm-to-BTU conversion exact rather than approximate.
Is 1 therm always exactly 100 cubic feet of gas?
No, only approximately. A therm is fixed by its BTU definition, but the actual heat content of 100 cubic feet of natural gas varies by region, supplier, and season depending on the gas's exact composition, according to the US Energy Information Administration. That variability is exactly why this page converts therms to BTU using the fixed definition rather than estimating from a cubic-feet figure.
Why do I need BTU instead of just reading my therms off the bill?
Because gas appliances -- furnaces, water heaters, ranges, dryers -- are almost always rated in BTU or BTU per hour, not therms. Converting your billed therms to BTU puts your actual usage in the same unit as the equipment's rated output, which is what you need to sanity-check a furnace size or compare usage across different-sized homes.
How does this differ from converting Ccf (hundred cubic feet) to therms?
Ccf-to-therms involves an estimate, since it multiplies a gas volume by a heating-value assumption (commonly the EIA's cited national average of about 1.036 therms per Ccf) that varies by region and year. This page instead converts an already-billed therm figure to BTU using the fixed 100,000-BTU-per-therm definition, so no assumption or average is involved.