Therm
What is a therm?
A therm is one of those old units that should have disappeared with metrication but has stubbornly hung on in pockets of the UK gas industry. It is an imperial unit of energy equal to 100,000 British Thermal Units (BTU), which works out to about 29.31 kWh. UK retail business gas bills almost always price energy in kWh now. But therms are still around in wholesale gas pricing, in some industrial contracts, and in the legacy documentation that older sites work from. You will sometimes see a wholesale price quoted in pence per therm even though the customer eventually gets billed in pence per kWh. The two units are interchangeable for the same amount of energy, but the conversion still trips people up.
Therms still pop up in some business gas contracts in the UK, though kWh has largely taken over as the billing unit on most modern supplies.
On this page
Therm is a unit of energy. Pence per therm is a price for that energy. Both turn up in business gas contracts, wholesale references, and older bill formats. Understanding when therms appear and how they convert to kWh helps you read gas bills, compare wholesale references, and avoid the small but irritating arithmetic mistakes that happen when the two units sit side by side without anyone explaining the difference.
What a therm actually is
A therm is an imperial unit of energy.
- 1 therm = 100,000 British Thermal Units (BTU).
- A BTU is the energy required to raise one pound of water by one degree Fahrenheit at sea level.
- 1 therm equals approximately 105.5 megajoules (MJ).
- 1 therm equals approximately 29.31 kilowatt-hours (kWh).
The unit is American in origin, has been used across the UK gas industry historically, and survives in selective use today even after metric units have taken over for billing.
Therms to kWh conversion
The conversion is simple but worth committing to memory.
| Therms | kWh equivalent |
|---|---|
| 1 therm | 29.31 kWh |
| 10 therms | 293.1 kWh |
| 100 therms | 2,931 kWh |
| 1,000 therms | 29,310 kWh |
| 1 MWh (1,000 kWh) | 34.13 therms |
The constant is 29.31. Multiply therms by 29.31 to get kWh. Divide kWh by 29.31 to get therms.
Where therms still appear
Therms turn up in three main places in the UK gas market today.
- Wholesale gas pricing. NBP (National Balancing Point) wholesale gas prices are sometimes quoted in pence per therm.
- Industrial contracts. Some long-running pass-through contracts reference unit rates in pence per therm.
- Older bill formats. Sites that have been on continuous supply for many years sometimes still see therms on bills, particularly on imperial meters.
Outside these three contexts, therms are rare. The retail UK business gas market is firmly in kWh.
Therms in wholesale gas pricing
The UK wholesale gas market trades around the National Balancing Point (NBP), which is a virtual hub where ownership of gas changes hands.
- NBP gas can be quoted in pence per therm (p/th) or pence per kWh.
- Both are common. Different brokers, trading platforms, and reference services use one or the other.
- A NBP price of 60 p/therm equals about 2.05 p/kWh. A NBP price of 100 p/therm equals about 3.41 p/kWh.
For UK business customers on pass-through or flex contracts, the wholesale price reference in the contract usually defines whether the supplier passes through in p/therm or p/kWh. The maths is identical; the unit just needs to match what the customer is being billed in.
Therms on retail bills
UK retail business gas bills almost always show kWh as the energy unit.
- The volume from the meter (in cubic metres or hundreds of cubic feet) is converted to kWh on the bill.
- Calorific value and volume correction handle the conversion behind the scenes.
- The unit rate is shown in p/kWh.
If you see therms on a retail business bill in 2026, it is normally one of three things. A legacy contract that has not been updated. A bill from a specialist industrial supplier that has retained the old unit. Or a misformatted bill that should be challenged.
Therms in industrial contracts
Some larger industrial UK gas contracts (particularly on DM gas sites with long-term pass-through arrangements) still use p/therm for the unit rate.
- The site’s daily kWh consumption is converted to therms (divide by 29.31).
- Therms are multiplied by the unit rate in p/therm.
- The result is the variable energy cost for that period.
The maths works exactly the same way as a p/kWh contract; only the unit of measure differs. For a customer comparing competing offers, one in p/therm and one in p/kWh, the conversion is essential to avoid an apples-to-oranges comparison.
Why therms have survived
The UK started moving away from therms in the 1990s as part of broader metrication. Retail gas bills switched to kWh in stages. Wholesale gas references followed less consistently, partly because the international gas market continued to use a mix of units (Btu, therms, kWh, MMBtu) and partly because long-running industrial contracts were rarely renegotiated purely to update the unit.
The result is a market where the consumer-facing side is fully metric and the wholesale and industrial side is mixed. Therms persist in the latter because the contracts persist, and the contracts persist because changing the unit would not change anything material about the deal.
Worked example. Therms to kWh
Illustrative example. A wholesale gas trader quotes the NBP day-ahead price at 65 p/therm. A UK business customer with a pass-through gas contract asks the broker what that translates to in p/kWh on their bill, and what it means for their monthly cost on a 100,000 kWh per month consumption.
| Step | Calculation | Result |
|---|---|---|
| Convert p/therm to p/kWh | 65 ÷ 29.31 | 2.218 p/kWh |
| Add supplier margin (illustrative) | 2.218 + 0.5 p/kWh | 2.718 p/kWh |
| Apply to consumption | 100,000 kWh × 0.02718 | £2,718 |
| Add standing charge, CCL, VAT | (separate) | (additional) |
Example only. Real wholesale prices and supplier margins vary. The point is the conversion, not the precise numbers.
Practical implications
For most UK business gas customers, therms are background trivia. Three situations where they matter.
- Comparing competing offers. If one supplier quotes in p/therm and another in p/kWh, convert both to the same unit before comparing.
- Reading wholesale price references. If your contract references NBP day-ahead in p/therm but your bill shows p/kWh, knowing the 29.31 conversion lets you tie the two together.
- Legacy contracts. If you inherit a long-running gas contract that uses therms, renewal is a natural moment to switch to kWh for cleaner ongoing comparison.
Comparing therm prices to kWh prices
To convert p/therm to p/kWh, divide by 29.31.
| p/therm | p/kWh equivalent |
|---|---|
| 30 p/therm | 1.024 p/kWh |
| 60 p/therm | 2.047 p/kWh |
| 90 p/therm | 3.071 p/kWh |
| 120 p/therm | 4.095 p/kWh |
| 150 p/therm | 5.118 p/kWh |
Related entries. kWh, calorific value, Annual Quantity (AQ), daily metered gas, NBP, bill validation.
For the kWh-per-m³ multiplier that sits behind UK gas billing, see the gas conversion factor.
Frequently asked questions
What is a therm?
An imperial unit of energy equal to 100,000 British Thermal Units (BTU), approximately 105.5 megajoules or 29.31 kilowatt-hours. Still used in UK wholesale gas pricing and some industrial contracts.
How many kWh is a therm?
Approximately 29.31 kWh. Multiply therms by 29.31 to get kWh. Divide kWh by 29.31 to get therms.
Are therms still used on UK gas bills?
On retail business gas bills, almost never. Bills are normally in kWh. Therms persist in wholesale gas pricing, in some industrial contracts, and on very old or specialist bill formats.
What is the conversion from p/therm to p/kWh?
Divide pence per therm by 29.31. So 60 p/therm equals about 2.05 p/kWh, 90 p/therm equals about 3.07 p/kWh.
Why is wholesale gas sometimes priced in p/therm?
Historic convention. NBP wholesale prices have traditionally been quoted in p/therm, and many brokers, trading platforms, and reference services have retained the unit even as retail prices moved to p/kWh.
What is NBP?
National Balancing Point. The virtual hub for UK wholesale gas trading where ownership of gas changes hands. Prices are quoted in pence per therm or pence per kWh.
Are therms and BTUs the same thing?
Related but different. 1 therm equals 100,000 BTU. A BTU is the energy needed to raise one pound of water by one degree Fahrenheit. The therm groups BTUs into a more usable industrial-scale unit.
Why did the UK switch from therms to kWh?
As part of metrication. Retail gas bills switched to kWh in stages during the 1990s and 2000s. Wholesale and industrial contracts followed less consistently, leaving therms in selective use today.
How do I convert therms to MJ?
1 therm equals approximately 105.5 megajoules. So 100 therms equals about 10,550 MJ.
How is the therm related to calorific value?
CV decides how much volume of gas equals one therm of energy. Higher CV means fewer cubic metres per therm. CV is published per LDZ daily; the therm itself is a fixed energy unit.
Is a therm the same in the UK and the US?
Yes. 1 therm equals 100,000 BTU in both systems. Both countries use the unit historically, though the UK has mostly migrated to kWh while US gas billing still uses therms widely.
Can I see therms on my smart gas meter?
No. UK smart gas meters report volume (cubic metres) and energy (kWh). Therms do not appear on the meter face.
What is MMBtu in international gas pricing?
Million British Thermal Units (10 therms). MMBtu is the unit commonly used in international LNG and US gas pricing. 1 MMBtu equals 10 therms or about 293.1 kWh.
Should I switch a legacy therm-based contract to kWh?
There is no operational benefit to changing units within the same contract; the maths is identical. At renewal, moving to kWh produces cleaner ongoing comparison with the wider market and avoids the small conversion-error risk on every bill.