Home  /  Glossary  /  What is calorific value?

Calorific value

What is calorific value?

Gas does not arrive at your meter with a single fixed energy content. Some days it is a touch richer, some days a touch leaner, because the underlying mix of methane, ethane, propane and other components varies with the source. Calorific value, almost always shortened to CV, is the figure the UK gas industry uses to convert the cubic metres your meter records into the kilowatt-hours you actually pay for. The supplier multiplies your volume by the CV applicable to your area on that day, applies the volume correction factor, and divides by 3.6 to convert megajoules to kWh. The result is the energy figure on your bill. Without CV, none of that conversion would work.

CV is the silent input that shapes every business gas bill in the UK, sitting behind the conversion from cubic metres of gas to billable kWh.

CV sits in the middle of the gas bill calculation chain. Volume in, energy out, with CV doing the translation. The figure changes every day in every local distribution zone and is published by the gas network operator. Understanding CV matters when you are validating a gas bill, comparing energy use across periods, or trying to make sense of why two months with similar volumes produced slightly different kWh totals.

What calorific value actually is

Calorific value is the heat energy released when a defined quantity of fuel is burned under standard conditions.

• For UK gas billing, CV is expressed per cubic metre of gas.
• The value depends on the chemical composition of the gas being delivered.
• Methane has a CV around 39.7 MJ per cubic metre. Other natural gas components have different values.
• The actual CV for any given supply depends on the mix delivered.

The figure is what turns volume into energy. Two different gases can occupy the same volume at the meter but deliver different amounts of useful heat. CV captures that difference.

How CV is measured and expressed

UK gas CV is published in megajoules per cubic metre (MJ/m³).

• Typical UK gas CV. 38.5 to 40.5 MJ/m³ depending on region and time of year.
• To convert to kWh, divide by 3.6 (1 kWh = 3.6 MJ).
• So gas with a CV of 39.6 MJ/m³ gives 11.0 kWh per cubic metre after volume correction.

The figure is updated daily and published per Local Distribution Zone (LDZ) by the gas network operator. The bill uses the CV applicable to your LDZ on each day of the billing period.

Why CV varies day to day

The CV of the gas delivered to a UK meter depends on where the gas came from. The UK gas network is fed from a mix of sources.

• North Sea production. Slightly leaner gas, around 38.7 MJ/m³.
• Interconnectors from continental Europe. Slightly different mix again.
• LNG imports. Higher-CV gas in most cases.
• Biomethane. Domestic biogas at lower CV.
• Storage withdrawal. Gas previously injected, with its own original CV.

The mix changes day to day based on which sources are flowing into the network. The daily CV reflects that. Over a year the variation is typically within a couple of per cent for any one LDZ.

The CV conversion chain on your bill

The UK gas bill calculation from meter reading to kWh.

1. Meter records volume. Cubic metres for modern meters. Hundreds of cubic feet for older imperial meters.
2. Imperial volume (if applicable) is converted to cubic metres by multiplying by 2.83.
3. Volume correction factor is applied (default 1.02264) to adjust for temperature and pressure at the meter.
4. Daily CV figures are applied to each day’s share of volume.
5. The result is divided by 3.6 to convert megajoules to kilowatt-hours.
6. kWh is multiplied by the unit rate to give the variable cost.

The bill shows every step explicitly so the customer can audit each one.

Worked example of the conversion

Illustrative example. A small commercial site records 200 cubic metres of gas in a one-month billing period in an LDZ where the average CV that month was 39.5 MJ/m³.

Example only. Real CV varies daily by LDZ. Real bills apply the daily CV figure to the share of volume consumed each day rather than the monthly average. The point is the structure of the conversion, not the precise number.

Where the CV figure comes from

National Gas Transmission (the UK gas system operator) measures the gas composition at every entry point into the national transmission system and publishes the daily CV for each LDZ.

• Measurements are taken at key network points using gas chromatography.
• Daily CV per LDZ is published on the National Gas data portal.
• Suppliers and shippers pull the figure automatically through industry data systems.
• Your gas bill uses the published CV for your LDZ on each day of the period.

Gross CV vs net CV

There are two ways of expressing CV.

• Gross CV (also called higher heating value). Includes the energy released when the water vapour from combustion condenses. UK billing uses gross CV.
• Net CV (also called lower heating value). Excludes the condensation energy. Used in some engineering contexts and in some non-UK billing systems.

The two differ by around 10 per cent. UK customers will always see gross CV on the bill. Confusion arises only when comparing UK billing figures to international engineering data.

CV and biomethane

Biomethane injected into the UK gas grid is required to meet a CV specification (Gas Safety Management Regulations) to ensure compatibility with appliances.

• Biomethane is upgraded from biogas (which has lower CV due to higher CO&sub2; content).
• The upgrade process removes CO&sub2; and other impurities until the gas reaches grid-quality CV.
• Once injected, biomethane is indistinguishable from natural gas for billing purposes.
• The CV figure published for the LDZ reflects the blended mix including any biomethane contribution.

Practical implications for businesses

For most UK business gas customers, CV is a background fact that does not require active management. Two situations where it matters.

• Bill validation. When checking a gas bill, the conversion chain (volume to kWh via CV) is where calculation errors can occur. Knowing the typical CV range for your LDZ (38.5 to 40.5 MJ/m³) lets you sanity-check the numbers.
• Year-on-year energy comparisons. If you compare gas consumption in kWh across two periods, small CV variations can shift the comparison by 1 to 2 per cent. For most operational purposes this is noise. For ESOS or SECR reporting where precision matters, the underlying volume figures are more comparable than the kWh figures.

Using CV in bill validation

A quick mental check on any gas bill.

1. Take the volume in cubic metres from the bill.
2. Multiply by 1.0226 for volume correction.
3. Multiply by 11 (a rough kWh-per-m³ figure for UK gas).
4. The result should be within a per cent or so of the kWh on the bill.

If the gap is materially larger, dig into the daily CV figures and volume correction applied by the supplier. The deeper article on the Clearsight site, What Is Calorific Value in Gas Billing, walks through the same calculation with more worked detail. Related entries. kWh, Annual Quantity (AQ), volume correction, MPRN, therm, bill validation.

For the bundled multiplier that combines CV and volume correction in a single working number, see the gas conversion factor.

Frequently asked questions