How this gas cost calculator works
Your gas meter does not measure energy. It measures volume. Suppliers charge for energy, so every bill hides a conversion from cubic metres (or cubic feet) to kilowatt-hours. This calculator performs that conversion the same way suppliers are legally required to, then builds the bill line by line.
- Units used. Enter two meter readings, or a consumption figure you already know. The calculator accepts metric meters (m³), imperial meters (hundreds of cubic feet), plain cubic feet, kWh and therms (the units used on US bills).
- Volume to energy. Volumes are converted to standard cubic metres, multiplied by a volume correction factor (temperature and pressure) and by the calorific value of the gas, then divided by 3.6 to turn megajoules into kWh.
- The bill. Energy is priced per kWh, per m³ or per therm, the standing charge is added for each day of the billing period, along with any separate levies, a discount if you have one, and finally VAT or sales tax.
- Annual estimate. If you choose the billing month, the calculator scales your bill up to a year using a typical temperate-climate monthly profile, so a single winter bill tells you roughly what a full year will cost.
The formula
V(m³) = imperial units × 100 × 0.0283168 (hundreds of ft³) kWh = therms × 29.3071
Bill = (kWh × unit price + kWh × levies + days × standing charge − discount) × (1 + VAT)
The UK fixes the correction factor at 1.02264 for domestic meters. In Germany the same step uses the Zustandszahl (typically 0.95–0.97), and the calorific value is called the Brennwert. Elsewhere the factor may be folded into a single “conversion coefficient” printed on the bill. Enter it as the correction factor and set the calorific value to match.
Worked example
The default inputs describe a metric meter going from 4,215 to 4,369, so 154 m³ over a 31-day January billing period:
- 154 m³ × 1.02264 = 157.49 m³ corrected
- 157.49 × 39.5 MJ/m³ = 6,221 MJ
- 6,221 ÷ 3.6 = 1,728 kWh (about 55.7 kWh a day, or 59 therms)
At €0.10/kWh the energy charge is €172.80. Thirty-one days of standing charge at €0.30/day add €9.30, giving a bill of about €182.10, an all-in price of €0.105 per kWh. Because January typically accounts for around 15.5% of a year’s gas, this bill suggests annual use of roughly 10,900 kWh, or about €1,200 a year at the same prices. Burning that January gas released about 316 kg of CO₂ (0.183 kg/kWh).
How to read the results
- Total bill is what you should expect to pay for the period, including fixed charges and tax.
- Energy used (kWh) is the figure to compare with your bill and with other fuels. If it differs from your supplier’s by more than a few percent, check the calorific value and whether your meter is imperial.
- All-in price per kWh spreads the standing charge over your consumption. It is always above the unit rate, and much higher in summer when usage is small.
- Estimated annual use is a rough guide based on one bill. Homes that use gas only for cooking or hot water have a much flatter profile, so the estimate will be too high for them.
- Conversion steps show every multiplication so you can check a disputed bill.
Typical values
| Item | Typical range |
|---|---|
| Calorific value (gross) | 37.5–43 MJ/m³ (UK usually 39–40) |
| Energy per m³ | 10.5–11.5 kWh |
| Energy per 100 ft³ | 30–32 kWh |
| 1 therm | 29.31 kWh |
| Annual use, flat | 4,000–8,000 kWh |
| Annual use, 3-bed house | 10,000–15,000 kWh |
| Annual use, large or old house | 18,000–30,000 kWh |
| January share of annual use | 14–17% |
| July share of annual use | 2–4% |
Tips to cut your gas bill
- Read the meter yourself. Estimated bills drift; actual readings every month or two keep you in control and make this calculator’s annual estimate meaningful.
- Turn the thermostat down 1 °C. Each degree typically saves 5–10% of heating gas.
- Lower the boiler flow temperature. Condensing boilers only condense, and reach their rated efficiency, when return water is below about 55 °C. Setting the flow to 50–60 °C often saves several percent at no cost.
- Insulate first. Loft and cavity wall insulation have the shortest paybacks; see the insulation savings calculator.
- Compare alternatives. Use the heating cost comparison and heat pump vs gas boiler calculators to check whether switching fuel would pay off.
Limitations
Calorific value changes slightly from one billing period to the next; using a single value introduces an error of about ±1–2%. Some countries correct for local altitude or meter temperature in ways not captured by a single correction factor. The annual estimate assumes a heating-dominated temperate profile and ignores the actual weather of the year. Tariffs with tiered gas rates, time-of-use pricing or monthly caps are not modelled. To convert gas units without building a bill, use the gas unit converter.
Method & assumptions
Meter volume (m³, or ft³ × 0.0283168) is converted to energy with kWh = V × correction factor × calorific value ÷ 3.6. The bill is (kWh × unit price) + (days × standing charge) + levies, then VAT. A therm is 29.3071 kWh (100,000 BTU).
Frequently asked questions
How do I convert gas meter readings to kWh?
How many kWh are in 1 m³ of gas?
What is the calorific value on my gas bill?
What is a standing charge?
How much gas does an average house use per year?
Why is my winter gas bill so much higher than my summer bill?
Sources & references
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