What Is a UK Imperial Gas Meter?
An imperial gas meter measures gas volume in cubic feet (ft³) rather than cubic metres (m³). All meters installed in UK homes before the mid-1990s were imperial — and many remain in service today, particularly in pre-war terrace houses, Victorian-era flats and older semi-detached properties. If your gas meter shows four main digits labelled ft³, CF, cubic feet or Hcf (hundreds of cubic feet), you have an imperial meter and this is the correct calculator for you.
Despite measuring in cubic feet, your energy bill will still show kilowatt hours (kWh) — because all UK suppliers are legally required to convert to kWh before charging you. The conversion uses the Ofgem-mandated formula, which first converts ft³ to m³ (by multiplying by 0.0283168), then applies the same calorific value and correction factor used for metric meters. Our calculator above does all of this automatically.
The Ofgem Formula: Cubic Feet to kWh
Every UK energy supplier — from British Gas to Octopus — applies this exact formula when converting your imperial meter reading to kWh on your bill.
For Hcf Meters — Add One Extra Step
Some older imperial meters display gas in hundreds of cubic feet (Hcf) rather than individual cubic feet. If your meter shows Hcf, multiply your reading by 100 first to get ft³, then apply the formula above. Our calculator handles this automatically — just select the "Hcf" mode at the top of the calculator.
How to Convert Cubic Feet to kWh: Step by Step
Read Your Imperial Gas Meter
Note your current reading from the meter display. For digital imperial meters, read the 4 main digits shown. For dial meters, read each dial left to right, taking the lower number when the needle is between two digits. Subtract your previous reading to find units used. Ignore red dials and any digits after a decimal point.
If Hcf: Multiply by 100
If your meter shows Hcf (hundreds of cubic feet), multiply your units used by 100 to convert to ft³. This step is only needed for Hcf meters — skip it if your meter already shows individual ft³.
Convert ft³ to m³
Multiply your ft³ figure by 0.0283168 to convert to cubic metres. This is the standard conversion factor (1 ft³ = 0.0283168 m³). The Ofgem formula works in cubic metres, so this step is always required for imperial meters.
Multiply by Calorific Value × VCF
Multiply your m³ figure by the calorific value (from your gas bill — UK average 39.5 kWh/m³) then by 1.02264 (Ofgem's volume correction factor). These two multiplications can be done in any order.
Divide by 3.6 for kWh
Divide by 3.6 to convert to kilowatt hours. This is your final answer — the energy used figure your supplier will show on your bill. Use it directly with our gas bill calculator to work out your cost in pounds.
Worked Examples — Cubic Feet to kWh
All examples use CV 39.5 kWh/m³ and VCF 1.02264. Cost estimate at Ofgem 2026 rate of 7.42p/kWh.
CV: 39.5 kWh/m³
2.832 × 39.5 × 1.02264 ÷ 3.6
= 31.76 kWh · £2.36
CV: 39.5 kWh/m³
12.176 × 39.5 × 1.02264 ÷ 3.6
= 136.62 kWh · £10.14
CV: 40.2 kWh/m³
42.475 × 40.2 × 1.02264 ÷ 3.6
= 484.72 kWh · £35.97
CV: 39.5 kWh/m³
85.94 × 39.5 × 1.02264 ÷ 3.6
= 964.0 kWh · £71.53
CV: 38.8 kWh/m³
198.22 × 38.8 × 1.02264 ÷ 3.6
= 2,179 kWh · £161.68
CV: 39.5 kWh/m³
121.76 × 39.5 × 1.02264 ÷ 3.6
= 1,366 kWh · £101.35
Cubic Feet to kWh Conversion Table
Quick reference using UK standard CV 39.5 kWh/m³ and VCF 1.02264. Cost at Ofgem 2026 rate of 7.42p/kWh (excluding standing charge and VAT). For your exact figures, use the calculator above with your bill's stated CV.
| ft³ Used | Hcf Equiv. | m³ Equiv. | kWh | Est. Cost (7.42p) | Typical Context |
|---|---|---|---|---|---|
| 10 ft³ | 0.1 Hcf | 0.283 m³ | 3.18 kWh | £0.24 | ~30 min heating |
| 50 ft³ | 0.5 Hcf | 1.416 m³ | 15.88 kWh | £1.18 | ~half day usage |
| 100 ft³ | 1 Hcf | 2.832 m³ | 31.76 kWh | £2.36 | ~1 day winter |
| 200 ft³ | 2 Hcf | 5.663 m³ | 63.52 kWh | £4.71 | ~2 days winter |
| 300 ft³ | 3 Hcf | 8.495 m³ | 95.28 kWh | £7.07 | ~3 days winter |
| 430 ft³ | 4.3 Hcf | 12.176 m³ | 136.62 kWh | £10.14 | ~1 week typical |
| 500 ft³ | 5 Hcf | 14.158 m³ | 158.80 kWh | £11.78 | ~1 week winter |
| 750 ft³ | 7.5 Hcf | 21.238 m³ | 238.20 kWh | £17.67 | ~10 days heavy |
| 1,000 ft³ | 10 Hcf | 28.317 m³ | 317.60 kWh | £23.56 | ~2 weeks winter |
| 1,500 ft³ | 15 Hcf | 42.475 m³ | 476.40 kWh | £35.35 | Monthly summer |
| 2,000 ft³ | 20 Hcf | 56.634 m³ | 635.20 kWh | £47.13 | Monthly shoulder |
| 3,035 ft³ | 30.35 Hcf | 85.94 m³ | 964.0 kWh | £71.53 | 🇬🇧 UK avg monthly |
| 3,500 ft³ | 35 Hcf | 99.109 m³ | 1,112 kWh | £82.51 | Monthly winter avg |
| 5,000 ft³ | 50 Hcf | 141.584 m³ | 1,588 kWh | £117.83 | Cold winter month |
| 7,000 ft³ | 70 Hcf | 198.218 m³ | 2,223 kWh | £164.95 | Very cold Jan/Feb |
| 10,000 ft³ | 100 Hcf | 283.168 m³ | 3,176 kWh | £235.66 | ~3 cold months |
| 36,300 ft³ | 363 Hcf | 1,028 m³ | 11,535 kWh | £855.90 | 🇬🇧 UK avg annual |
All values: CV 39.5 kWh/m³, VCF 1.02264. Cost excludes standing charge (~29p/day) and 5% VAT. For your bill cost use the gas bill calculator.
How to Read an Imperial Dial Gas Meter
Imperial dial meters have four or five mechanical dials, each numbered 0–9 with a pointer. Reading them correctly is the most common source of errors for imperial meter users. Follow these rules for every dial:
Read all four main dials left to right, write down each digit, and that 4-digit number is your meter reading in ft³ or Hcf. Example: dials show 1-4-2-5 = reading of 1,425. If your previous reading was 1,382, you used 43 units. If the meter is in Hcf, that is 43 × 100 = 4,300 ft³ used.
🔢 Skip the Maths — Use the Calculator Above
Enter your ft³ or Hcf readings and get an instant kWh result with full formula breakdown. Also try our other gas tools:
UK Regional Calorific Values 2026
The calorific value affects every cubic feet to kWh conversion. Since imperial meters are most common in older properties — which tend to be in urban areas with historically higher gas demand — many imperial meter users are in London, the Midlands and Northern England. Check your region below and compare with your bill's stated CV.
| UK Region | CV Range (kWh/m³) | Band | kWh per ft³ | kWh per 100 ft³ (1 Hcf) |
|---|---|---|---|---|
| 🇬🇧 UK National Average | 38.5 – 40.5 | Average | 0.309 – 0.325 | 30.9 – 32.5 kWh |
| London & South East | 39.0 – 41.5 | Above Avg | 0.313 – 0.333 | 31.3 – 33.3 kWh |
| South West England | 38.8 – 41.0 | Average | 0.311 – 0.329 | 31.1 – 32.9 kWh |
| East Anglia & East Midlands | 39.0 – 40.8 | Average | 0.313 – 0.327 | 31.3 – 32.7 kWh |
| West Midlands | 38.5 – 40.5 | Average | 0.309 – 0.325 | 30.9 – 32.5 kWh |
| Yorkshire & Humber | 38.0 – 40.2 | Average | 0.305 – 0.322 | 30.5 – 32.2 kWh |
| North West England | 37.8 – 40.0 | Below Avg | 0.303 – 0.321 | 30.3 – 32.1 kWh |
| North East England | 37.5 – 39.8 | Below Avg | 0.301 – 0.319 | 30.1 – 31.9 kWh |
| Scotland (Central) | 37.5 – 40.0 | Below Avg | 0.301 – 0.321 | 30.1 – 32.1 kWh |
| Scotland (North) | 37.2 – 39.5 | Below Avg | 0.298 – 0.317 | 29.8 – 31.7 kWh |
| Wales | 38.0 – 40.2 | Average | 0.305 – 0.322 | 30.5 – 32.2 kWh |
| Northern Ireland | 38.0 – 40.0 | Average | 0.305 – 0.321 | 30.5 – 32.1 kWh |
kWh/ft³ calculated using mid-range CV × 0.0283168 × 1.02264 ÷ 3.6. Source: National Grid Gas Transmission. Always use your bill's stated CV for maximum accuracy.
Imperial Gas Meters in the UK
Are Imperial Meters Still Legal in the UK?
Yes — imperial gas meters are still fully legal and approved for use in the UK. There is no requirement to replace a working imperial meter simply because it measures in cubic feet. Ofgem requires all suppliers to be capable of billing from both imperial and metric readings. However, imperial meters are older technology and may be less accurate than modern metric or smart meters. If your meter is over 20 years old, it may benefit from testing — contact your supplier or National Grid's meter helpline.
Getting a Smart Meter to Replace Your Imperial Meter
Under the UK government's smart meter rollout, every household is entitled to a free smart meter installation. When you request a smart meter, your old imperial meter is removed and replaced with a modern SMETS2 smart meter — which measures in cubic metres and transmits readings automatically every 30 minutes. You'll also receive a free In-Home Display showing your gas usage in kWh and cost in real time. To arrange a free replacement, contact your energy supplier directly. Most major suppliers including British Gas and Octopus Energy offer online booking.
Why Imperial Meter Readings Sometimes Differ From Your Bill
- Dial reading error — Misreading one dial by 1 digit creates a 100-unit error, equivalent to 317 kWh or ~£23 on a typical bill. Double-check every dial carefully.
- Estimated readings — Suppliers who cannot access your meter may estimate usage from historical patterns. This is more common with dial meters in hard-to-access locations.
- Meter rollover — If a 4-digit meter rolls over from 9,999 to 0, add 10,000 to your current reading before calculating the difference.
- CV changes — The calorific value changes quarterly. If your billing period straddles a quarter boundary, two different CVs may apply to different portions of the period.
- Meter creep — Older mechanical meters can run fast or slow. If your calculated kWh consistently differs from your bill by more than 5%, request a meter test from your supplier.