Gas Rate Calculator
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What a gas rate calculator actually does
Two jobs in one: (1) appliance gas rating from the meter, and (2) bill/usage conversion into kWh.
- Appliance gas rating: measures the volumetric gas flow through your meter and converts it to input power (kW), so engineers can check an appliance against its data plate. For imperial test dials you time one revolution; for metric meters you record how many cubic metres pass in a timed interval, then scale to m�/h.
- Bill/usage conversion: turns a meter volume (m� or ft�/CCF) into kWh using your area�s calorific value (CV) and a correction factor (CF) for temperature/pressure � the same steps suppliers use on bills.
What you�ll need (meter, CV & CF)
Simple inputs, but accuracy matters.
- Meter type: metric (m�) or imperial (ft�).
- Calorific value (CV): typically 37.5�43.0 MJ/m� in the UK; the exact average for your zone appears on your bill. Our default 39.5 is a realistic mid-range.
- Correction factor (CF): accounts for temperature/pressure at your property; most suppliers show 1.02264 unless unusual conditions apply.
Metric vs imperial meters: how to measure
Two common field methods, both supported in the calculator.
- Imperial (ft�) with a test dial:
Time one full revolution of the test dial (usually marked 1, 2, 5 or 10 ft�).
Formula for flow:
ft�/h = (3600 � ft� per revolution) / seconds ? then convert to m�/h. - Metric (m�) timed draw:
Record the meter, run the appliance for e.g. 120 seconds, record again, subtract to get m� used, then scale to m�/h.
Tip: Longer timing windows (2�3 minutes) reduce error on low flows.
Formulas (flow ? kW, volume ? kWh)
These are the exact steps suppliers and trade guides publish.
From flow to kW (appliance gas rating)
- Convert your measured flow to m�/h.
- Apply correction factor (CF) and calorific value (CV).
- Convert megajoules per hour to kW:
kW (gross) = (m�/h � CF � CV) � 3.6.
From volume to kWh (bill/usage)
- If you start in ft�, convert to m� first: m� = ft� � 0.0283168 (or CCF � 100 � 0.0283168).
- Then: kWh = (m� � CF � CV) � 3.6.
Typical values (for context, check your bill): CV usually lies between 37.5 and 43.0 MJ/m�; CF is commonly 1.02264.
Gross vs net input (and cost per hour)
Know which figure your appliance plate uses.
- Gross (GCV) includes the latent heat of the water vapour in exhaust gases; Net (NCV) doesn�t. Many plate ratings quote net; some manuals quote gross.
- A practical rule-of-thumb is Gross ? Net � 1.11 (so Net ? Gross � 1.11). The calculator shows both and lets you change the factor.
- Cost per hour uses your entered price per kWh � net kW (billing units).
Worked examples
These match what the calculator returns (rounded).
Example A � Imperial test dial (appliance rating):
Dial = 1 ft�, one revolution in 45 s.
- Flow: ft�/h = (3600 � 1)/45 = 80 ft�/h ? m�/h ? 80 � 0.0283168 ? 2.265.
- With CV 39.5 MJ/m�, CF 1.02264:
Gross kW = (2.265 � 1.02264 � 39.5)/3.6 ? 25.5 kW.
Net kW ? 25.5 � 1.11 ? 23.0 kW.
(Imperial flow & dial timing per trade pocket guide.)
Example B � Metric timed draw (appliance rating):
In 120 s, meter advances 0.020 m�.
- m�/h = 0.020 � (3600/120) = 0.6 m�/h.
- With CV 40.0, CF 1.02264: Gross kW = (0.6 � 1.02264 � 40)/3.6 ? 6.82 kW; Net ? 6.15 kW.
Example C � Bill conversion:
You used 100 m� in the billing period; CV 39.5, CF 1.02264.
- kWh = (100 � 1.02264 � 39.5)/3.6 ? 1122.8 kWh.
This matches the supplier equation used in Ofgem�s guidance and GOV.UK explanations for meter ? kWh.
<h2 id=”troubleshoot”>Troubleshooting & best practice</h2> <p class=”subtitle”>Reduce measurement noise and avoid common mistakes.</p>
- Time longer on low flows. For small appliances, use 2�3 minutes rather than 60 s.
- Use your bill�s printed CV/CF. CV varies by zone and day (~37.5�43.0 MJ/m�). CF typically 1.02264 but can differ slightly.
- Imperial to metric: Don�t forget ft� ? m� (� 0.0283168) before applying CV/CF.
- Gross vs net: Plate ratings can be net while service manuals quote gross � check the documentation.
- Billing sanity-check: Many suppliers show the exact CV and CF used on each statement; your kWh should follow (m� � CF � CV)/3.6.
FAQs
Fast answers from the standards and supplier guides.
What is the correction factor (CF)?
An allowance for temperature and pressure at your property; UK suppliers commonly use 1.02264, printed on bills.
What CV should I enter?
Use the CV on your bill. Typical UK values span 37.5�43.0 MJ/m�.
Do I time one revolution on imperial meters?
Yes�time one full test-dial revolution (1, 2, 5 or 10 ft�), then scale to ft�/h with (3600 � ft� per rev)/seconds.
How do I convert ft� or CCF to kWh for bills?
Convert to m� first (ft� � 0.0283168 or CCF � 100 � 0.0283168), then kWh = (m� � CF � CV)/3.6.
Why do suppliers use 3.6?
Because 1 kWh = 3.6 MJ, so dividing MJ by 3.6 gives kWh.
What�s a �therm�?
US billing sometimes uses therms; 1 therm ? 29.3 kWh. (Not used for UK billing, which is in kWh.)