# how do i calculate my gas meter reading?

ringo110879 | 13:52 Wed 15th Mar 2006 | Home & Garden
Hi. I have just been to read my gas meter to get an idea of how much gas i am using, only just switched from pre-payment. I know that i am paying 3.2p per kw of gas i use but my meter is displayed in cubic meters (m3), the reading is 317.75 cubic meters, how many kw does this work out as? Thanks

No best answer has yet been selected by ringo110879. Once a best answer has been selected, it will be shown here.

Since your meter appears to measure in m� (metres cubed), we don�t have to worry about converting to metric, however, we do need to convert it to a standardised volume. (Volume varies with pressure so before any calculations are done, the actual volume you have used is converted to the gas company's standardised volume unit.) This is the Volume Conversion Factor shown on your bill.

We now need to work out how much energy was in the gas you have used. This energy is released when you burn it and is known as the calorific value. The SI unit of energy is the Joule, and the calorific value given on your bill is measured in Mega Joules per metre cubed of gas. (1 Mega Joule (MJ) = 1 000 000 Joules.
The gas company works out the average calorific value over the period you have used it, (since it can, and does vary).
They will give you a figure for the average calorific value on your bill. This figure will probably be around 38 � 40 MJ per m� .

So, now multiply your converted volume of gas used (in m�) by the calorific value given on your bill to get the absolute amount of energy contained in the gas you have used (in MJ).

[cont...]

...[cont.]

You are actually charged by a unit known as the KWh (KiloWatt hour) � the same unit used in your electricity bill. It is a derived unit that represents the amount of energy used by a 1KW device in one hour. The amount of energy this represents can be calculated;

A Watt is the unit of power, and represents a rate of energy equal to 1 Joule per second.
Therefore, 1 KW = 1000 Joules per second (J/s)
There are 3600 seconds in an hour, so using energy at a rate of 1KW for a duration of 1 hour would use 3,600,000 J or 3.6 MJ (Mega Joules).

Since we know the total amount of energy in the volume of gas you have used in MJ, and that there are 3.6 MJ in a KWh, then divide your total by 3.6 to get the total of KWh you have used.

[cont...]

So (finally !!) this can be summarised by;

{ Your Meter Reading (m�) x Conversion factor* x Calorific Value* (MJ/m�) } divided by 3.6

Using typical values

317.75 x 1.022640* x 39.4*
3.6

= 3555.85 KWh

Therefore, since you pay 3.2p per KWh, your bill at this point would be;

3555.85 x 0.032 = �113.78

or, if you haven't paid your bill promptly, �113.78 !! ;o)

*These figures vary, check your last bill to get an idea what they should be.
This answer gets my vote for "The Most Comprehensively Answered Question - Ever" :)
Question Author

Lost for words. Thankyou

Sorry about long-windedness - I could have just said "multiply by this and that and divide by 3.6" - but I'm one of those people who likes to know 'why?'.

So, armed with nothing more than my gas bill and a calculator, I had to work it out from First Principals !!

Hope you've got an idea of how much your bill will be now, ringo - to skip all the 'reasoning behind it' rubbish, just go straight to post 3 !!

I wasnt being sarcastic Brach... I was impressed with the comprehensiveness of yer answer is all :)

wanna work out my tax returns for me? :)
thanks have you got a more up to date volume/calorific value. i am with british gas fixed dec 09. cheers
Hello
The above calculation is only part of the answer, and possibly the easiest bit.
Where it gets really tricky is if the gas supllier uses a threshold number of kW hrs so that you get gas at a cheaper rate once you have used a certain number of kW hrs - 1143 kW hrs is typical. This threshold is for a quarter of a year, but your quarter can vary a lot depending on when the reading was taken. I have never got a straight answer from a supplier what their quarter is, e.g. 90 days, 91 days, 91.25 days, but you need this figure to work out what proportion of the quarter you have actually used. Say your bill is for 100 days and the quarter is 90 days, then your threshold is 1143 x 100/90. I believe that every supplier has their own way of doing this, but by proportioning the threshold, you will get pretty close to the correct answer