Golan wrote:

is there a Calculator that i can put my old reading and my new one ?

Any calculator will do.

Assuming an imperial meter (still the more common):

1. Subtract previous reading from current one to get consumption in
100's of cu. ft.

2. Multiply by 2.83 to get consumption in cubic metres. (Omit this
step if you have a metric meter, which reads directly in m^3.)

3. Multiply by the volume conversion factor of 1.02264 to correct for
temperature and pressure. (Step can be omitted if you only want
a rough answer).

4. Multiply by the declared calorific value (CV - see last gas bill) to
get consumption in Megajoules.

5. Divide by 3.6 to get consumption in kilowatt-hours.

6. Multiply by price per kWh to get billed amount, ex. VAT, taking
any price thresholds into account (e.g. x p/kWh for first 200 kWh
and y p/kWh for the remainder).

7. Add standing charge, if any, and 5% VAT.


Very roughly one "unit" (100 ft^3) on an imperial meter corresponds to
about 32 kWh, assuming a typical CV of 39.5 MJ/m^3.

--
Andy