Did my bill go up because I used more or because the price rose?

Line up the bill details

An itemised bill can separate a change in usage from a change in unit price. Put two bills side by side and write their service periods, day counts and currencies. Copy the usage quantity, unit name, price per unit and fixed charges from each. Use the same units for the comparison. A payment total alone does not show these separate parts.

Worked example with made-up figures

Repeating

Fictional sample rent: A$270.97 / week
Telco One: A$11.06 / week
Fictional sample groceries: A$9.65 / week
Streamly: A$3.16 / week

Together A$294.84 a week.

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Rebuild each usage charge

Multiply the quantity by the listed price per unit on paper. Compare the result with the usage charge printed on the bill. Record any rounding difference. Keep fixed charges, credits and other itemised amounts on their own lines. If the bill uses several price bands or different rates within a period, copy each band separately before adding the charges together.

Hold one number steady

Use the earlier unit price to calculate what the later quantity would cost at that price. Subtract the earlier usage charge: that gives the quantity contribution using the earlier price. Next multiply the later quantity by the difference between the two unit prices. That gives the price contribution using the later quantity. These two contributions add to the usage-charge change when each bill has one unit price.

Example: made-up figures

Imagine the earlier bill lists 100 units at 2 currency units each, making 200. The later bill lists 120 units at 3 each, making 360. At the earlier price, 120 units would cost 240. Extra quantity contributes 40, calculated as 240 minus 200. The price change contributes 120, calculated as 120 times the increase of 1. Together 40 plus 120 explains the increase of 160.

Add the other bill lines

In that example, a fixed charge of 20 on each bill makes totals of 220 and 380. The fixed charge contributes zero to the difference. Copy any changed fixed charge, credit or separately stated amount and calculate its difference too. Keep the labels printed on the bill. A total is explained only when all those differences add to the change between the complete bill totals.

Keep the comparison reproducible

Tick each line used in the calculation and write its page reference. Different service lengths can explain different quantities; show units per day as a separate calculation if useful. This is an average across the stated period, not a reading for a particular day. Retain the original quantities and totals beside it. Where a rate or quantity is absent, mark which part the bill does show and leave the remaining cause unassigned.