Meter Reading and Energy Bill Calculator: kWh a Day, Gas m³ to kWh, Therms, the Bill Line by Line

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Opens on the book's house. Change any number to your own and press Calculate.

Two electricity reads (kWh), the same time of day

First read kWh

Last read kWh

Days between them

Two gas reads

The gas meter reads in

First read

Last read

Calorific value MJ/m³, on a UK bill

Volume correction UK

Therm factor therms a CCF, on a US bill

The bill: units × unit rate + days × standing charge

Electricity unit rate a kWh

Electricity standing charge a day

Gas unit rate a kWh

Gas standing charge a day

Electricity kWh on the bill blank = from the reads

Gas kWh on the bill blank = from the reads

Power × time = energy (a kettle, a heater, anything)

Power kW (1,000 W = 1 kW)

For how long minutes

Enter two meter reads and the days between them, then press Calculate.

The formula

kWh used = last read − first read; kWh a day = used ÷ days; average power (kW) = kWh a day ÷ 24
UK gas: kWh = m³ × 1.02264 (the volume correction) × the calorific value (MJ/m³) ÷ 3.6
US gas: therms = CCF × the therm factor on your bill; 1 therm = 100,000 Btu ≈ 29.3 kWh
The bill = kWh × unit rate + days × standing charge, for each fuel
Energy (kWh) = power (kW) × hours: watts are a rate, watt-hours an amount (the speedometer and the odometer)

The book's worked example, Chapter 1. Over 28 days the electricity meter went from 41,236.4 to 41,505.2: 268.8 kWh, 9.6 kWh a day, 0.4 kW on average. The gas meter moved 232.8 m³: × 1.02264 × 39.5 ÷ 3.6 = 2,612 kWh, 93.3 a day (US: 89.1 therms, 86 CCF). At the book's tariff the four-week bill is 69.89 + 15.40 + 208.96 + 8.40 = 302.65. A 3 kW kettle for 3.5 minutes uses 0.175 kWh.

Check it. Read both meters at the same time two mornings running: the day's kWh should land near your bill's kWh ÷ its days. Is each read on the bill marked A (actual) or E (estimated)?

Honest limits. A reading is only as good as the moment you took it: read at the same time of day, and photograph the meter. UK calorific values change a little from bill to bill; US therm factors differ by utility. Copy yours from the bill. The book's house, its meters and its bill were worked out on paper from published figures, not measured by us: read your own meters and log your own night. Tariffs are illustrative, as seen in November 2026 (shaped like the GB price cap of October to December 2026, with US prices beside), shown without a currency sign; put in your own. No saving is promised. Everything stays in your browser: no sign-up, nothing stored.

These calculators go with Watt Happens Next (Make It Add Up, Book 3), where every number on this page is worked by hand.