Methodology & editorial policy
How we build calculators you can trust — and how to tell us when we get something wrong.
1. Formulas are separate, tested code
Each calculator's maths lives in its own small module, separate from the page design. Every module has automated tests built from cases worked out by hand or taken from standard textbook results — for example, a $200,000 mortgage at 6% over 30 years must give a payment of $1,199.10, and 30 US mpg must convert to 7.84 L/100 km. The site can't be published if any of these tests fail.
2. Units and currencies follow your country
Choosing the US, UK, Canada or Australia switches currency (USD, GBP, CAD, AUD), distance (miles or km), fuel economy (US mpg, UK mpg or L/100 km), how pump prices are quoted ($/gal, p/L, ¢/L, c/L) and how gas is billed (therms, kWh, m³ or MJ). Internally every calculation uses metric units with exact conversion factors (1 mile = 1.609344 km; 1 US gallon = 3.785411784 L; 1 imperial gallon = 4.54609 L; 1 therm = 100,000 Btu).
3. Reference rates: sourced, dated, or absent
We pre-fill a price only when it comes from an official publisher — such as the U.S. Energy Information Administration, Ofgem, GOV.UK, Statistics Canada or the Australian Bureau of Statistics — and we show the source and the period right under the input. We never invent energy prices, taxes, interest rates or official statistics. If there is no trustworthy national figure (for example, electricity prices in Canada and Australia are set by province, state and retailer), the field starts empty and asks for your own number.
| Default | Country | Value | Source and period |
|---|---|---|---|
| Electricity unit rate | US | 18.31¢/kWh | U.S. EIA, Electric Power Monthly, Table 5.6.A, July 2026 (preliminary) |
| Electricity unit rate | UK | 26.32p/kWh | Ofgem, 1 Oct – 31 Dec 2026 |
| Electricity unit rate | Canada | — | No verified national figure on file — the field asks for your own rate. |
| Electricity unit rate | Australia | — | No verified national figure on file — the field asks for your own rate. |
| Electricity standing charge | US | — | No verified national figure on file — the field asks for your own rate. |
| Electricity standing charge | UK | 54.83p/day | Ofgem, 1 Oct – 31 Dec 2026 |
| Electricity standing charge | Canada | — | No verified national figure on file — the field asks for your own rate. |
| Electricity standing charge | Australia | — | No verified national figure on file — the field asks for your own rate. |
| Natural gas | US | $1.481/therm | U.S. EIA, Natural Gas Prices (annual), 2025 (annual average) |
| Natural gas | UK | 7.97p/kWh | Ofgem, 1 Oct – 31 Dec 2026 |
| Natural gas | Canada | — | No verified national figure on file — the field asks for your own rate. |
| Natural gas | Australia | — | No verified national figure on file — the field asks for your own rate. |
| Gas standing charge | US | — | No verified national figure on file — the field asks for your own rate. |
| Gas standing charge | UK | 29.68p/day | Ofgem, 1 Oct – 31 Dec 2026 |
| Gas standing charge | Canada | — | No verified national figure on file — the field asks for your own rate. |
| Gas standing charge | Australia | — | No verified national figure on file — the field asks for your own rate. |
| Gasoline / petrol | US | $4.331/gal | U.S. EIA, Gasoline and Diesel Fuel Update, week of 12 Oct 2026 |
| Gasoline / petrol | UK | 175.28p/L | GOV.UK (DESNZ), Weekly road fuel prices, week of 12 Oct 2026 |
| Gasoline / petrol | Canada | 173.80¢/L | Statistics Canada, Table 18-10-0001-01, August 2026 (monthly) |
| Gasoline / petrol | Australia | — | No verified national figure on file — the field asks for your own rate. |
| Heating oil | US | $6.138/gal | U.S. EIA, Heating Oil and Propane Update, week of 12 Oct 2026 |
| Heating oil | UK | — | No verified national figure on file — the field asks for your own rate. |
| Heating oil | Canada | — | No verified national figure on file — the field asks for your own rate. |
| Heating oil | Australia | — | No verified national figure on file — the field asks for your own rate. |
| Propane | US | $2.638/gal | U.S. EIA, Heating Oil and Propane Update, week of 12 Oct 2026 |
| Propane | UK | — | No verified national figure on file — the field asks for your own rate. |
| Propane | Canada | — | No verified national figure on file — the field asks for your own rate. |
| Propane | Australia | — | No verified national figure on file — the field asks for your own rate. |
Rates are checked against their publishers' release schedules (weekly for fuel, monthly or quarterly for energy) and a stale rate is updated or removed. Historical inflation uses official consumer price indexes from the BLS, ONS, Statistics Canada and ABS.
4. Assumptions are labelled
Some inputs need a starting value to make a calculator usable — a typical battery size, an appliance wattage, a boiler efficiency. These are marked as examples or assumptions, with a source where one exists (for example, the U.S. Department of Energy's ~16% EV charging loss, or NREL's 0.5%/year median solar panel degradation). Replace them with your own figures whenever you can.
5. Your data
Calculations run in your browser; we don't store what you type. Share links include only non-personal inputs by default. Fields such as income, savings balances and budget lines are marked “private” and are only added to a link if you tick the box.
6. Independence and advertising
We may show advertising in future to fund the site. Advertisers have no influence over formulas, defaults or content, ads will never appear inside a calculator's inputs or results, and any affiliate link will be clearly labelled.
7. Corrections
If you find an error, please tell us. We aim to verify reports promptly, fix confirmed errors, add a test so the same mistake can't return, and update the “last reviewed” date on the page.