Methodology, Constants and Data Sources
Every conversion on this site uses an exact or documented factor, listed here with its source. Decision calculators take the values that matter from you, and say so next to each result.
Last updated · Next scheduled review January 2027 · Maintained by the SnapUnitConverter editorial team
Kinds of numbers
Definitional factors are fixed by international agreement and carry no uncertainty. One kilowatt-hour is exactly 3.6 megajoules; one therm is exactly 100,000 BTU. These never change and are safe to use for billing, specification and engineering.
Real-world inputs — prices, efficiencies, energy use, emissions factors — vary by country, tariff, season and installation. The calculators ask for them rather than assuming them; see how assumptions are handled below.
A third category deserves explicit mention: conversions that are not fixed unit pairs at all. Watts to amps depends on supply voltage, and for alternating current also on power factor and phase. Milliamp-hours to watt-hours depends on cell voltage. Those converters require you to enter the missing quantity rather than presenting a single answer built on a hidden assumption.
Exact conversion factors
| Quantity | Value used | Basis / source |
|---|---|---|
| 1 kWh in megajoules | 3.6 MJ (exact) | SI definition: 1 W = 1 J/s, 1 h = 3,600 s BIPM SI Brochure (9th ed., 2019) |
| 1 kWh in joules | 3,600,000 J (exact) | SI definition BIPM SI Brochure (9th ed., 2019) |
| 1 BTU (International Table) | 1,055.05585262 J (exact) | International Steam Table 1956, as tabulated by NIST NIST SP 811 (2008) |
| 1 kWh in BTU | 3,412.1416 BTU | Derived from the IT BTU definition (3,600,000 J ÷ 1,055.05585262 J) NIST SP 811 (2008) |
| 1 therm (US) | 100,000 Btu(IT) = 29.307107 kWh | US gas billing convention. The UK therm is defined as 105,506,000 J = 29.307222 kWh — four parts per million apart, but a different definition. NIST SP 811 (2008) |
| 1 thermochemical calorie | 4.184 J (exact) | IUPAC / NIST convention NIST SP 811 (2008) |
| 1 mechanical horsepower | 745.699872 W | 550 ft·lbf/s, NIST SP 811 NIST SP 811 (2008) |
| 1 metric horsepower (PS/CV) | 735.49875 W | 75 kgf·m/s NIST SP 811 (2008) |
| 1 ton of refrigeration | 12,000 BTU/h = 3.516853 kW | ASHRAE convention ASHRAE Handbook — Fundamentals |
| European nominal supply voltage | 230 V ±10% single-phase, 400 V three-phase | Harmonised standard voltage IEC 60038 — Standard voltages (230/400 V) |
How assumptions are handled
There is no hidden sitewide table of household assumptions. Every number a calculator uses is one of three kinds:
- Defined conversion factors — exact unit definitions and physical relationships, such as 1 kWh = 3.6 MJ. These are fixed because they are definitions, not assumptions, and are listed in the tables on this page.
- User inputs — in the decision calculators, anything that materially changes the result comes from you: electricity and fuel prices, system cost, efficiency (COP/SCOP), annual energy use, driving distance, home/public charging mix, solar generation or specific yield, and the emissions factor where it can be edited.
- Example values — where a calculator opens with demonstration figures, they are labelled as examples and can be edited. They are not typical or universal values and are not recommendations.
Where a calculator offers emissions factors, each is shown with its source, year and geography, and you can enter your own figure instead.
Each tool uses its own model
- Solar Payback uses your system cost, generation estimate, self-consumption share, import price and export rate.
- Heat Pump Savings uses your heating demand or fuel use, fuel and electricity prices, and the efficiency values you enter.
- EV Charging Cost uses your charging prices, charging efficiency and home/public mix.
- Household Carbon Footprint uses stated emissions factors, each with its source, year and region.
- Home Energy Audit reconciles your appliance list against the annual electricity use on your bill.
Each calculator page explains its own formula, assumptions and limitations next to the result.
Every unit-conversion constant, with its provenance
This is the complete registry the conversion engine uses. Each factor is labelled exact (fixed by definition or treaty), derived (calculated from exact definitions, so also exact but not itself a published figure), or conventional (a value in documented common use that is not a legal definition). No factor on this site is an approximation typed from memory, and no page claims a blanket “based on international standards” without naming which one.
| Quantity | Relation | Provenance | Source |
|---|---|---|---|
| Length | 1 in = 0.0254 m | EXACT | 1959 International Yard and Pound Agreement |
| Length | 1 ft = 0.3048 m | EXACT | 1959 International Yard and Pound Agreement |
| Length | 1 yd = 0.9144 m | EXACT | 1959 International Yard and Pound Agreement |
| Length | 1 mi = 1609.344 m | EXACT | NIST SP 811 App. B.9 (5280 ft) |
| Length | 1 nmi = 1852 m | EXACT | International nautical mile, 1929 |
| Mass | 1 lb = 0.45359237 kg | EXACT | 1959 International Yard and Pound Agreement |
| Mass | 1 oz = 28.349523125 g | DERIVED | lb ÷ 16 |
| Mass | 1 st = 6.35029318 kg | DERIVED | 14 lb |
| Volume | 1 US gal = 3.785411784 L | EXACT | 15 USC 205 (231 in³) |
| Volume | 1 imp gal = 4.54609 L | EXACT | UK Weights and Measures Act 1985 |
| Volume | 1 US fl oz = 29.5735295625 mL | DERIVED | US gal ÷ 128 |
| Volume | 1 imp fl oz = 28.4130625 mL | DERIVED | imp gal ÷ 160 |
| Area | 1 ft² = 0.09290304 m² | DERIVED | (0.3048 m)² |
| Area | 1 acre = 4046.8564224 m² | DERIVED | 4840 yd² |
| Energy | 1 kWh = 3.6 MJ | EXACT | BIPM SI Brochure (W = J/s) |
| Energy | 1 Btu(IT) = 1055.05585262 J | EXACT | NIST SP 811 App. B.9 |
| Energy | 1 cal(th) = 4.184 J | EXACT | NIST SP 811 (thermochemical calorie) |
| Energy | 1 therm (US) = 29.307107 kWh | CONVENTIONAL | 100 000 Btu(IT); US gas billing convention |
| Power | 1 hp (mechanical) = 745.699872 W | CONVENTIONAL | NIST SP 811 (550 ft·lbf/s) |
| Power | 1 PS (metric hp) = 735.49875 W | CONVENTIONAL | DIN 66036 / NIST SP 811 |
| Pressure | 1 bar = 100 000 Pa | EXACT | BIPM SI Brochure Table 8 |
| Pressure | 1 atm = 101 325 Pa | EXACT | BIPM SI Brochure Table 8 |
| Pressure | 1 psi = 6894.757293 Pa | DERIVED | lbf ÷ in², g_n = 9.80665 m/s² |
| Data | 1 kB = 1000 B, 1 KiB = 1024 B | EXACT | IEC 80000-13; BIPM SI prefixes |
| Temperature | 0 °C = 273.15 K | EXACT | BIPM SI Brochure (kelvin definition) |
| Fuel economy | US mpg × L/100 km = 235.214583 | DERIVED | 100 × US gal(L) ÷ mile(km) |
| Fuel economy | imp mpg × L/100 km = 282.480936 | DERIVED | 100 × imp gal(L) ÷ mile(km) |
Digital storage: decimal and binary are different units
A kilobyte and a kibibyte are not the same size, and treating them as interchangeable is the single most common error in storage conversion. Under IEC 80000-13 and the SI prefix system:
- Decimal (SI): 1 kB = 1,000 B · 1 MB = 106 B · 1 GB = 109 B · 1 TB = 1012 B
- Binary (IEC): 1 KiB = 1,024 B · 1 MiB = 220 B · 1 GiB = 230 B · 1 TiB = 240 B
Converters on this site labelled kB, MB, GB or TB use the decimal powers of 1,000, which is what drive manufacturers, network operators and mobile data plans mean. Converters labelled KiB, MiB, GiB or TiB use the binary powers of 1,024. The GB to GiB converter crosses between the two families explicitly — so 1 TB (1012 B) is about 931 GiB.
Display precision and rounding
Calculations run at full double precision using the constants above — never a pre-rounded factor. Rounding happens once, at display time, to at most 9 significant figures rather than a fixed number of decimal places. Significant figures matter here: a fixed four-decimal display turns 0.00001 kg into “0”, and a truncated 2.2046 factor introduces a visible error of thousands of units once you convert millions of kilograms.
Values too large or too small to read as plain decimals are shown in exponential notation instead of a row of zeroes. Round-tripping a conversion and converting back returns the original value to within floating-point tolerance; this is enforced by an automated test over every converter in the library rather than trusted by inspection.
Physically impossible inputs are refused rather than answered. Negative lengths, masses, volumes, areas and durations, and temperatures below absolute zero (0 K, −273.15 °C, −459.67 °F), produce an explanation instead of a confident-looking number.
Reference bodies we follow
- BIPM — SI Brochure, 9th edition (2019) — definitions of the joule, watt, ampere and volt, including the 2019 redefinition of the base units.
- NIST Special Publication 811 (2008) — imperial-to-SI conversion factors, including horsepower, BTU and the thermochemical calorie.
- IEC 60038 — Standard voltages — supply voltage harmonisation (230 V ±10% in Europe, 400 V three-phase).
- ASHRAE Handbook — Fundamentals — refrigeration ton and BTU/h conventions.
Source links point to the publishing body rather than a mirrored copy, so they stay valid when a body republishes an updated edition.
Where these numbers stop being reliable
- Grid carbon intensity varies by more than a factor of thirty between countries and by the hour within a country. Use your own grid figure for any reporting purpose.
- Electricity and gas prices change with tariff, region and contract. Cost figures use the tariff you enter; no average price is assumed.
- Heat pump performance depends on climate, emitter temperature and installation quality. A quoted SCOP is not a guarantee.
- Electrical results assume a resistive single-phase load. Motors, three-phase supplies and inverter loads need power factor and phase corrections.
- Battery capacity degrades with age and falls in cold weather; usable capacity is always below nameplate.
- Nothing here replaces a qualified electrician, heating engineer or accredited energy assessor for work that will be installed or certified.
Corrections and updates
If a factor here is wrong or out of date, we want to fix it. Constants are reviewed whenever a source body publishes an update, and every calculator states its assumptions inline so errors are visible rather than hidden.