Methodology, Constants and Data Sources
Every number this site produces comes from either an exact definitional factor or a documented average. This page lists all of them, states which category each belongs to, links the source, and explains where the assumption stops being universal.
Last updated · Next scheduled review January 2027 · Maintained by the SnapUnitConverter editorial team
Two 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.
Modelling assumptions are averages used to give a result meaning — the price of electricity, the carbon intensity of a grid, the efficiency of a heat pump. These vary by country, tariff, season and installation quality. Wherever this site shows a real-world equivalence, the assumption behind it is stated next to the figure and listed 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) |
Modelling assumptions used in decision context
These constants power the “What does this result mean?” panels, the flagship calculators and the guided journeys. They are deliberately conservative and rounded to avoid implying more precision than the underlying data supports.
| Assumption | Value used | Source / basis |
|---|---|---|
| Average household electricity use | 10.5 kWh/day (≈3,800 kWh/year) | EU household consumption average Eurostat — Energy consumption in households (2024) |
| Electricity price | €0.25/kWh including taxes | Eurostat household electricity price band, EU average Eurostat — Electricity price statistics (2025 H1) |
| Grid carbon intensity | 0.35 kg CO₂e/kWh | Approximate global average generation intensity for 2024 — local grids range from 0.02 to 0.8 IEA — Emissions from electricity (2024 data) |
| EV usable battery capacity | 77 kWh | Representative long-range passenger EV pack |
| EV consumption | 18 kWh/100 km | WLTP mixed-cycle figures for mid-size EVs EEA / WLTP electric vehicle consumption data |
| Heat pump seasonal COP | 3.2 | Typical air-source SCOP for a well-installed system in a temperate climate NREL — Heat pump performance research |
| Condensing gas boiler efficiency | 90% | Seasonal efficiency of a modern condensing boiler |
| Whole-house peak heat demand | 5 kW for ~120 m² | Typical insulated dwelling; a proper heat-loss survey overrides this |
| Kettle boil | 0.11 kWh (1.7 L) | 2.2 kW element for approximately three minutes |
| Reference battery sizes | Phone 12 Wh, laptop 60 Wh, 18 V tool pack 90 Wh | Common consumer product capacities |
| Air-transport battery thresholds | 100 Wh / 160 Wh | Carry-on limits and airline-approval band for lithium batteries IATA — Lithium battery guidance (100/160 Wh) |
| E-bike consumption | 12 Wh/km | Mid-level pedal assistance on mixed terrain |
| Continuous circuit loading limit | 80% of breaker rating | Standard derating practice for loads exceeding three hours |
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, which is what Windows reports and what RAM is sized in. Bridge pages convert between the two families reports and what RAM is sized in. The GB to GiB converter crosses between the two families explicitly — that is why a “1 TB” drive shows as 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 tons, BTU/h conventions and HVAC load practice.
- IEA — Data & statistics (2024 data year) — electricity generation carbon intensity and global energy statistics.
- Eurostat — Electricity price statistics (2025 H1) and Energy consumption in households (2024) — the European price and consumption averages.
- IATA — Lithium battery guidance — watt-hour thresholds for air transport (100 Wh and 160 Wh).
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. Every cost figure on this site is illustrative.
- 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.