Common Unit Conversions
Use this reference for common physical-unit relationships: pounds to kilograms, Celsius to Fahrenheit, mph to km/h, and atmosphere to kilopascals. Each section links back to a live converter when a precise table is needed.
Quick check
1 lb = 0.45359237 kg
Exact factors and offset formulas matter most when values move into specifications, reports, or lab work.
Common physical-unit relationships
| Quantity | Common conversion | Use case | Tool |
|---|---|---|---|
| Mass | 1 lb = 0.45359237 kg | Body weight, shipping, product labels | Open converter |
| Temperature | °F = °C x 9 / 5 + 32 | Weather, ovens, labs, classroom formulas | Open converter |
| Speed | 1 mph = 1.609344 km/h | Road speed, route comparisons, vehicle specs | Open converter |
| Pressure | 1 atm = 101.325 kPa | Weather, tires, gauges, lab pressure | Open converter |
Mass: use kilogram factors
Mass units such as grams, kilograms, pounds, ounces, and tons can be normalized through kilograms. Be careful with ton labels because short tons, long tons, and metric tonnes are different.
Temperature: include the offset
Temperature conversion needs an offset because scales do not share the same zero point. Celsius, Fahrenheit, kelvin, and Rankine should not be treated like simple unit factors.
Speed: normalize to m/s
Meters per second keeps physics calculations consistent. Road-speed conversions still remain available for mph and km/h comparisons.
Pressure: normalize to pascals
Pressure units span weather, lab, tire, and engineering contexts. Pascals provide the base route for kPa, bar, atm, psi, Torr, and mercury-column units.
How to choose the correct conversion method
Start by identifying the physical quantity. If the value is a mass or common weight label, use a kilogram factor. If it is a temperature, use an offset-aware formula. If it is speed, convert through meters per second. If it is pressure, convert through pascals. The base route protects the result from unit-system shortcuts that only work in one narrow context.
The next step is to preserve the source label. A ton can mean a short ton, long ton, or metric tonne. A gallon can be U.S. or imperial. Mach depends on local speed of sound. When a unit label is ambiguous, a good conversion note keeps the original source unit visible alongside the converted result.
References for conversion constants
NIST Guide to the SI: Conversion Factors
Official SI definitions, accepted unit guidance, and conversion-factor references.
NIST SI Units
Official SI definitions, accepted unit guidance, and conversion-factor references.
BIPM SI Brochure
Official SI definitions, accepted unit guidance, and conversion-factor references.
Common conversion questions
Why do physical unit converters use different base units?
Different physical quantities have different SI base or derived units. Mass converts through kilograms, temperature through kelvin with offsets, speed through meters per second, and pressure through pascals. A single generic multiplication method would fail for temperature and would hide important context for speed and pressure units.
What is the most common pounds to kilograms factor?
The international avoirdupois pound is exactly 0.45359237 kilogram. That factor is used for practical body weight, shipping, and product-label conversions. Ounces and stones are related to the pound, so those conversions should follow the same avoirdupois system unless a source says otherwise.
What is the quickest Celsius to Fahrenheit formula?
Use °F = °C x 9 / 5 + 32. The reverse formula is °C = (°F - 32) x 5 / 9. Temperature conversions need the offset because Celsius and Fahrenheit have different zero points, unlike length or mass units that usually use multiplication factors only.
How do you convert mph to km/h?
Multiply miles per hour by 1.609344 to get kilometers per hour. That relationship comes from the exact international mile definition of 1609.344 meters. For physics formulas, converting speed into meters per second is often more useful than converting between road-speed labels.
What pressure units should I know first?
The most useful pressure units are pascals, kilopascals, bar, atmospheres, and psi. Weather references may use millibars or inches of mercury, tire labels often use psi and kPa, and laboratory vacuum work may use Torr or millimeters of mercury.