Engineering Unit Converter

SI vs imperial and when each system is used

The SI (International System) is the global standard in science, engineering, and most countries' everyday life. Every unit is derived from seven base units: the metre, kilogram, second, ampere, kelvin, mole, and candela. SI prefixes express decimal multiples and submultiples across all of them.¹² Imperial units such as inches, pounds, gallons, and BTU remain in active use in the United States and for specific domains like aviation (feet, nautical miles, knots) and some trades across the UK.

Why two systems persist side by side

Mixing systems is a real source of error in multi-team engineering projects. NASA's Mars Climate Orbiter was lost in 1999 because one team in Colorado used SI while the navigation team in California used imperial units without converting between them.³ The rule for engineering work is to pick one system for a project and convert at the boundary between disciplines or teams, never in the middle of a calculation where a missed factor can compound.

Temperature scales explained

Four temperature scales remain in practical use today. Celsius (°C) sets 0 at the freezing point of water and 100 at its boiling point, making it the everyday metric standard. Fahrenheit (°F) uses a finer step size where 32°F equals 0°C and 212°F equals 100°C. Kelvin (K) is the SI base unit that starts at absolute zero, or negative 273.15°C, with the same step size as Celsius.⁴ Rankine (°R) is the Fahrenheit-based absolute scale still used in some US thermodynamic engineering work.⁵

Why temperature conversion needs more than a single factor

Because zero means a different physical point on each scale, converting a temperature always requires both an offset and a multiplication, unlike a simple length or mass conversion where only the factor changes. You should always use an absolute scale such as Kelvin or Rankine inside thermodynamic equations, because inserting a Celsius or Fahrenheit value directly into a formula that involves ratios or differences will produce results that are quietly wrong in ways hard to spot on inspection.

Fuel consumption: L/100km vs mpg

Europe and most of the world rate fuel consumption as litres per 100 kilometres, where a lower figure means better efficiency, while the United States commonly uses miles per gallon, where a higher figure means better efficiency.⁶ UK passenger-car information must also list mpg alongside L/100km or km/L.⁷ These two ratings are inverses of each other rather than a simple scale factor. A car rated at 6 L/100km equals roughly 39.2 mpg US or 47.1 mpg UK, and cutting consumption to 3 L/100km, which is twice as efficient, doubles those mpg figures.

How the gallon difference distorts cross-market comparisons

The gallon size itself also differs between the two mpg standards: a US gallon is 3.785 litres while a UK imperial gallon is 4.546 litres.⁶ Because of that size difference alone, a car rated at 40 mpg US would show roughly 48 mpg on UK labelling, even before test-cycle differences are considered. Keeping both gallon definitions in mind prevents a misleading comparison when you read efficiency specs from different markets side by side.

L/100km also has the practical advantage of being linear: halving the litres always means halving the fuel cost for the same distance, while mpg changes by smaller absolute steps at the efficient end of the scale and by bigger steps at the wasteful end. That asymmetry makes mpg feel more flattering for thirsty vehicles and less rewarding than it looks for efficient ones, which is one reason regulators preferring transparency tend to favour L/100km.

Domain-specific units in practice

Several units in this converter are conventions of a specific domain rather than strict SI or imperial. Knots remain the standard for speed in aviation and maritime work.⁸⁹ Torr and mmHg are still common in laboratory vacuum measurement and medical blood pressure readings.¹⁰¹¹ Centipoise (cP) is a common dynamic viscosity unit in fluid specs.¹² Standard gravity (g₀ = 9.80665 m/s²) is a conventional reference acceleration.¹²

For torque and energy the units can be especially confusing because Newton-metres (N·m) and joules (J) are dimensionally identical, both reducing to kg·m²/s², even though they are not interchangeable.¹³ Engineers reserve N·m for torque, which is a rotational force applied over a radius, and reserve J for energy, which is work done. Calling a torque value a number of joules in a specification is a communication error that can mislead a manufacturing team, even when the numeric value happens to be identical in both units.

Pressure units across engineering disciplines

Pressure arrives at engineers in half a dozen units depending on the field, and none of them has displaced the others. The SI unit is the pascal (Pa), but because one pascal is very small, kilopascals and megapascals dominate in most engineering specs. Hydraulic systems are commonly specified in kPa or MPa depending on scale, and tire pressure is commonly expressed in kPa or psi.¹⁰ Standard atmospheric pressure is 101.325 kPa, the same value as 1.01325 bar or 760 mmHg, expressed in different conventions and each entrenched in a specific domain.

Medical and laboratory work uses millimetres of mercury (mmHg) because it remains standard in clinical practice for blood pressure and vacuum measurement.¹¹ US industry commonly uses PSI in automotive and HVAC specifications. Converting between these systems requires exact factors: 1 atm is 14.696 PSI, 101.325 kPa, 1.01325 bar, and 760 mmHg simultaneously.⁵ Therefore, keeping a unit converter open when reading specifications from different countries or disciplines is simply good engineering practice, not a sign of unfamiliarity with the subject.