Engineering Unit Calculator: 29 Categories + Custom Units, Free

Engineering Unit Calculator

Live conversion across 29 categories · exact SI constants

Conversions use internationally defined constants where they exist, so the inch, pound, atmosphere and calorie values here are exact by definition rather than rounded approximations. Where units share a name but not a value the tool keeps them separate: mechanical horsepower and metric PS differ by about ten watts, US and imperial gallons differ by roughly twenty percent, decimal and binary storage units diverge increasingly with scale, and thermochemical and IT calories are not the same. Temperature is converted through an absolute scale rather than a simple ratio, which is why a Celsius-to-Fahrenheit conversion involves an offset and why zero in one scale is not zero in another. Fuel economy needs care because miles per gallon measures distance per volume while litres per hundred kilometres measures volume per distance, so the relationship is reciprocal and halving one does not halve the other. Custom units are defined relative to the category base unit and exist only for the current session. For legal, trade or safety-critical work, confirm against the governing standard for your jurisdiction.

Engineering Unit Calculator: 29 Categories, Exact Constants and Your Own Custom Units

Most everyday unit converters treat mechanical horsepower and metric PS as the same thing or quietly round the inch to something close enough, but close enough isn’t good enough for real engineering work. This engineering unit converter uses the internationally defined exact constants wherever they exist, an inch really is exactly 2.54 centimetres, a pound really is exactly 0.45359237 kilograms and it keeps genuinely different units apart even when they get lumped together elsewhere, mechanical horsepower and metric PS differ by about ten watts and a gigabyte and a gibibyte differ by over seventy megabytes once you actually do the math. As a full metric to imperial converter, it covers 29 categories, length, mass, pressure, energy, torque, viscosity, digital storage and plenty more, each with metric, imperial, US customary and CGS units where they genuinely apply. The interface stays simple on the surface, pick a category, type in either box, watch the other update live, but the depth is there when you need it, a searchable unit finder, the exact conversion factor shown so you can check the math yourself, adjustable precision, a batch mode for converting a whole list at once and the ability to define your own unit right against the category base.

How to Use

Step 1: Pick your category and convert
  • Select a Category from the dropdown, length, mass, pressure, torque, viscosity, digital storage and 29 in total.
  • Type a value into the From box and use the search units field or the list below it to pick your starting unit.
  • Pick your target unit in the To box the same way. The converted value updates live the moment you change either side, no submit button needed.
  • Tap the ⇄ swap button anytime to flip which unit is which without retyping anything.
  • Check the factor bar underneath, it shows you the exact conversion factor being used for full transparency, so you can verify the math instead of just trusting a black box.
  • Your last few conversions show up in the recent pairs bar too, tap one to jump straight back to it.
Step 2: Adjust precision, if you want
  • Tap the ⚙ Precision tab to open that panel.
  • Choose Decimal places or Significant figures as your Rounding Mode, then set how many Digits you want shown.
Step 3: Define your own custom unit, if you need one
  • Tap the ★ Custom Unit tab to open that panel.
  • Type a Custom Unit Name and enter how many base units it equals for that category.
  • Tap Add Unit. Your custom unit now shows up right alongside all the built-in ones for that category and you can convert against it just like any other unit, it just only lasts for your current session.
Step 4: Convert a whole list at once
  • Tap the ≡ Batch Convert tab to open that panel.
  • Paste or type your Values, one per line or separated by commas or spaces.
  • Tap Convert Batch to get every single value converted at once using your currently selected From and To units.
Step 5: Export
  • Use Print / PDF for a clean printable copy or Copy to paste the numbers elsewhere.

Key Features

  • 29 categories, length, mass, volume, area, temperature, pressure, energy, power, speed, force, torque, density, viscosity, digital storage and plenty more
  • Exact, internationally defined constants, not rounded approximations, so an inch really is 2.54 cm and a pound really is 0.45359237 kg
  • Genuinely different units kept distinct, mechanical HP versus metric PS, decimal versus binary digital storage, US versus imperial gallons, none of them silently merged
  • Live bi-directional conversion, type in either box and the other updates instantly, with a swap button to flip them
  • Searchable unit finder for the longer categories, so you don’t have to scroll through a huge list
  • Exact factor shown on screen, for full transparency on exactly what math is happening
  • Unlimited custom units per session, defined relative to the category base and usable against everything else in it
  • Batch mode, converting a whole list of values in one go
  • Correct temperature offsets and fuel-economy inversion, both handled as the special cases they actually are, not treated like a plain ratio

Formula / Logic Used

Standard Linear Conversion

Valueto=Valuefrom×FactorfromFactortoValue_{to} = Value_{from} \times \frac{Factor_{from}}{Factor_{to}}

Where every unit’s factor is it’s exact relationship to the category’s chosen base unit.

Temperature (Offset, Not a Plain Ratio)

°F=°C×95+32,K=°C+273.15°F = °C \times \frac{9}{5} + 32, \qquad K = °C + 273.15

Temperature can’t just be scaled like other units because zero doesn’t mean the same thing across scales, zero Celsius isn’t zero Fahrenheit or zero Kelvin, which is exactly why an offset has to be applied on top of the ratio.

Fuel Economy (Reciprocal Relationship)

L/100km=100MPG×0.425144L/100km = \frac{100}{MPG \times 0.425144}

Miles per gallon measures distance per volume, while litres per 100km measures volume per distance, the exact opposite way round, so the relationship between them is reciprocal, halving one does not halve the other, it roughly doubles it instead.

Custom Unit Definition

Valuebase=Valuecustom×Custom FactorValue_{base} = Value_{custom} \times Custom\ Factor

Once defined, your custom unit converts against every other unit in that category through this same base-unit relationship.

Who Should Use This Tool

Engineers, engineering students and technical writers who need genuinely accurate conversions across a wide range of specialised categories, viscosity, torque, digital storage and more, not just the handful covered by a typical everyday converter. Also useful for anyone converting a long list of values at once via the batch mode or working with a plant-specific or company-specific unit that needs it’s own custom definition.

Frequently Asked Questions (FAQs)

1. How is this different from a normal, everyday unit converter?

Most everyday converters only cover common categories like length, weight and temperature, and often quietly round or merge units that are actually different, like mechanical horsepower and metric PS. This tool covers 29 categories including specialised engineering ones like viscosity, torque, and thermal conductivity, uses exact internationally defined constants throughout and keeps commonly-confused units genuinely distinct.

2. Why does this tool treat mechanical horsepower and metric PS as different values?

They’re genuinely different units with different underlying definitions, mechanical horsepower equals 745.699872 watts while metric PS equals 735.49875 watts, a difference of about ten watts. Treating them as interchangeable, which a lot of simpler converters do, quietly introduces a small but real error into any power calculation.

3. Can I convert Celsius to Fahrenheit the same way as converting metres to feet?

No, and this tool handles it correctly, temperature conversion needs an offset on top of a ratio, since zero on one scale doesn’t correspond to zero on another, while length conversion is a pure ratio with no offset needed. This distinction is exactly why temperature has it’s own dedicated conversion logic here rather then being treated like every other category.

4. How do I define my own custom unit for a value my industry uses?

Open the ★ Custom Unit panel, give your unit a name and enter exactly how many base units of that category it equals. Once added, it shows up right alongside the built-in units and converts against every one of them using that same relationship, though it only lasts for your current browser session.

5. Can I convert a whole spreadsheet column of values at once instead of one at a time?

Yes, open the ≡ Batch Convert panel, paste your values in, one per line or separated by commas or spaces, and tap Convert Batch. Every value gets converted at once using whatever From and To units you’ve currently got selected.

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