Engineering Unit Conversion Calculator
Thirteen groups of engineering units, converted instantly: acceleration, angle, couple unbalance, density, length, rotational speed, surface roughness, temperature, time, unbalance amount, velocity, voltage and weight.
Engineering Unit Conversion Calculator
On this page
What each group covers
| Group | Units |
|---|---|
| Acceleration | mm/sec², cm/sec², m/sec², in/sec², ft/sec², in/min² |
| Angle | radians, degrees, arcminutes |
| Couple unbalance | g-mm², g-cm², g-m², g-in², kg-mm², kg-cm², kg-m², oz-in², lb-ft², N-m² |
| Density | g/mm³, g/cm³, kg/m³, oz/in³, lb/in³ |
| Length | mm, cm, m, micron, in, ft |
| Rotational speed | Hz, Cps, Rad/Sec, RPM |
| Surface roughness | micron, µinch |
| Temperature | Celsius, Fahrenheit, Kelvin, Rankine, Réaumur |
| Time | s, ms, µs, min, hr |
| Unbalance amount | g-mm, g-cm, g-m, g-in, kg-mm, kg-cm, kg-m, mg-in, oz-in, lb-ft, N-m, grain-mm, grain-cm, grain-m, grain-in, grain-ft |
| Velocity | mm/s, cm/s, m/s, in/s, ft/s, mm/min, cm/min, m/min, in/min, ft/min, km/h, mi/h |
| Voltage | Volts, millivolts, µvolts |
| Weight | g, kg, mg, oz, lb, grain |
Unbalance amount: the conversion that causes the most trouble
Unbalance is a mass multiplied by a radius, so every unit in that group is a mass unit paired with a length unit. That is why the list looks unusual next to ordinary engineering units, and why converting between them trips people up.
The common pairs and where you meet them:
g-mm and g-cm — the metric defaults, and what most balancing machines report. ISO 21940-11 tolerances come out in g·mm naturally.
oz-in — the North American convention, and still what a great many drawings specify. 1 g·mm is about 0.00139 oz·in, so the numbers look dramatically different for the same physical unbalance. A tolerance that reads as a comfortable 500 in one unit is a demanding 0.69 in the other.
grain-based units — used in some armature and small-motor work, where grams produce inconveniently small numbers. A grain is 1/7000 of a pound.
lb-ft and N-m — these look like torque units and are not being used as torque here. In this context they are a mass or weight paired with a radius. If a drawing specifies unbalance in N·m, confirm what the supplier means before converting, because the same notation is used for torque elsewhere on the same print.
Velocity, and why vibration is measured in mm/s
Vibration severity is conventionally expressed as a velocity, usually mm/s RMS, because velocity correlates better with damage across a wide frequency range than displacement or acceleration do. Displacement dominates at low frequency, acceleration at high frequency; velocity sits in between and is roughly flat across the range where most rotating machinery faults appear.
That is why the machinery vibration standards are written in mm/s, and why converting a measurement to in/s is often the only thing standing between a European specification and an American instrument.
Temperature is not a simple factor
Every other group on this page converts by multiplication alone. Temperature does not, because the scales have different zero points as well as different degree sizes.
Celsius and Kelvin share a degree size but differ in origin by 273.15. Fahrenheit and Rankine share a degree size, five-ninths of a Celsius degree, and differ in origin by 459.67. Réaumur uses a degree four-fifths the size of a Celsius degree with the same freezing point.
The calculator converts through Kelvin internally and refuses values below absolute zero on whichever scale you entered — −273.15°C, −459.67°F, 0 K, 0°R or −218.52°Ré.
Notes on precision
Conversions here use exact definitions where exact definitions exist. One inch is exactly 25.4 mm; one pound is exactly 0.45359237 kg; one grain is exactly 1/7000 pound. Results are shown to ten significant figures, switching to scientific notation for very large or very small values rather than rounding them to zero.
For the unbalance units that pair a force unit with a length — N-m and lb-ft — standard gravity of 9.80665 m/s² is used to relate mass and weight.
Frequently asked questions
Why does the unbalance group mix mass units with force units?
Because industry does. Some specifications express unbalance as a mass at a radius, others as a weight at a radius. The conversion between them requires an assumed gravity, which is why the two families are related here by standard gravity rather than treated as interchangeable.
What is the difference between Hz and Cps?
None. Cycles per second is the older name for the hertz. Both appear on older machine documentation and they convert one to one.
Is surface roughness in microns the same as Ra?
The unit is the same; Ra is a particular parameter expressed in that unit. Converting the unit does not convert between roughness parameters — Ra, Rz and Rq are different measurements of the same surface, not different units of the same measurement. Our surface measurement systems page covers which parameter to specify.
Can I convert couple unbalance the same way as single-plane unbalance?
No, and they are kept in separate groups here for that reason. Couple unbalance is a mass times a radius times a distance between planes, so its units carry a squared length. A conversion factor from the unbalance amount group will give you a wrong answer.
What if my unit is not listed?
Tell us what you are working from and we will add it. The list reflects what our customers actually specify on drawings, and it grows.
Related resources
ISO 21940-11 balance tolerance calculator · Vector calculator · Process capability calculator · GD&T symbols reference
