Unit & engineering glossary

Plain-language definitions of the measurement and engineering terms that come up most often across this site — from ampacity to torr. Each term links to the concepts behind our converters, calculators, and cheat sheets.

Ampacity: The maximum electric current a conductor can carry continuously without exceeding its temperature rating. Set by conductor size, insulation type, and ambient conditions (e.g., NEC Table 310.16).
AWG (American Wire Gauge): A logarithmic wire-sizing system where a larger number means a thinner wire. Each 6-gauge step roughly halves or doubles the cross-sectional area.
Atmosphere (atm): A pressure unit defined as exactly 101,325 pascals — approximately the average air pressure at sea level.
Bar: A metric pressure unit equal to exactly 100,000 pascals. Close to one atmosphere and widely used in engineering despite being non-SI.
BTU (British Thermal Unit): The energy needed to raise one pound of water by 1 °F, about 1,055 joules. Common in HVAC and heating capacity ratings.
Celsius (°C): A temperature scale on which water freezes at 0° and boils at 100° at standard pressure. Related to kelvin by °C = K − 273.15.
Duty cycle: The fraction of time a device operates within a repeating period, usually expressed as a percentage.
Fahrenheit (°F): A temperature scale on which water freezes at 32° and boils at 212°. Convert with °F = (°C × 9/5) + 32.
Foot-pound (ft·lbf): A unit of torque or energy equal to the effect of one pound-force acting at a one-foot lever arm (about 1.356 N·m).
Gauge (sheet metal): A thickness designation for sheet metal where a higher number means thinner material. Values differ by metal (steel vs. aluminum use different standards).
Horsepower (hp): A unit of power. Mechanical (imperial) horsepower ≈ 745.7 W; metric horsepower ≈ 735.5 W — the two are not identical.
Kelvin (K): The SI base unit of thermodynamic temperature. 0 K is absolute zero; the scale has no negative values.
Kilogram-force (kgf): The force exerted by one kilogram of mass under standard gravity — exactly 9.80665 newtons. Non-SI but common in industry.
K-factor (fasteners): An empirical friction coefficient in the bolt torque relation T = K × D × P, linking applied torque to clamp load.
Knot (kn): A speed of one nautical mile per hour, about 0.514 m/s. Used in marine and aviation navigation.
Mebibyte vs Megabyte: A mebibyte (MiB) is 1,048,576 bytes (1024²); a megabyte (MB) is 1,000,000 bytes (1000³). Confusing them causes storage-size discrepancies.
Metric (SI) prefixes: Decimal multipliers applied to units — kilo (10³), milli (10⁻³), micro (10⁻⁶), mega (10⁶), and so on.
Newton (N): The SI unit of force: the force that accelerates one kilogram at one meter per second squared (1 N = 1 kg·m/s²).
Nominal vs actual size: Nominal is the labeled/reference size used for ordering; actual is the true measured dimension, which often differs (e.g., pipe and lumber).
NPS (Nominal Pipe Size): A North American pipe-size designation. For small sizes it does not equal the actual outside diameter; wall thickness is set by the schedule number.
NPT (National Pipe Taper): A tapered US pipe thread standard (ASME B1.20.1) that seals through thread interference, usually with sealant.
Pascal (Pa): The SI unit of pressure: one newton per square meter. Small in everyday terms, so kilopascals and megapascals are common.
Pound-force vs pound-mass: Pound-mass (lbm) measures mass; pound-force (lbf) measures the weight of one lbm under standard gravity. They are numerically equal only at standard gravity.
PSI (pounds per square inch): An imperial pressure unit — pound-force per square inch, about 6,895 pascals.
R-value: A measure of thermal resistance to conductive heat flow. Higher R-value means better insulation; SI and imperial R-values differ by a factor of 5.678.
Schedule (pipe): A number (e.g., Sch 40, Sch 80) indicating pipe wall thickness for a given nominal size — higher schedule means thicker wall.
Significant figures: The digits in a number that carry meaningful precision. Conversions should not report more significant figures than the input justifies.
Slug: The imperial unit of mass in the foot-pound-second system: the mass accelerated at 1 ft/s² by 1 lbf (about 14.59 kg).
Standard gravity (g₀): The conventional acceleration of free fall, defined as exactly 9.80665 m/s², used to relate mass and force units.
Torque: A rotational (twisting) force equal to force times lever-arm distance, measured in newton-meters (N·m) or pound-feet (lbf·ft).
Torr: A pressure unit defined as exactly 1/760 of a standard atmosphere (≈133.322 Pa), historically the height of a mercury column in millimeters.