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SI base and derived units, their symbols, and common conversions.
27 of 27 units & measurements
metre (m)
Defined using the speed of light: the distance light travels in a vacuum in 1/299,792,458 of a second.
kilogram (kg)
Since 2019 defined using Planck's constant, replacing a platinum-iridium cylinder kept near Paris.
second (s)
Defined by the vibrations of the caesium-133 atom (9,192,631,770 cycles), the basis of atomic clocks.
ampere (A)
Named after André-Marie Ampère. Measured with an ammeter, connected in series.
kelvin (K)
0 K is absolute zero, the lowest possible temperature (−273.15 °C). Named after Lord Kelvin.
mole (mol)
One mole contains about 6.022 × 10²³ particles (Avogadro's number).
candela (cd)
Measures how bright a light source appears in a given direction; roughly the brightness of one candle.
newton (N)
The force needed to accelerate 1 kg by 1 m/s². Named after Isaac Newton.
joule (J)
Work done when a force of 1 newton moves something 1 metre. Named after James Prescott Joule.
watt (W)
One joule per second. Named after James Watt.
pascal (Pa)
One newton per square metre. Named after Blaise Pascal. Air pressure at sea level is about 101,325 Pa.
volt (V)
Named after Alessandro Volta. Measured with a voltmeter, connected in parallel. Pakistan's household supply is 220–230 V.
ohm (Ω)
Named after Georg Ohm, whose law relates voltage, current and resistance.
coulomb (C)
The charge carried by a current of 1 ampere in 1 second. Named after Charles-Augustin de Coulomb.
hertz (Hz)
One cycle per second. Named after Heinrich Hertz. Pakistan's mains electricity runs at 50 Hz.
tesla (T)
Named after Nikola Tesla. MRI scanners use magnetic fields of about 1.5–3 T.
weber (Wb)
Measures the total magnetic field passing through an area; one weber per square metre equals one tesla. Named after Wilhelm Weber.
farad (F)
Named after Michael Faraday; measures how much charge a capacitor stores.
becquerel (Bq)
One nuclear decay per second. Named after Henri Becquerel, who discovered radioactivity.
light-year (ly)
The distance light travels in one year, about 9.46 trillion km. Despite its name it measures distance, not time.
knot (kn)
One nautical mile per hour.
decibel (dB)
A logarithmic scale: every 10 dB means ten times more intensity. Normal conversation is about 60 dB; long exposure above about 85 dB can damage hearing.
calorie / kilocalorie (kcal)
Food labels use kilocalories (often written 'Calories'). One kilocalorie raises the temperature of 1 kg of water by 1 °C.
acre, hectare (ha)
Land in Pakistan is often measured in acres, kanals and marlas.
bit, byte (B)
A bit is a single 0 or 1; a byte is 8 bits. Storage is measured in kilobytes, megabytes, gigabytes and terabytes.
barrel (bbl)
The standard unit for crude oil prices and production.
carat (ct) / karat (K)
A carat is 0.2 g for gemstones. Gold purity uses karats: 24K is pure gold, 22K is about 91.7% gold.
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