Science uses one shared system of units of measurement: the International System of Units (SI). It has 7 base units (meter, kilogram, second, ampere, kelvin, mole and candela), 22 derived units with special names such as the newton and joule, and 24 prefixes from quecto (10^-30) to quetta (10^30).
Every measurement in a lab notebook has two parts: a number and a unit. Without the unit, however, the number means nothing. For example, “12” could be 12 millimeters or 12 kilometers. That is why scientists everywhere agree on the same units, so that a lab in Texas can check a result from Tokyo without guessing.
That agreement is the SI, which is also the modern form of the metric system. Today, the International Bureau of Weights and Measures (BIPM) near Paris maintains it. Meanwhile, in the United States, the National Institute of Standards and Technology (NIST) explains the same system in its guide to SI units. That page also describes 7 base units that define 22 derived units with special names, plus 24 recognized prefixes.
In short: learn the seven base units, a handful of derived units and the common prefixes, and you can read almost any science textbook or lab report. After all, everything else is a combination of those pieces.
What This Guide Covers
This guide, therefore, stays focused on the units themselves. If you want the bigger picture of how labs make measurements traceable and trustworthy, read our explainer on the science of measurement (metrology). Similarly, chemistry students who need molarity, the mole in practice and significant figures should go to our guide to measurement units in chemistry.
Next, you will find a converter, a table of the base units with their modern definitions, the most useful derived units, the full prefix list, and a simple method for converting between metric and US customary values.
Convert Metric and US Length Units
First, length is the easiest place to see prefixes at work. So type a value below and pick its unit. The converter then uses the exact definition 1 inch = 25.4 mm and shows millimeters, centimeters, meters and kilometers next to inches, feet and miles. Because the site has no dedicated prefix converter, this length tool is the closest fit.
Recommended Tools for Measuring in SI Units
However, you do not need a national lab to work in SI. First, a graduated cylinder covers volume in milliliters. Next, a digital caliper reads length to 0.01 mm, and a 0.01 g scale handles small masses. Finally, a probe thermometer covers temperature in degrees Celsius, which converts straight to kelvin. In addition, all of these picks come from established brands, based on manufacturer specs and buyer reports.
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Key Takeaways
- The SI has exactly 7 base units: meter, kilogram, second, ampere, kelvin, mole and candela.
- Since May 20, 2019, each base unit rests on an exact constant of nature, not on a physical object.
- Next, derived units such as the newton, joule, watt and pascal come from combining base units.
- In addition, prefixes scale any unit by powers of ten; there are 24 of them, including four added in 2022.
- The liter, minute and degree Celsius are fine to use alongside SI, but they are not base units.
- Also, US law defines customary units from SI values, for example 1 inch = 2.54 cm exactly.
- Finally, always write the unit symbol after a number, with a space: 5 kg, not 5kg.
The 7 SI Base Units of Measurement
First, each base unit covers one basic quantity. In other words, these seven stand on their own, and every other SI unit comes from multiplying or dividing them. The table also shows one everyday comparison in US customary terms.
| Quantity | Unit (symbol) | Everyday sense of scale |
|---|---|---|
| Length | meter (m) | 1 m = 39.37 in, a little more than a yard |
| Mass | kilogram (kg) | 1 kg = 2.205 lb |
| Time | second (s) | Same second used everywhere |
| Electric current | ampere (A) | A phone charger draws roughly 1 to 3 A |
| Thermodynamic temperature | kelvin (K) | 0 K = -273.15 °C = -459.67 °F |
| Amount of substance | mole (mol) | 6.02214076 x 10^23 particles |
| Luminous intensity | candela (cd) | Roughly the glow of one ordinary candle |
Also notice that the kilogram is the only base unit with a prefix already built into its name. As a result, the gram is not a base unit, even though prefixes attach to “gram” (milligram, microgram) rather than to “kilogram”.
How Scientists Define the Base Units of Measurement Today
For most of the 20th century, a metal cylinder in a vault in France defined the kilogram. As a result, if that cylinder gained or lost a few atoms, then the kilogram changed with it. Therefore, to fix this, the General Conference on Weights and Measures voted in 2018 to define every base unit from fixed constants of nature. Then the new definitions took effect on May 20, 2019.
| Unit | Fixed constant it relies on | Exact value |
|---|---|---|
| second | Cesium-133 hyperfine frequency | 9,192,631,770 Hz |
| meter | Speed of light in vacuum | 299,792,458 m/s |
| kilogram | Planck constant | 6.62607015 x 10^-34 J s |
| ampere | Elementary charge | 1.602176634 x 10^-19 C |
| kelvin | Boltzmann constant | 1.380649 x 10^-23 J/K |
| mole | Avogadro constant | 6.02214076 x 10^23 per mol |
| candela | Luminous efficacy of 540 THz green light | 683 lm/W |
So the meter is now simply the distance light travels in vacuum in 1/299,792,458 of a second. Because the speed of light now has an exact value, any well-equipped lab can realize the meter without borrowing a reference bar. Similarly, the same idea applies to the other six units.
Derived Units of Measurement With Special Names
Once you have the base units, you can then build everything else. For instance, speed is length divided by time, so its SI unit is meters per second (m/s). However, some combinations are so common that they get their own names. Altogether, there are 22 of these special names, and the ones below also show up in almost every physics or chemistry course.
| Quantity | Unit (symbol) | In base units | US customary comparison |
|---|---|---|---|
| Force | newton (N) | kg m/s^2 | 1 N = 0.2248 lbf |
| Energy, work, heat | joule (J) | N m = kg m^2/s^2 | 1 J = 0.2390 cal (thermochemical) |
| Power | watt (W) | J/s | 746 W = about 1 mechanical hp |
| Pressure | pascal (Pa) | N/m^2 | 1 atm = 101,325 Pa = 14.70 psi |
| Frequency | hertz (Hz) | 1/s | – |
| Electric charge | coulomb (C) | A s | – |
| Voltage | volt (V) | W/A | – |
| Resistance | ohm | V/A | – |
| Celsius temperature | degree Celsius (°C) | K – 273.15 | °F = °C x 1.8 + 32 |
Plenty of useful derived units have no special name, however. For example, area is square meters (m^2), volume is cubic meters (m^3), and density is kilograms per cubic meter (kg/m^3). Water, in turn, has a density close to 1,000 kg/m^3, which is the same as 1 g/cm^3.
SI Prefixes: Scaling Any Unit by Powers of Ten
In short, prefixes are what make the system fast to use. For example, instead of writing 0.000001 meters, you write 1 micrometer. Each prefix is a fixed power of ten, so converting is just moving the decimal point. Then, in November 2022, the CGPM added four new prefixes (ronna, quetta, ronto and quecto), bringing the total to 24, as listed on the BIPM prefix page.
| Prefix (symbol) | Factor | Prefix (symbol) | Factor |
|---|---|---|---|
| quetta (Q) | 10^30 | deci (d) | 10^-1 |
| ronna (R) | 10^27 | centi (c) | 10^-2 |
| yotta (Y) | 10^24 | milli (m) | 10^-3 |
| zetta (Z) | 10^21 | micro (µ) | 10^-6 |
| exa (E) | 10^18 | nano (n) | 10^-9 |
| peta (P) | 10^15 | pico (p) | 10^-12 |
| tera (T) | 10^12 | femto (f) | 10^-15 |
| giga (G) | 10^9 | atto (a) | 10^-18 |
| mega (M) | 10^6 | zepto (z) | 10^-21 |
| kilo (k) | 10^3 | yocto (y) | 10^-24 |
| hecto (h) | 10^2 | ronto (r) | 10^-27 |
| deca (da) | 10^1 | quecto (q) | 10^-30 |
SI vs Metric vs US Customary Units of Measurement
People often use “metric” and “SI” as if they mean the same thing. They overlap; however, they are not identical. The metric system is the older family of decimal units, while the SI is the precisely defined, internationally agreed core of it. Meanwhile, US customary units are a separate system, although US law defines them from SI values.
| Feature | SI | Wider metric family | US customary |
|---|---|---|---|
| Length | meter | meter, plus older units like the angstrom | inch, foot, yard, mile (1 in = 2.54 cm exactly) |
| Mass | kilogram | kilogram, gram, metric ton | pound (1 lb = 0.45359237 kg exactly) |
| Volume | cubic meter | liter (1 L = 1 dm^3) | gallon (1 gal = 3.785411784 L exactly) |
| Temperature | kelvin (and °C) | degree Celsius | degree Fahrenheit |
| Steps between sizes | Powers of ten | Powers of ten | 12, 3, 1,760, 16 and others |
Because the conversion factors above are exact definitions, a US engineer and a European engineer can always agree on a number. In practice, the trouble only starts when someone mixes the two without converting.
Non-SI Units Scientists Still Use
Some units sit outside the SI, but the SI still accepts them, mostly because they are too convenient to drop. For instance, the liter is the best example: chemists measure solutions in milliliters every day. Similarly, the minute, hour and day stay in use for time, and the astronomical unit, the electronvolt and the dalton stay in use in their own fields.
Other units, however, are simply traditional. For example, a light-year is the distance light travels in one Julian year, about 9.461 x 10^15 m. Likewise, a calorie is 4.184 J, and many weather maps still show pressure in millibars, where 1 mbar equals 100 Pa (1 hectopascal). Even so, when you report results, convert these to SI unless your field has a clear convention.
How to Convert Units of Measurement Step by Step
Most mistakes, after all, happen in the arithmetic, not the physics. So follow the same routine every time:
- Write the value with its unit. First, start with something like 2.5 km, never a bare 2.5.
- Find the exact factor. Next, use a definition such as 1 km = 1,000 m or 1 in = 2.54 cm.
- Set up a ratio that cancels the old unit. Then multiply 2.5 km by (1,000 m / 1 km) so that km cancels out.
- Do the math and keep the new unit. In this case, 2.5 x 1,000 = 2,500 m.
- Check the size makes sense. In general, a smaller unit should give a bigger number, and vice versa.
- Round only at the end. Then match the precision of your original measurement.
The same chain also works for compound units. For instance, to turn 60 mph into meters per second, multiply by 1,609.344 m per mile and divide by 3,600 seconds per hour: the answer is about 26.82 m/s.
Do and Don’t When Writing SI Units
Do
- First, put a space between the number and the symbol: 25 kg.
- Next, write symbols in upright type and without a period.
- Keep one prefix per unit: ng, not mµg.
- Also, convert to base units before plugging values into a formula.
- Finally, label every table column with its unit.
Don’t
- Add an “s” to symbols: write 5 kg, not 5 kgs.
- Write “degrees kelvin”; the unit is just kelvin (K).
- Also, avoid mixing unit names and symbols, such as kilo-m.
- Assume “ton” means the same thing in every country.
- Round too early, such as halfway through a long calculation.
Honest Limits of Units of Measurement
A unit definition is perfect; a measurement never is. The Planck constant fixes the kilogram, yet your kitchen scale still has a tolerance, and your thermometer still drifts. In other words, using SI units does not make a number accurate by itself. Instead, accuracy depends on the instrument, its calibration and the way you take the reading.
Similarly, conversions have limits. Exact factors (such as 2.54 cm per inch) introduce no error, but rounded ones do. For example, if you convert 1 m to 3.28 ft and back, you get 0.99974 m, not 1 m. Also, some units with the same name differ by region: the US ton is 2,000 lb, the British long ton is 2,240 lb, and the metric ton is 1,000 kg. Therefore, always state which one you mean.
When to Call a Calibration Lab
For homework, cooking and DIY projects, a decent instrument and careful conversion are enough. However, some work needs measurements traceable to national standards. For example, this includes pharmaceutical dosing, legal-for-trade scales, aerospace parts and regulated environmental testing. In those cases, then, send instruments to an accredited calibration lab (look for ISO/IEC 17025 accreditation) and keep the certificates on file.
Frequently Asked Questions About Units of Measurement
What are the 7 basic units of measurement in science?
They are the meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance) and candela (luminous intensity).
Why do scientists use SI units instead of US customary units?
SI units are decimal and shared worldwide, so results can be compared and checked anywhere without conversion errors. By contrast, US customary units take their definitions from SI values but use irregular steps.
Is the liter an SI unit?
No. The liter is a non-SI unit that the SI accepts for use alongside it. In other words, one liter equals one cubic decimeter, or 0.001 cubic meters.
What is the difference between base and derived units of measurement?
First, base units are the seven independent units the SI starts from. Derived units, such as the newton or meters per second, then come from multiplying or dividing base units.
Why is the kilogram a base unit instead of the gram?
For historical reasons: the international prototype adopted in 1889 was a kilogram, not a gram, and the kilogram has stayed the base unit for mass ever since. Prefixes still attach to gram, so you write milligram rather than microkilogram.
How many SI prefixes are there?
There are 24. The CGPM added the newest four, ronna, quetta, ronto and quecto, in November 2022, and they extend the range from 10^-30 to 10^30.
Is Celsius or kelvin used in science?
Both. Kelvin is the SI base unit, and gas laws and thermodynamics require it. Celsius is an SI derived unit with the same size degree, so a change of 1 degree Celsius equals a change of 1 K.
When did the SI units of measurement get redefined?
The CGPM voted for the redefinition in November 2018, and it took effect on May 20, 2019. Since then, all seven base units rest on exact constants of nature.
How do I convert metric units quickly?
First, move the decimal point by the difference in powers of ten. For example, kilo to base is three places, so 4.2 km is 4,200 m and 350 mg is 0.35 g.
Does the United States use SI units of measurement?
Yes, in science, medicine, the military and many industries. However, everyday life mostly uses US customary units, and US law defines those in SI terms, such as 1 inch = 2.54 cm.
Units of Measurement in Science: The Bottom Line
To sum up, the SI gives science one language for numbers: seven base units, 22 named derived units and 24 prefixes. Moreover, since 2019 those base units rest on constants of nature, so they will not drift over time. In practice, therefore, you only need the base units, a few derived ones and the common prefixes to handle most coursework and lab work.
So next time you convert, write the unit at every step, use exact factors, and round only at the end. Finally, use the converter above for quick length checks, and see our chemistry and metrology guides when you need to go deeper.