Everything you should know about derived units of measurement!

Do you remember the article on measurement unit systems? As promised, we’re back with another piece on the basics of physics. In our previous post, we discussed physical quantities, measurement systems, the origin of the International System of Units (SI), and the fundamental units of measurement. Today, we’re diving into the topic of derived units of measurement within the SI system — not only those related to electronics. Let’s get started!

Derived SI units of measurement used in electronics and electrical engineering

Derived SI units of mesurement are created by combining fundamental units. In electronics and electrical engineering, the following units hold particular significance:

1. Coulomb (C) – the unit of electric charge (q), essential in the context of capacitor charging and determining battery capacity.
2. Watt (W) – the unit of power (P), representing the rate of energy transfer (1 W = 1 joule per second). In electronics, power (in watts) determines how much energy a device consumes or delivers — for example, how much heat a processor generates.
3. Volt (V) – the unit of electric voltage (U), a fundamental quantity in electronics that plays a crucial role in regulating stabilizers and converters.
4. Ohm (Ω) – the unit of electrical resistance (R), known as resistance. Excessively high resistance can cause electronic connections to overheat.

Some people confuse resistance (R) with resistivity (ρ) — two entirely different concepts, although related. The unit of resistivity (ρ) is the ohm-meter (Ω⋅m). If you’d like to learn more about this physical quantity, check out the blog post Resistivity — what is it and why is it important?

5. Farad (F) – the unit of capacitance (C), representing the ability to store electric charge. It plays a key role in analog circuits, especially in signal filtering.
6. Henry (H) – the unit of inductance (L), influencing the characteristics of circuits containing coils (such as switching power supply coils and low-pass filters).
7. Hertz (Hz) – the unit of frequency (f), essential in radio and digital technology, defining the rate of signal changes.
8. Siemens (S) – the unit of conductance (σ), known as conductivity, which is the inverse of resistivity. It helps analyze the efficiency of conductors.
9. Weber (Wb) or volt-second (Vs) – the unit of magnetic flux (Φ), closely related to tesla and the magnetic field. It is used in transformer and motor design.
10. Tesla (T) – the unit of magnetic induction (B), also applied in measuring magnets and electromagnets.

Other SI derived units of measurement

Several other important derived units of measurement include:

• Meter per second (m/s) – the unit of velocity (v), used in analyzing electron flow or active cooling.
• Meter per second squared (m/s²) – the unit of acceleration (a), applied in motion sensors and accelerometers.
• Cubic meter (m³) – the unit of volume (V), relevant when selecting hermetic enclosures or thermal insulation materials.
• Kilogram per cubic meter (kg/m³) – the unit of density (d, ρ), significant in mechanical and thermal design.
• Newton (N) – the unit of force (F).
• Pascal (Pa) – the unit of pressure (p), used in hermetisitation sealing systems or liquid cooling applications.
• Joule (J) – the unit of energy (E), commonly used in power calculations.

Derived units of measurement with special names and symbols

It’s worth mentioning two lesser-known units of measurement — previously classified as supplementary units but now considered derived units of measurement with special names and symbols. These units correspond to specific geometric phenomena:

• Radian (rad) – the unit of plane angle measurement.
• Steradian (sr) – the unit of solid angle measurement.

These measurement units are commonly used in photometry, LED optics design, and light emission analysis.

Non-standard SI units of measurement

By now, you are familiar with the basic classification of physical quantity units in the SI system. However, this classification can be further refined. Units in the International System of Units (SI) can be divided into the following categories:

• Coherent units – erived units whose definitions equal exactly 1, such as the newton (N).
• Principal units – all base units and some derived units of measurement without prefixes, directly resulting from defining equations (they are not multiples).
• Secondary (multiple and submultiple) units – auxiliary measurement units referring to the same physical quantity as the principal unit (g., 1 kilogram (kg)). These units indicate a much larger or smaller value by adding a prefix to the name of the principal unit. Example: kilometer (km) — a unit of length — equals 1,000 meters (m).

You can learn more about scaling principles and prefix names from the relevant unit system.

Non-SI units of measurement

We can also distinguish non-SI units of measurement, which are not defined within the SI system but are more intuitive and widely accepted in specific regions. These include:

• units used in engineering practice (g., revolutions per second – rev/s),
• units used in trade, economy, and commerce (g., piece, item, package, crate),
• currency units (g., euro, pound, dollar),
• currency conversion rates (g., złoty/euro).

Many non-SI units of measurement have been established based on long-standing practical use — they represent unwritten metric conventions accepted in most countries worldwide.
Example: a matchbox may contain 24 to 60 matches (depending on the package size), while a crate of matches holds 1,000 bundles of ten matchboxes.

Legally recognized non-SI units of measurement

Among non-SI units of measurement, we can also identify legally recognized non-SI units of measurement — those justified by tradition or convenience.

A great example is temperature measurement. In the SI system, the official unit of temperature is the kelvin (K). However, it is not commonly used in everyday applications. Instead, alternative units of measurement, such as degrees Celsius (°C) and degrees Fahrenheit (°F), are widely accepted.

What non-SI units are used in Poland?

The most commonly used non-SI units of measurement in Poland include:

• degree Celsius (°C),
• calorie (1 cal = 4.1868 J),
• kilowatt-hour (kWh) (1 kWh = 3,600,000 J),
• metric horsepower (KM) (1 KM = 735.49875 W),
• atmosphere (at) (1 at = 98,066.5 Pa),
• millimeter of mercury (mmHg) (1 mmHg = 133.3224 Pa),
• bit (b) – a unit of measurement that can take only the values 0 (zero) or 1 (one),
• byte (B) – consists of 8 bits,
• kilobyte (kB), megabyte (MB), gigabyte (GB), terabyte (TB) – derived units of measurement from bytes, where 1 kB = 1,024 B.

Calories, kilowatt-hours, metric horsepower, atmosphere, and millimeters of mercury are widely used in industry, classical physics, and energy sectors. In contrast, bit, byte, kilobyte, megabyte, gigabyte, and terabyte are prevalent in computer science, particularly for memory and bandwidth calculations.

Non-SI units of measurement in other countries – english-speaking countries

Compared to the International System of Units (SI), english (imperial) units of measurement stand out significantly:

• 1 foot = 12 inches (48 cm),
• 1 inch = 1/12 of a foot (54 cm),
• 1 yard = 3 feet (9144 m),
• 1 mile = *609 km.

Imperial units of measurement are widely used in the United Kingdom and the United States.

Conclusion

Understanding units of measurement and knowing how to apply them in practice is fundamental for anyone working with electronics and electrical engineering. The most important system to learn is the SI system, a reliable, internationally recognized framework that allows scientists and engineers worldwide to communicate and compare measurement results effectively. It ensures that discussions about power supply design, electrical resistance measurements, or thermoconductivity testing all happen in a universal language, no matter the country.