SI Units and Measurement Notes for Competitive Exams

Measurement is the foundation of science. To measure anything, such as the length of a road, the mass of a bag of rice or the time taken by a train, we compare it with a fixed standard called a unit. Different regions once used different units, which caused confusion in trade and science. To solve this, the world adopted the International System of Units, known as SI (from the French "Système International d'Unités"), in 1960. Questions on SI units, derived units, units named after scientists and conversions are among the most common in the general science sections of SSC CGL, CHSL, MTS, railway (RRB NTPC, Group D), defence and state PCS exams. In UPSC Prelims, units appear in physics and current affairs questions, for example on the redefinition of the kilogram. This note explains the system clearly with tables and simple examples.

Quick Facts at a Glance

• SI adopted: 1960, by the General Conference on Weights and Measures (CGPM).
• Number of base units: 7.
• Since 20 May 2019, all seven base units are defined using fixed values of fundamental constants of nature (for example, the kilogram is defined through the Planck constant).
• India's custodian of standards: CSIR-National Physical Laboratory (NPL), New Delhi, which maintains Indian Standard Time.
• World Metrology Day: 20 May.

1. The Seven SI Base Units

Memory trick: "L-M-T-C-T-A-L" (Length, Mass, Time, Current, Temperature, Amount, Luminous intensity). A sentence like "Little Monkeys Take Chocolate To Aunt Lily" helps.

Earlier, the radian (plane angle) and steradian (solid angle) were called supplementary units. They are now treated as dimensionless derived units.

2. Important Derived Units

Derived units are formed by combining base units. For example, speed is distance divided by time, so its unit is metre per second (m/s).

Units Named After Scientists

Many units honour scientists: newton (Isaac Newton), joule (James Prescott Joule), watt (James Watt), pascal (Blaise Pascal), hertz (Heinrich Hertz), ampere (André-Marie Ampère), volt (Alessandro Volta), ohm (Georg Ohm), coulomb (Charles-Augustin de Coulomb), kelvin (Lord Kelvin), tesla (Nikola Tesla), farad (Michael Faraday), becquerel (Henri Becquerel) and siemens (Werner von Siemens, unit of conductance). Rule: When written in full, unit names are in lowercase (newton), but symbols named after people start with a capital letter (N).

3. SI Prefixes

In 2022, four new prefixes were added: ronna (10²⁷), quetta (10³⁰), ronto (10⁻²⁷) and quecto (10⁻³⁰), to describe very large data volumes and very small quantities.

4. Important Non-SI and Special Units

Note: A light year is a unit of distance, not time, a common trap in exams.

5. Fundamental and Derived Quantities Explained

Physical quantities are divided into two groups. Fundamental (base) quantities are independent; they cannot be expressed in terms of other quantities. Length, mass and time are the most familiar examples. Derived quantities are obtained by combining fundamental quantities through formulas. Area is length × length (m²), volume is length × length × length (m³), density is mass ÷ volume (kg/m³), and acceleration is change in velocity ÷ time (m/s²). Understanding this relationship lets you work out the unit of almost any quantity from its formula, which is a frequently tested skill.

Example: Momentum = mass × velocity, so its unit is kg·m/s. Work = force × distance, so its unit is N·m, which is the joule. Pressure = force ÷ area, so its unit is N/m², which is the pascal.

6. Temperature Scales and Conversion

Three temperature scales are commonly asked:

• Celsius (°C): Water freezes at 0 °C and boils at 100 °C at normal atmospheric pressure.
• Fahrenheit (°F): Water freezes at 32 °F and boils at 212 °F.
• Kelvin (K): The SI scale; 0 K is absolute zero, the lowest possible temperature (−273.15 °C).

Conversion formulas: K = °C + 273.15 (often rounded to 273); °F = (9/5 × °C) + 32. At −40, the Celsius and Fahrenheit scales show the same reading. Normal human body temperature is about 37 °C, which is 98.6 °F or about 310 K.

7. Measuring Instruments

8. Why Standard Units Matter

Standard units make trade fair, science reliable and engineering safe. A famous example of what goes wrong without them is NASA's Mars Climate Orbiter, lost in 1999 partly because one team used imperial units while another used metric units. In India, the Legal Metrology Act, 2009 ensures that weights and measures used in trade are accurate, protecting consumers from short weighing. CSIR-NPL maintains national standards and disseminates Indian Standard Time, which is important for banking, telecom and navigation systems.

9. Rules for Writing SI Units Correctly

• Unit names are written in lowercase, even when named after a person: newton, pascal, kelvin (but "degree Celsius" keeps a capital C).
• Symbols of units named after people begin with a capital letter: N, Pa, W, J, Hz. Other symbols are lowercase: m, kg, s, mol, cd.
• Symbols do not take a plural "s": write 10 kg, not 10 kgs.
• Symbols are not abbreviations, so do not put a full stop after them in the middle of a sentence: write "a 5 m rope", not "a 5 m. rope".
• Leave a space between the number and the symbol: 25 kg, 100 W (an exception is the degree sign for angles).
• Use only one prefix at a time: write nanometre (nm), not millimicrometre.

10. Practice Questions

1. What is the SI unit of electric charge? Answer: Coulomb.
2. Which unit measures the frequency of a wave? Answer: Hertz.
3. What is the SI unit of magnetic flux density? Answer: Tesla.
4. How many metres make one nautical mile? Answer: 1,852 metres.
5. Which instrument measures the purity of milk? Answer: Lactometer.
6. At what temperature are Celsius and Fahrenheit readings equal? Answer: −40.

11. Quick Revision Strategy

First, memorise the seven base units with the "Little Monkeys" trick. Next, learn derived units in pairs with their formulas, such as force (newton) with F = ma and power (watt) with P = W/t, so that you can rebuild them if you forget. Then learn the prefixes from tera to pico, since conversion questions use them often. Finally, revise the special units like light year, nautical mile, horsepower and kilowatt-hour, which are favourite exam traps. Spending fifteen minutes on each of these four steps is enough to master the topic.

Remember: Every derived unit can be traced back to the seven base units, so if you understand the formula, you can always work out the unit.

Related Notes

• Physics Basics: Motion, Force, Gravitation and Light
• Everyday Science: Heat, Sound and Electricity
• Atomic Structure, Carbon Compounds and Fuels
• All General Science Notes

Frequently Asked Questions

How many base units are there in the SI system?

Seven: metre, kilogram, second, ampere, kelvin, mole and candela.

What is the SI unit of force?

The newton (N), equal to kg·m/s².

What is the SI unit of work and energy?

The joule (J).

What is the SI unit of power?

The watt (W), equal to one joule per second.

What is the SI unit of pressure?

The pascal (Pa), equal to one newton per square metre.

What is the SI unit of temperature?

The kelvin (K).

Is a light year a unit of time?

No. It is a unit of distance, the distance light travels in one year.

How much is one horsepower in watts?

About 746 watts.

What does one "unit" of electricity mean?

One kilowatt-hour, equal to 3.6 million joules.

What is the SI unit of luminous intensity?

The candela (cd).

When was the kilogram redefined?

On 20 May 2019, using the Planck constant instead of a physical metal prototype.

What is a nautical mile?

A unit of distance used at sea and in aviation, equal to 1,852 metres.