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The first chapter of Class 11 physics opens with a table of SI base units whose definitions look intimidating: fixed numerical values of constants with nine or ten significant figures. Many students skip it. That is a mistake, not because you must memorise the numbers, but because the ideas behind them explain a lot of physics and chemistry you will use later. This post explains the seven units, what changed in 2019, and some calculations the new definitions make easy.
What are the seven SI base units?
The BIPM's list of SI base units gives:
second (s), for time
metre (m), for length
kilogram (kg), for mass
ampere (A), for electric current
kelvin (K), for thermodynamic temperature
mole (mol), for amount of substance
candela (cd), for luminous intensity
All other SI units are derived from these by multiplying powers of the base units. For example, the newton is kg m s⁻² and the joule is kg m² s⁻². Our post on dimensional analysis shows how to use these combinations to check equations.
This is Chapter 1, Units and Measurements, in the CBSE Physics curriculum for 2026-27, and Unit 1 of the JEE (Main) 2026 syllabus.
What changed in 2019?
At its 26th meeting in November 2018, the General Conference on Weights and Measures adopted Resolution 1, which decided that "effective from 20 May 2019" the SI is defined by fixing the values of seven constants:
the caesium-133 hyperfine transition frequency, ΔνCs = 9 192 631 770 Hz
the speed of light in vacuum, c = 299 792 458 m/s
the Planck constant, h = 6.626 070 15 × 10⁻³⁴ J s
the elementary charge, e = 1.602 176 634 × 10⁻¹⁹ C
the Boltzmann constant, k = 1.380 649 × 10⁻²³ J/K
the Avogadro constant, NA = 6.022 140 76 × 10²³ mol⁻¹
the luminous efficacy of 540 × 10¹² Hz radiation, Kcd = 683 lm/W
The BIPM notes that in this revision the definitions of four base units changed: the kilogram, the ampere, the kelvin and the mole. The second, metre and candela were already tied to constants.
How is each changed unit now defined?
Kilogram: fixed by h. Since h has units kg m² s⁻¹, and the metre and second are fixed by c and ΔνCs, fixing h fixes the kilogram. The BIPM kilogram page explains this relation.
Ampere: fixed by e. One ampere is a flow of 1 / e elementary charges per second.
Kelvin: fixed by k, which links temperature to energy.
Mole: one mole contains exactly 6.022 140 76 × 10²³ entities. It is no longer defined through 12 g of carbon-12.
The NCERT Class 11 Physics chapter on Units and Measurement gives these definitions in Table 1.1 and adds a reassuring footnote: the values "need not be remembered or asked in a test". What you should understand is the idea that each unit is now tied to a constant of nature.
What used to be exact and is now measured?
Resolution 1 also notes that some quantities that were exact by definition became measured values from 20 May 2019:
the mass of the international prototype, now 1 kg only within an uncertainty;
the magnetic constant μ₀, still very close to 4π × 10⁻⁷ H/m but now determined experimentally;
the triple point of water, very close to 273.16 K;
the molar mass of carbon-12, very close to 0.012 kg/mol.
This surprises students who learned μ₀ = 4π × 10⁻⁷ as exact. For exam calculations, the value you use does not change.
Worked numbers from the fixed constants
Because these constants are now exact, some useful values follow directly. Here are four, each a one-line calculation.
Electrons per second in 1 A: 1 / e = 1 / (1.602 176 634 × 10⁻¹⁹) ≈ 6.24 × 10¹⁸ electrons per second.
Gas constant: R = NA × k = 6.022 140 76 × 10²³ × 1.380 649 × 10⁻²³ ≈ 8.314 J mol⁻¹ K⁻¹. Since both factors are exact, R is now exact too.
Faraday constant: F = NA × e ≈ 6.022 × 10²³ × 1.602 × 10⁻¹⁹ ≈ 96 485 C mol⁻¹, the value you use in electrolysis.
Thermal energy at room temperature: kT at 300 K = 1.380 649 × 10⁻²³ × 300 ≈ 4.14 × 10⁻²¹ J. Dividing by e gives about 0.0259 eV.
These connect Chapter 1 of physics to kinetic theory, electrochemistry and semiconductors.
Which mistakes do students make?
Saying the kilogram is defined by a prototype object. That definition ended in 2019.
Listing the newton or the joule as base units. They are derived.
Writing unit symbols wrongly. The BIPM list uses kg with a lower-case k, K for the kelvin and s for the second.
Confusing the mole with a mass. It is a count of entities.
For more on writing measured values correctly, see our post on significant figures and units.
A practice routine
Write the seven base units and their quantities from memory.
For each changed unit, write one sentence saying which constant fixes it.
Recalculate R, F and 1 / e yourself from the constants above.
Express five derived units (newton, joule, watt, pascal, coulomb) in base units.
When an idea such as "fixing h fixes the kilogram" feels abstract, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can read up on the constant itself.
Tonight, calculate R from NA and k and confirm you get 8.314 J mol⁻¹ K⁻¹.
