{"id":694,"date":"2026-07-31T23:54:15","date_gmt":"2026-07-31T23:54:15","guid":{"rendered":"https:\/\/physicsfundamentalsinfo.com\/blog\/?p=694"},"modified":"2026-07-31T23:54:17","modified_gmt":"2026-07-31T23:54:17","slug":"si-units-physics","status":"publish","type":"post","link":"https:\/\/physicsfundamentalsinfo.com\/blog\/mechanics\/si-units-physics\/","title":{"rendered":"SI Units of Measurement in Physics"},"content":{"rendered":"\n
Definition<\/div>

\n\nThe SI units physics runs on are seven base units: the second (s), metre (m), kilogram (kg), ampere (A), kelvin (K), mole (mol) and candela (cd). Since 2019 each one is fixed by an exact constant of nature, and every other unit \u2014 newton, joule, volt, pascal \u2014 is built from these seven.\n\n<\/p><\/div>\n\n

In September 1999 a NASA Mars orbiter costing about $125 million vanished into the Martian atmosphere. The hardware worked. The navigation maths worked. One team had supplied thruster impulse data in pound-force seconds while the flight software expected newton seconds.<\/p>\n\n

That is the case for a single agreed system of measurement, made in one very expensive sentence. Physics is never just numbers \u2014 it is numbers welded to units, and the SI is the welding standard the scientific world agreed to share.<\/p>\n\n

What Are SI Units in Physics?<\/h2>\n\n

SI units are the international system of measurement used in physics, built from seven base units and defined by seven exact constants of nature. SI stands for Syst\u00e8me International d’Unit\u00e9s<\/em>. Every measurement in a physics paper, exam paper or engineering drawing can be expressed in it.<\/p>\n\n

Think of it as a language with seven letters. You cannot spell “pressure” without borrowing from mass, length and time \u2014 and once you know how those letters combine, you can read any unit you have never met before.<\/p>\n\n

Two features make the SI different from a random collection of units. It is coherent<\/strong>: multiply and divide base units together and you get the right derived unit with no fudge factor of 60 or 5,280 hiding in the middle. And it is defined<\/strong> rather than stored: the units come from constants of nature, not from objects in a vault.<\/p>\n\n

The 7 SI Base Units and What Each One Measures<\/h2>\n\n

The seven SI base units are the second, metre, kilogram, ampere, kelvin, mole and candela, measuring time, length, mass, electric current, thermodynamic temperature, amount of substance and luminous intensity respectively.<\/p>\n\n

\n\n\n\n\n\n\n\n\n\n\n\n
Base quantity<\/th>\nUnit<\/th>\nSymbol<\/th>\nFixed by<\/th>\nIn plain English<\/th>\n<\/tr>\n<\/thead>\n
Time<\/td>\nsecond<\/td>\ns<\/strong><\/td>\n\u0394\u03bdCs<\/sub> = 9 192 631 770 Hz<\/td>\n9 192 631 770 ticks of a caesium-133 atom<\/td>\n<\/tr>\n
Length<\/td>\nmetre<\/td>\nm<\/strong><\/td>\nc = 299 792 458 m\/s<\/td>\nHow far light travels in 1\/299 792 458 of a second<\/td>\n<\/tr>\n
Mass<\/td>\nkilogram<\/td>\nkg<\/strong><\/td>\nh = 6.626 070 15 \u00d7 10-34<\/sup> J s<\/td>\nFixed by the Planck constant, measured on a Kibble balance<\/td>\n<\/tr>\n
Electric current<\/td>\nampere<\/td>\nA<\/strong><\/td>\ne = 1.602 176 634 \u00d7 10-19<\/sup> C<\/td>\nA set number of elementary charges flowing per second<\/td>\n<\/tr>\n
Thermodynamic temperature<\/td>\nkelvin<\/td>\nK<\/strong><\/td>\nk = 1.380 649 \u00d7 10-23<\/sup> J\/K<\/td>\nA fixed amount of energy per degree of freedom<\/td>\n<\/tr>\n
Amount of substance<\/td>\nmole<\/td>\nmol<\/strong><\/td>\nNA<\/sub> = 6.022 140 76 \u00d7 1023<\/sup> mol-1<\/sup><\/td>\nExactly 6.022 140 76 \u00d7 1023<\/sup> specified entities<\/td>\n<\/tr>\n
Luminous intensity<\/td>\ncandela<\/td>\ncd<\/strong><\/td>\nKcd<\/sub> = 683 lm\/W<\/td>\nBrightness weighted for the human eye at 540 THz<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n\n

Notice what is missing. There is no base unit for force, energy, pressure, voltage or speed \u2014 every one of those is assembled from the seven above. The list is deliberately minimal.<\/p>\n\n

The candela is the odd one out, and students often ask why brightness gets its own base unit. It is the only one weighted by human physiology: the same radiant power looks brighter to your eye at green wavelengths than at deep red, and the candela bakes that response in.<\/p>\n\n

How the 2019 Redefinition Rebuilt the SI on Constants of Nature<\/h2>\n\n

On 20 May 2019 the SI switched from artefacts and recipes to fixed constants, so every unit is now defined by a number that can never drift. Four base units changed definition that day: the kilogram, ampere, kelvin and mole.<\/p>\n\n

Until then, the kilogram was a lump. A platinum-iridium cylinder held near Paris was<\/em> the kilogram by definition, and its official copies around the world slowly disagreed with it by tens of micrograms over a century. Nobody could say which one had changed.<\/p>\n\n

The fix inverts the logic. Instead of measuring the Planck constant in terms of the kilogram, the SI fixes h<\/em> at exactly 6.626 070 15 \u00d7 10-34<\/sup> J s and lets the kilogram fall out of it. A Kibble balance then realises that definition by comparing mechanical power against electrical power.<\/p>\n\n\n\n<\/path><\/marker>\n<\/defs>\n<\/rect>\nSeven constants of nature fix the seven SI base units<\/text>\nNumerical values fixed exactly, with zero uncertainty, since 20 May 2019<\/text>\n\n<\/rect>\n\u0394\u03bdCs<\/tspan> = 9 192 631 770 Hz<\/tspan><\/text>\n<\/line>\n<\/rect>\nsecond s<\/tspan> \u2014 time<\/text>\n\n<\/rect>\nc = 299 792 458 m\/s<\/text>\n<\/line>\n<\/rect>\nmetre m<\/tspan> \u2014 length<\/text>\n\n<\/rect>\nh = 6.626 070 15 \u00d7 10-34<\/tspan> J s<\/tspan><\/text>\n<\/line>\n<\/rect>\nkilogram kg<\/tspan> \u2014 mass<\/text>\n\n<\/rect>\ne = 1.602 176 634 \u00d7 10-19<\/tspan> C<\/tspan><\/text>\n<\/line>\n<\/rect>\nampere A<\/tspan> \u2014 current<\/text>\n\n<\/rect>\nk = 1.380 649 \u00d7 10-23<\/tspan> J\/K<\/tspan><\/text>\n<\/line>\n<\/rect>\nkelvin K<\/tspan> \u2014 temperature<\/text>\n\n<\/rect>\nNA<\/tspan> = 6.022 140 76 \u00d7 10<\/tspan>23<\/tspan> mol<\/tspan>-1<\/tspan><\/text>\n<\/line>\n<\/rect>\nmole mol<\/tspan> \u2014 amount<\/text>\n\n<\/rect>\nKcd<\/tspan> = 683 lm\/W at 540 THz<\/tspan><\/text>\n<\/line>\n<\/rect>\ncandela cd<\/tspan> \u2014 luminous intensity<\/text>\n\nPairings follow the SI Brochure. Realising a unit can chain several constants \u2014 the kilogram also uses c and \u0394\u03bdCs<\/tspan>.<\/text>\n<\/svg>\n\n

Each SI base unit is now defined by fixing the numerical value of one constant of nature.<\/p>\n\n

One practical consequence is easy to miss: the speed of light<\/a> is no longer something we measure. It is a defined number, so measuring “the speed of light” now really means calibrating your metre.<\/p>\n\n

\n\n \"Kibble\n\n
A Kibble balance realises the kilogram by balancing mechanical power against electrical power, linking mass directly to the Planck constant.<\/figcaption>\n\n<\/figure>\n\n

What is still changing<\/h3>\n\n

The second is next in line. Optical clocks now beat caesium clocks by orders of magnitude in stability, and the international timekeeping community has been working towards a new definition based on an optical transition, with roughly 2030 as the working target. Nothing you calculate today changes \u2014 the numerical value of the second is designed to carry over.<\/p>\n\n

SI Derived Units: How Every Other Unit Is Built<\/h2>\n\n

A derived unit is any SI unit made by multiplying or dividing base units together, and 22 of them are important enough to have their own name and symbol. Force, energy and power are the three that matter most in an introductory course.<\/p>\n\n

1 N = 1 kg \u00b7 m \/ s\u00b2<\/div>\n\n