The answer first, because it is the one people actually want: nothing you weigh, measure, or cook changed value. Your kitchen scale is not wrong. A kilogram of flour is the same kilogram it always was, and every conversion factor you have ever memorised still holds.
What changed is where the units come from. The metric system used to be anchored partly to physical objects and laboratory recipes; it is now anchored entirely to fixed numerical values of constants of nature. That sounds like a physics-department detail, and for a supermarket scale it is. But it quietly settled a question that comes up in conversion work constantly: which factors are exact by definition, and which are measurements that could shift? That answer did move, and in a direction worth knowing.
The old problem: a unit made of metal
For most of the metric system's life, the kilogram was defined by an object — a single cylinder of platinum-iridium kept under glass, with official copies distributed to national laboratories. The kilogram was, by definition, the mass of that cylinder. Whatever it weighed was correct.
That is a neat trick and a serious flaw. An object can be scratched, contaminated, or cleaned. It can gain or lose a trace of surface material over decades. And because the object was the definition, there was no higher authority to check it against. If it drifted, the kilogram drifted with it, and every mass measurement in the world drifted along quietly.
The metre had already escaped this trap. It went from a metal bar, to a count of wavelengths of a particular light, to its present form: the distance light travels in vacuum in a fixed fraction of a second. The second went the same way, defined by a fixed frequency of a caesium atom's hyperfine transition. Once you fix a constant, anyone with the right equipment can rebuild the unit from scratch — in any laboratory, on any continent, forever.
The redefinition extended that idea to everything else.
The new system in one sentence
Fix the number, derive the unit. Rather than describing the kilogram and then measuring the Planck constant, the SI now fixes the Planck constant at an exact value with no uncertainty and lets the kilogram follow from it. The same move was made for the rest of the base units.
| Unit | Now defined by fixing… | Fixed value |
|---|---|---|
| second | caesium-133 hyperfine frequency | 9 192 631 770 Hz |
| metre | speed of light in vacuum | 299 792 458 m/s |
| kilogram | Planck constant | 6.626 070 15 × 10⁻³⁴ J s |
| ampere | elementary charge | 1.602 176 634 × 10⁻¹⁹ C |
| kelvin | Boltzmann constant | 1.380 649 × 10⁻²³ J/K |
| mole | Avogadro constant | 6.022 140 76 × 10²³ mol⁻¹ |
Those values are not rounded. They are exact by decree, chosen so that the new definitions match the old units as closely as measurement allowed at the moment of the switch — which is why nothing you own needed recalibrating.
What genuinely changed: the direction of "exact"
Here is the part that matters for conversion work. Constants and units trade places on the exact/measured ledger, and the redefinition swapped several of them.
Now exact, with zero uncertainty: the Planck constant, the elementary charge, the Boltzmann constant, the Avogadro constant — plus the speed of light and the caesium frequency, which already were.
Now measured, where they used to be exact: a few quantities that the old definitions had pinned. The magnetic constant of vacuum used to be exactly 4π × 10⁻⁷ in SI units, because the old ampere was defined by the force between two idealised wires; it is now an experimentally determined value. Similarly, the molar mass of carbon-12 is no longer exactly 12 g/mol by definition, because the mole no longer refers to a mass of carbon. Both are still known far beyond any precision an everyday conversion needs — but "exact" and "extremely well measured" are different statements, and the labels moved.
Unaffected entirely: every imperial-to-metric factor you use. That surprises people, but the reason is simple. Those factors are defined against the metre and the kilogram, not against any particular realisation of them. An international agreement fixed the yard and the pound in metric terms, and those definitions did not change:
| Conversion | Status |
|---|---|
| 1 in = 2.54 cm | Exact by definition |
| 1 ft = 0.3048 m | Exact by definition |
| 1 mi = 1.609344 km | Exact by definition |
| 1 lb = 0.45359237 kg | Exact by definition |
| 1 L = 1 dm³ | Exact by definition |
| 0 °C = 273.15 K | Exact by definition |
| °F = °C × 9/5 + 32 | Exact by definition |
The pillar guide on unit conversions leans on that table constantly, and the redefinition left every row of it standing.
The trap inside "exact": exact ratios still print rounded
An exact definition does not mean a tidy decimal in the other direction. Because 1 lb = 0.45359237 kg exactly, the reverse factor is 1 ÷ 0.45359237 = 2.204622621… kilograms to pounds, which never terminates. So:
- 68 kg × 2.20462262 = 149.91 lb, rounded to 2 decimal places.
- 150 lb × 0.45359237 = 68.04 kg, rounded to 2 decimal places.
Both directions are exact as fractions and rounded as printed numbers. When a converter shows you eight decimal places on a value you typed to two, it is inventing precision the input never had. Round to what the measurement justifies, and say what you rounded.
The other standards shift, in a different corner
Constants are not the only place where a definition quietly got tightened. Data sizes went through their own version of this: a kilobyte meant 1 000 bytes to storage manufacturers and 1 024 bytes to operating systems, until a separate standards body gave the binary sizes their own names and symbols — kibibyte, mebibyte, gibibyte, written KiB, MiB, GiB. That is why a drive sold as 500 GB shows up as roughly 465 GiB, and the full order of data sizes walks through the arithmetic.
The pattern is the same in both cases: an ambiguity that everyone tolerated for years got resolved by making the definition explicit rather than by changing any quantity. Nothing got bigger or smaller. The naming just stopped lying.
What this actually means for you
If you are converting for a recipe, a workshop, or a spreadsheet: nothing. Carry on. The factors in converting cups to grams and everywhere else are unchanged.
If you care whether a factor is trustworthy: you can now check its status, and the answer is usually clean. Metric-to-metric relationships are exact by construction. The main imperial-to-metric factors are exact by agreement. Anything involving a physical property — the density of water, the mass of an atom, gravity at your location — is measured, and measured values carry uncertainty and context.
If you are teaching or writing about this: the useful sentence is that the SI stopped defining units by things and started defining them by numbers. Things drift and can be lost. Numbers do not.
FAQ
Did the kilogram get heavier or lighter? No. The new definition was set to agree with the old one to within the best measurement available at the time, so no scale, weight, or product specification changed.
Do I need to recalibrate anything? No. Calibration chains changed at the top — national laboratories realise the kilogram with instruments rather than by comparing to a stored object — but nothing downstream shifted.
Is 1 inch still exactly 2.54 cm? Yes, and it always will be. The inch is defined in terms of the metre by international agreement, so it inherits whatever the metre is, exactly.
Was the Celsius scale redefined too? Indirectly. The kelvin is now fixed by the Boltzmann constant, and the degree Celsius is defined off the kelvin with an exact offset of 273.15. The size of a degree did not change.
Why bother, if nothing everyday changed? Because the old definitions had a ceiling. An object-based kilogram is hard to use for very small or very large masses, and a definition you cannot check is a definition you have to trust. Constants can be realised anywhere, at any scale, indefinitely.
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