The kilogram is no longer defined by a metal cylinder.

The modern mass standard is established by fixing the numerical value of the Planck constant.

The fixed value is exactly 6.62607015 × 10⁻³⁴ joule seconds.

This makes the mass unit reproducible through measurements tied to constants rather than one artifact.

For more than a century, the international prototype kilogram had an unusual status: its mass was the kilogram by definition. The platinum-iridium cylinder was preserved at the International Bureau of Weights and Measures near Paris, while national copies carried the standard into other countries. Periodic comparisons showed small differences between artifacts. Such comparisons could reveal that the family of masses was changing relative to itself, but could not establish an independent absolute history for the object that defined the unit.

The replacement took effect on 20 May 2019 as part of a broader revision of the SI. Instead of assigning authority to one cylinder, the system fixed a numerical value for the Planck constant, which connects quantum energy with frequency. The chosen value maintained continuity with the old kilogram within measurement uncertainty. Existing one-kilogram weights remained useful. What changed was the ultimate reference behind their calibration, not the everyday meaning of a kilogram of flour or a person's recorded mass.

The units of the Planck constant explain how this can define mass. A joule second can also be written as kilogram meter squared per second. Once the meter and second are already anchored to other constants, fixing the Planck constant determines the kilogram. This is a relationship among units, not a claim that mass and energy have become interchangeable in every practical measurement. Laboratories still need an experimental method for translating the defining relationship into a mass they can compare with a physical object.

A Kibble balance does that using two complementary modes. In one, an electrical force balances a weight; in another, moving a coil through a magnetic field generates a measurable voltage. Combining the results eliminates a difficult geometric factor and connects mechanical quantities with electrical measurements. Quantum electrical standards make those measurements depend on fundamental constants. Local gravitational acceleration, coil velocity, alignment, and environmental effects still need careful evaluation. “Comparing power” describes the mathematical combination of the two modes, not necessarily simultaneous mechanical and electrical energy delivery.

A different realization route uses highly characterized silicon spheres and measurements related to their crystal structure and atomic content. Having more than one method is valuable because independent techniques expose different systematic errors. National laboratories can realize the standard and pass it down through calibrated masses to industrial balances and shop scales. Most users will never operate either primary instrument. They benefit from a reference that does not depend on the continued condition of a single irreplaceable artifact and can improve as experimental methods become more precise.

Mass should not be confused with weight: weight is a force that depends on the local gravitational field. A calibrated balance must handle that distinction appropriately.

Replacing the prototype did not make ordinary masses suddenly heavier or lighter. The redefinition was designed to preserve continuity while removing dependence on one stored object. It changed the foundation of the unit, not the intended mass of goods already being measured in kilograms.