The next revision to the International System of Units will emphasize the relationship between the base units (kilogram, metre, second, ampere, kelvin, candela and mole) and fundamental constants of nature (the speed of light, c, the Planck constant, h, the elementary charge, e, the Boltzmann constant, kB, the Avogadro constant, NA, etc). The redefinition cannot proceed without consistency between two complementary metrological approaches to measuring h: a ‘physics’ approach, using watt balances and the equivalence principle between electrical and mechanical force, and a ‘chemistry’ approach that can be viewed as determining the mass of a single atom of silicon. We report the first high precision physics and chemistry results that agree within 12 parts per billion: h (watt balance) = 6.626 070 63(43) × 10−34 J s and h(silicon) = 6.626 070 55(21) × 10−34 J s. When combined with values determined by other metrology laboratories, this work helps to constrain our knowledge of h to 20 parts per billion, moving us closer to a redefinition of the metric system used around the world.

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