The Avogadro Project's silicon-28 spheres are so round that at Earth's scale their surface would deviate only 3–5 meters

The single-crystal silicon spheres built for the Avogadro Project — each roughly a 94-mm ball of "99.9995 percent silicon-28" massing 1 kg — are among the most precisely spherical objects ever manufactured. NIST renders the achieved roundness with a scale analogy: "if they were blown up to the size of Earth, the distance between the tallest mountain and deepest ocean would be just 3–5 meters." At true planetary scale, Earth's actual mountain-to-trench range is on the order of 20 kilometers, so the analogy dramatizes a departure from a perfect sphere thousands of times smaller than the planet's own.

The extreme roundness was not decorative but load-bearing for the measurement: to count atoms in the crystal and back out the Avogadro constant, the sphere's volume must be known with matching precision, which requires its diameter to be uniform to nanometer tolerances in every direction. This physical fact is the consequence of the metrology program described at claim-avogadro-silicon-28-spheres-realized-2019-kilogram-redefinition, and the final nanometer-scale figuring that got the spheres this round is the subject of claim-leistner-atomic-feeling-hand-polished-avogadro-spheres.