Where the definitions come from

Cited, not fetched.

Unitii's sibling apps download datasets and refuse to fabricate a missing number. Unit definitions are different: they aren't a dataset, they're authoritative texts. So this app's honesty mechanism is the citation — every page is an authored summary with its source one click away.

The authorities

BIPM (the International Bureau of Weights and Measures) maintains the SI itself; NIST maintains the US guidance and the customary-unit conversions; CODATA publishes the recommended values of measured constants. 32 units on this map, each tagged with the source its page summarizes.

The rules this app keeps

• Every factor states its status. Conversion factors marked "exact" are exact by definition (an inch is 25.4 mm, period). The one measured constant shown — the atomic mass unit's kilogram value — is labeled measured, because it carries uncertainty and pretending otherwise would be fabrication.
• No fake conversions. The decibel gets an explanation instead of a converter, because a logarithmic ratio with a hidden reference cannot honestly sit in one.
• Definitions are exact; measurements are not. The 2019 SI pins definitions to fixed constants, but realizing a unit in a lab still carries uncertainty — the deep-dive tab says so instead of blurring the two.
• Authored means owned. The plain-language summaries are ours; any error in them is ours, not the sources' — that's why every page links straight to the text it summarizes.