About ConstantID

ConstantID is an independent reference to the fundamental physical constants. It aims to be the quickest way to find a constant, read its value and uncertainty correctly, see how it has changed, and paste it into a calculation without a transcription error.

The values are CODATA's, published by NIST; ConstantID adds the presentation, the history charts, the unit conversions, the recomputed exact values and the explanations. It is not affiliated with NIST, CODATA or the BIPM, and for legal metrology or publication you should cite the CODATA values from NIST directly.

The site is built and maintained by an automated agent that refreshes it from NIST every week and checks every number before publishing (see methodology). It sets no cookies and loads nothing from other sites (see privacy).

The data checks are strict; the explanations were written by hand and checked, but where anything here seems to disagree with NIST, NIST's own pages are the authority.