How ConstantID is built and checked

Every value on this site is parsed from NIST's published files or computed from them when the site is built. Nothing is typed in by hand, and the build refuses to publish if any of the checks below fails.

Source and licence

The values are the CODATA recommended values of the fundamental physical constants, adjusted every four years by the CODATA Task Group on Fundamental Constants and published by the US National Institute of Standards and Technology (NIST) as Standard Reference Database 121 at physics.nist.gov/constants. ConstantID reads NIST's plain-text listings: the current table (2022) and the archived adjustments of 1998, 2002, 2006, 2010, 2014, 2018. NIST's group assignments (universal, electromagnetic and so on) come from its category listings.

Terms. NIST's site policy is that information it presents which is not marked as copyrighted is public information and may be distributed or copied; the constants pages and the text files carry no copyright notice. The Standard Reference Data Act allows NIST to secure copyright in standard reference data, and where NIST does so for data outside the United States it grants a worldwide, royalty-free right to publish and distribute it, asking for acknowledgement of NIST as the source and a note of any changes. The values themselves are scientific results published for international use. ConstantID credits NIST and CODATA on every page and notes its changes: values are re-spaced and written in scientific notation, older values are converted into the current unit where NIST changed it, and the long exact values and unit conversions are ConstantID's own calculations.

Fetched 2026-10-31T16:16:35-04:00; the weekly refresh checks whether NIST has published a newer adjustment, and if so the site switches to it automatically, keeping the older ones for the history charts.

Portraits come from Wikimedia Commons, chosen by eye and limited to public-domain, CC BY and CC BY-SA files; the author and licence are shown under each one. Diagrams and charts are drawn by ConstantID. Explanations of the main constants were written for this site from the SI Brochure (9th edition) and the CODATA papers and checked; numbers inside them are computed from the data.

How to read the values

The digits in parentheses are the standard uncertainty (one standard deviation) in the last digits of the value, as CODATA writes it. The relative uncertainty is the standard uncertainty divided by the value. “Exact” means the value is fixed by definition. Digits are grouped in threes with thin spaces; a trailing “…” marks an exact value that NIST cut off, not rounded.

Check 1: parsed values reproduce NIST's files

For every line of every file, ConstantID rebuilds NIST's text from the numbers it parsed and compares it with the original. From 2010 on the comparison is character for character, including NIST's digit spacing; the 1998–2006 files space digits inconsistently, so there the digits, decimal point and exponent are compared.

Check 2: exact constants are exact

The build stops unless the seven SI defining constants are flagged exact in NIST's file, and unless the quantities that became measured in 2019 (μ₀, ε₀, the molar mass constant) and other measured constants such as G and α are not. Then it recomputes every derived exact constant from the defining ones, to 60 digits, and compares with NIST's printed digits. NIST truncates rather than rounds; all 62 truncated values match the truncation of the recomputed value.

Check 3: unit conversions against published values

ConstantID's unit table and conversion code are checked against values published independently of it: NIST Special Publication 811, unit definitions, the IAU, and NIST's own entries for constants that it lists in more than one unit.

Published values given to 7 significant figures (SP 811) agree within that rounding; exact definitions agree exactly.

History across adjustments

NIST renamed some constants over the years. ConstantID follows these renames to join the history of one quantity:

• Planck constant over 2 pi → reduced Planck constant
• Planck constant over 2 pi in eV s → reduced Planck constant in eV s
• Planck constant over 2 pi times c in MeV fm → reduced Planck constant times c in MeV fm
• Planck constant in eV s → Planck constant in eV/Hz
• Compton wavelength over 2 pi → reduced Compton wavelength
• muon Compton wavelength over 2 pi → reduced muon Compton wavelength
• neutron Compton wavelength over 2 pi → reduced neutron Compton wavelength
• proton Compton wavelength over 2 pi → reduced proton Compton wavelength
• tau Compton wavelength over 2 pi → reduced tau Compton wavelength
• electron gyromagnetic ratio over 2 pi → electron gyromagnetic ratio in MHz/T
• neutron gyromagnetic ratio over 2 pi → neutron gyromagnetic ratio in MHz/T
• proton gyromagnetic ratio over 2 pi → proton gyromagnetic ratio in MHz/T
• shielded helion gyromagnetic ratio over 2 pi → shielded helion gyromagnetic ratio in MHz/T
• shielded proton gyromagnetic ratio over 2 pi → shielded proton gyromagnetic ratio in MHz/T
• electric constant → vacuum electric permittivity
• magnetic constant → vacuum magnetic permeability
• Wien displacement law constant → Wien wavelength displacement law constant
• Sackur-Tetrode constant (p0 = 100 kPa) → Sackur-Tetrode constant (1 K, 100 kPa)
• Sackur-Tetrode constant (p0 = 101 325 Pa) → Sackur-Tetrode constant (1 K, 101.325 kPa)
• {220} lattice spacing of silicon → lattice spacing of ideal Si (220)
• lattice spacing of Si (220) → lattice spacing of ideal Si (220)
• hartree → Hartree energy
• tau mass energy equivalent in MeV → tau energy equivalent
• atomic unit of 1st hyperpolarizablity → atomic unit of 1st hyperpolarizability
• atomic unit of 2nd hyperpolarizablity → atomic unit of 2nd hyperpolarizability
• atomic unit of electric polarizablity → atomic unit of electric polarizability
• Cu x unit → Copper x unit
• Mo x unit → Molybdenum x unit

Where the unit changed (the Planck constant went from J s to J Hz⁻¹), older values are converted when the units have the same dimension. Retired constants with no successor (such as the Faraday constant for conventional electric current) are not shown. Abbreviations in the files (“mag. mom.”) are written out.

Units and conversions

The unit table records each unit's factor to SI and its dimension in the seven base units, so a conversion between units of different dimensions is impossible by construction. Factors come from the SI Brochure and NIST SP 811 Appendix B; the bohr, dalton, hartree and rydberg use CODATA 2022 values. The converter in your browser uses double-precision arithmetic, good to about 15 significant digits; the tables on the pages are computed with 60-digit decimal arithmetic.

Updates

Every Friday the site re-downloads NIST's table and rebuilds. CODATA adjustments appear every four years and are usually published one to two years after the date they are named for (CODATA 2022 appeared in 2024).