Fundamental scientific constants and their units
Fundamental physical constants connect measured quantities across physics and chemistry. Some constants have exact values because the SI defines units through them; others remain experimentally measured.
The table identifies exact and measured values and preserves the uncertainty notation published with measured CODATA values.
How to use the scientific constants reference
- Search: Enter a constant name, symbol, or related term.
- Filter: Show all values or only exact or measured constants.
- Copy carefully: Copy both value and SI unit into the calculation.
Formula and variables
A constant must be used with its unit and full exponent. Parentheses after measured values give standard uncertainty in the last displayed digits.
x = value × SI unit- c — Speed of light
- Exact defining constant (m s⁻¹)
- h — Planck constant
- Exact defining constant (J Hz⁻¹)
- G — Gravitational constant
- Measured constant (m³ kg⁻¹ s⁻²)
Photon energy from frequency
Use the Planck constant to calculate E = hf for frequency f.
- h
- 6.62607015 × 10⁻³⁴ J Hz⁻¹
- f
- 5.00 × 10¹⁴ Hz
- E = hf
- E = (6.62607015 × 10⁻³⁴)(5.00 × 10¹⁴)
Result: E ≈ 3.31 × 10⁻¹⁹ J.
The h value is exact, while the example frequency controls the shown precision.
Understanding your results
Exact versus measured constants
Exact values have no measurement uncertainty in the SI; measured values can change when CODATA publishes a new adjustment.
- Do not drop powers of ten or units when copying a constant.
- Carry sufficient digits during calculation and round the final result appropriately.
Assumptions
- Values use SI units unless the table explicitly states otherwise.
- Displayed measured values follow the 2022 CODATA adjustment.
Limitations
- This is a concise reference rather than the complete CODATA database.
- It does not provide covariance data or machine-readable uncertainty propagation.
Common mistakes
- Using an older measured value without checking the adjustment year.
- Confusing uppercase G with standard gravity g.
- Copying a value without its exponent or unit.
- Treating a parenthetical uncertainty as part of the central value.
Practical use cases
Physics calculations
Find constants for mechanics, electromagnetism, quantum physics, and relativity.
Chemistry calculations
Use Avogadro, Boltzmann, gas, and Faraday constants with their SI units.
Frequently asked questions
Why are some scientific constants exact?
The modern SI assigns exact numerical values to defining constants such as c, h, e, k, and Nₐ. Derived combinations of exact constants can also be exact.
What do digits in parentheses mean?
They state the standard uncertainty in the last digits shown. For example, 6.67430(15) means an uncertainty of 0.00015 at that coefficient scale.
Sources and review
- CODATA Recommended Values of the Fundamental Physical Constants: 2022 — NIST Physical Measurement Laboratory. Accessed 2026-07-14.
Reviewed 2026-07-14.