Physics Tools
What It Does
A searchable, categorized reference of fundamental physical constants — the speed of light, Planck’s constant, Avogadro’s number, and more — with one-click copy in several formats. Nothing is calculated and nothing leaves your device.
How to Use It
- Search by name, symbol, or alias (e.g. “avogadro”, “Nₐ”, “lightspeed”).
- Filter by category, or switch the notation shown.
- Copy a constant as a plain value, scientific form, value + unit, or a ready-to-paste JavaScript / Python snippet.
h, e, k_B, Nₐ) are exact by definition — look for the ✅ badge.| Symbol | Constant | Value | Unit | Status | Copy |
|---|---|---|---|---|---|
| c ★ | Speed of light in vacuumDefines the meter since 1983; exact by definition. | 2.99792458 × 108 | m/s | ✅ exact | |
| G ★ | Newtonian gravitational constantThe least precisely known fundamental constant. | 6.6743 × 10-11 | m³·kg⁻¹·s⁻² | ± 2.2e-5 | |
| h ★ | Planck constantExact since the 2019 SI redefinition; now defines the kilogram. | 6.62607015 × 10-34 | J·s | ✅ exact | |
| gₙ ★ | Standard gravityA conventional, defined value for free-fall acceleration on Earth.Used by: Projectile Motion, Kinematics (SUVAT) | 9.80665 × 100 | m/s² | ✅ exact | |
| ℏ | Reduced Planck constantEquals h / 2π; exact because h is exact. | 1.054571817 × 10-34 | J·s | ✅ exact | |
| atm | Standard atmosphereA defined reference pressure (101 325 Pa), exact by convention. | 1.01325 × 105 | Pa | ✅ exact |
| Symbol | Constant | Value | Unit | Status | Copy |
|---|---|---|---|---|---|
| e ★ | Elementary chargeExact since the 2019 SI redefinition; now defines the ampere. | 1.602176634 × 10-19 | C | ✅ exact | |
| ε₀ | Vacuum electric permittivityNo longer exact after 2019; carries the uncertainty of α. | 8.8541878128 × 10-12 | F/m | ± 1.5e-10 | |
| μ₀ | Vacuum magnetic permeabilityFormerly exactly 4π×10⁻⁷; now measured since the 2019 redefinition. | 1.25663706212 × 10-6 | N/A² | ± 1.5e-10 | |
| kₑ | Coulomb constantEquals 1 / (4π ε₀). | 8.9875517923 × 109 | N·m²·C⁻² | ± 1.5e-10 |
| Symbol | Constant | Value | Unit | Status | Copy |
|---|---|---|---|---|---|
| α | Fine-structure constantDimensionless; approximately 1/137.035999. | 7.2973525693 × 10-3 | — | ± 1.5e-10 | |
| mₑ | Electron massRest mass of the electron. | 9.1093837015 × 10-31 | kg | ± 3.0e-10 | |
| m_p | Proton massRest mass of the proton. | 1.67262192369 × 10-27 | kg | ± 3.1e-10 | |
| m_n | Neutron massRest mass of the neutron. | 1.67492749804 × 10-27 | kg | ± 5.7e-10 | |
| u | Atomic mass unitOne twelfth of the mass of a carbon-12 atom. | 1.6605390666 × 10-27 | kg | ± 3.0e-10 | |
| a₀ | Bohr radiusMost probable electron–proton distance in ground-state hydrogen. | 5.29177210903 × 10-11 | m | ± 1.5e-10 | |
| R∞ | Rydberg constantOne of the most precisely measured physical constants. | 1.097373156816 × 107 | m⁻¹ | ± 1.9e-12 |
| Symbol | Constant | Value | Unit | Status | Copy |
|---|---|---|---|---|---|
| k_B ★ | Boltzmann constantExact since the 2019 SI redefinition; now defines the kelvin. | 1.380649 × 10-23 | J/K | ✅ exact | |
| R ★ | Molar gas constantExact as the product of the (exact) Avogadro and Boltzmann constants. | 8.31446261815324 × 100 | J/(mol·K) | ✅ exact | |
| Nₐ ★ | Avogadro constantExact since the 2019 SI redefinition; defines the mole. | 6.02214076 × 1023 | mol⁻¹ | ✅ exact | |
| σ | Stefan–Boltzmann constantExact; derived from k_B, h, and c, which are all exact. | 5.670374419 × 10-8 | W/(m²·K⁴) | ✅ exact | |
| F | Faraday constantExact as the product of the (exact) Avogadro and elementary charge. | 9.648533212 × 104 | C/mol | ✅ exact |
About the Fundamental Physical Constants
Physical constants are fixed quantities of nature that appear in the laws of physics — the speed of light c, the Planck constant h, the elementary charge e, and so on. Since the 2019 revision of the SI, the base units are defined by fixing the numerical values of a handful of these constants. As a result h, e, k_B, and Nₐ no longer carry any uncertainty: they are exact by definition. Constants that are still determined by experiment — such as the gravitational constant G or the electron mass mₑ — are shown here with their relative uncertainty.
Example lookup
Searching for “avogadro” returns Nₐ = 6.02214076 × 10²³ mol⁻¹. Because the mole is now defined by fixing this value, the constant is exact — there is no ± uncertainty to quote.
Frequently Asked Questions
Why are some constants now “exact”?
The 2019 SI redefinition defines the kilogram, ampere, kelvin, and mole by assigning exact numerical values to h, e, k_B, and Nₐ. Once a constant’s value is fixed by definition, it has zero uncertainty.
What’s the difference between R and k_B?
The Boltzmann constant k_B relates energy to temperature for a single particle, while the molar gas constant R does the same per mole. They differ by exactly the Avogadro constant: R = Nₐ · k_B. Since both Nₐ and k_B are exact, R is exact too.
Where does Avogadro’s number come from?
It is now a defined value — exactly 6.02214076 × 10²³ entities per mole — chosen to match the historical count of atoms in 12 grams of carbon-12 as closely as the measurements allowed.
What are the units of G?
The Newtonian gravitational constant G has units of m³·kg⁻¹·s⁻² (equivalently N·m²·kg⁻²). It remains the least precisely measured fundamental constant, with a relative uncertainty around 2.2 × 10⁻⁵.
Is my data sent to a server?
No. This page is a static reference rendered entirely in your browser; nothing you search or copy is transmitted or stored.