Every term the site uses, in plain English, each linked to the chapter that explains it properly. Below the glossary: the fifteen equations that run the whole subject, the constants they need, and a converter for the numbers you’ll actually want.
Common mix-up: dB and dBm are not the same thing. A decibel is a ratio between two powers (“the wall cost 10 dB”). dBm is an absolute power, measured against one milliwatt: 0 dBm = 1 mW, 20 dBm = 100 mW, −70 dBm = 0.1 nanowatt. You can subtract dB from dBm, but you can’t add two dBm values.
Each one in the form you’d use, with what it means in a sentence and a worked number. Symbols are defined in the constants table below.
| Symbol | Name | SI value | Note |
|---|---|---|---|
| c | Speed of light in vacuum | 299,792,458 m/s | Exact; defines the meter |
| h | Planck constant | 6.626 070 15 × 10⁻³⁴ J·s | Exact since 2019; = 4.135 667 × 10⁻¹⁵ eV·s |
| e | Elementary charge | 1.602 176 634 × 10⁻¹⁹ C | Exact since 2019; 1 eV = this many joules |
| ε₀ | Vacuum permittivity | 8.854 187 8188 × 10⁻¹² F/m | Measured (CODATA 2022) |
| μ₀ | Vacuum permeability | 1.256 637 061 27 × 10⁻⁶ N/A² | Measured; within 10⁻¹⁰ of the old exact 4π × 10⁻⁷ |
| k | Coulomb constant, 1/(4πε₀) | 8.987 551 786 × 10⁹ N·m²/C² | Usually rounded to 8.988 × 10⁹ |
| Z₀ | Impedance of free space, √(μ₀/ε₀) | 376.730 Ω | Ratio of E to H in a plane wave |
| kB | Boltzmann constant | 1.380 649 × 10⁻²³ J/K | Exact; kBT at 310 K = 0.0267 eV |
| me | Electron mass | 9.109 383 7139 × 10⁻³¹ kg | Rest energy mec² = 510.999 keV |
| hc | Handy product | 1239.842 eV·nm | E[eV] = 1239.84 ÷ λ[nm] |
| b | Wien displacement constant | 2.897 771 955 × 10⁻³ m·K | Exact (derived from h, c, kB) |
| σ | Stefan–Boltzmann constant | 5.670 374 419 × 10⁻⁸ W/(m²·K⁴) | Total power radiated by a black body: σT⁴ |
Type in any box, or drag the slider; the others follow. The strip shows where you are on the spectrum and on the power scale.
Photon energy uses E = h f; wavelength uses c = f λ in vacuum. dBm = 10 log₁₀(P ÷ 1 mW). Numbers accept forms like 2.4, 1e-9 or 6.5e14.