Energy Unit Converter

Type a value into any field and every other unit updates instantly. Constants are the exact SI definitions plus CODATA 2022.

nm and fs scale as 1/E and are defined only for E > 0; every other unit is proportional to the energy and accepts negative values (binding energies, energy differences). Both 1.5e-3 and Fortran-style 1.5d-3 are accepted.

Documentation

Conversion formulas, the constants used, and a quick-reference table. Formula rendering: plain text.

1 · Conversion formulas

Every conversion pivots on eV: the input is first converted to eV, then out to all remaining units. The underlying relation is

E = hν = hc/λ = hc·ν̃ = kBT

UnitQuantityFrom E(eV)
2 · Constants used

The values below are exactly those the page computes with — they are generated from the same code, so this documentation cannot drift away from the arithmetic. Since the 2019 SI redefinition h, e, c, kB and NA are exact, so every conversion factor other than those involving Eh is an exact combination of defined values.

SymbolNameValueUnitSource

Derived quantities (computed from the table above):

QuantityValueUnit
3 · Quick reference

Each row reads «one of this unit equals the value in each column». Scroll the table sideways on a narrow screen.

1 ＝eVcm⁻¹KkJ/molkcal/molTHz
4 · Conventions and caveats
  • Definition of fs: here fs is the oscillation period τ = 1/ν = h/E, so 1 eV ↔ 4.135668 fs, matching the original Python script. If you want a lifetime / decoherence time ħ/E (1 eV ↔ 0.6582120 fs), divide by 2π.
  • nm and fs are inversely proportional to the energy and undefined for E ≤ 0; those two fields are then left blank and marked «undefined».
  • kcal is the thermochemical calorie: 1 cal ≡ 4.184 J (exact), not the 15 °C or IT calorie.
  • J refers to a single particle; kJ/mol and kcal/mol are per mole, the two differing by NA.
  • K is the equivalent temperature E/kB, with no 3/2 or other degree-of-freedom factor.
  • Hartree is the atomic unit (a.u.) of energy, 1 Ha = 2 Ry.
  • Small differences from the original Python script: energy_unit_conv.py uses pre-2018 CODATA values (h = 4.13566733e-15 eV·s, e = 1.60217733e-19 C) whereas this page uses the exact 2019 SI definitions; the two start to differ in the 8th significant digit.
  • Works offline: the page needs no server-side code at all — save it and open it locally. Only the formula typesetting is enhanced by KaTeX when online; offline it falls back to plain-text formulas and the conversion itself is unaffected.