Wavenumber Converter (cm⁻¹ to Wavelength, Frequency, eV)

Convert between wavenumber (cm⁻¹), wavelength (nm, µm), frequency (THz) and photon energy (eV, kJ/mol) for IR, Raman and UV-visible spectroscopy, using ν̃ = 1/λ, f = c·ν̃ and E = h·c·ν̃ with CODATA constants.

Last updated: October 2026

Value to convert

It is a

Convert to

Wavelength (µm)

6.0606 µm

Formula below · 3 sources (physics.nist.gov, www2.chemistry.msu.edu, viva.pressbooks.pub) · Updated Oct 2026

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About this calculator

Spectroscopists describe a wave by its wavenumber, the number of wavelengths per centimetre, because it is proportional to photon energy and reads directly off an IR or Raman spectrum:

ν̃ (cm⁻¹) = 1 ÷ λ (cm) = 10⁷ ÷ λ (nm) = 10⁴ ÷ λ (µm)

Frequency and energy follow from the wavenumber with the speed of light and Planck's constant:

f = c × ν̃ (with ν̃ in m⁻¹, so f (THz) = 0.029979 × ν̃ (cm⁻¹)) E = h × c × ν̃ = 1.239842 × 10⁻⁴ eV × ν̃ (cm⁻¹), so 1 eV = 8,065.54 cm⁻¹ E per mole = N_A × h × c × ν̃ = 0.011963 kJ/mol × ν̃ (cm⁻¹)

The calculator turns whatever you enter into a wavenumber first and then into the quantity you ask for; the tiles show all five at once. With the defaults, the carbonyl C=O stretch at 1,650 cm⁻¹ is a wavelength of 6.061 µm (6,060.6 nm), a frequency of 49.47 THz and a photon energy of 0.2046 eV (19.74 kJ/mol).

Constants (CODATA 2018, all exact by definition): c = 299,792,458 m/s, h = 6.62607015 × 10⁻³⁴ J·s, e = 1.602176634 × 10⁻¹⁹ C, N_A = 6.02214076 × 10²³ mol⁻¹, which give hc = 1,239.841984 eV·nm.

The wavenumber here is the spectroscopic one (1/λ, in cm⁻¹). The angular wavenumber k = 2π/λ used in wave physics is 2π times larger and in rad/m. All values are for vacuum; inside a material the wavelength shortens by the refractive index while frequency and energy stay the same, and wavenumbers quoted in spectra are vacuum values.

Common spectroscopic bands converted with this calculator (vacuum values)

Scroll sideways to see all columns →

Band	Wavenumber (cm⁻¹)	Wavelength	Frequency (THz)	Energy (eV)
CO₂ asymmetric stretch	2,349	4.257 µm	70.42	0.2912
C=O stretch (ketone)	1,715	5.831 µm	51.41	0.2126
Amide I (C=O)	1,650	6.061 µm	49.47	0.2046
Telecom laser 1,550 nm	6,452	1,550 nm	193.41	0.7999
He-Ne laser 632.8 nm	15,803	632.8 nm	473.76	1.9593
Green laser 532 nm	18,797	532 nm	563.52	2.3305
1 eV	8,065.54	1,239.84 nm	241.80	1.0000

Source: NIST CODATA: inverse metre-electron volt relationship

How to use

Enter a value, say what it is (wavenumber, wavelength, frequency or energy) and choose what to convert it to. The tiles below the result show the other four quantities. Example 1 (IR band): 1,650 cm⁻¹ to wavelength. λ = 10⁴ ÷ 1,650 = 6.0606 µm = 6,060.6 nm; f = 1,650 × 0.029979 = 49.47 THz; E = 1,650 × 1.239842 × 10⁻⁴ = 0.2046 eV = 19.74 kJ/mol. Example 2 (green laser): 532 nm to wavenumber. ν̃ = 10⁷ ÷ 532 = 18,797 cm⁻¹; E = 1,239.84 ÷ 532 = 2.3305 eV; f = 563.52 THz. Example 3 (O-H stretch): 3,000 cm⁻¹ = 3.333 µm, 0.3720 eV, 89.94 THz.

Frequently asked questions

How do I convert cm⁻¹ to nm?

Divide 10,000,000 by the wavenumber: λ (nm) = 10⁷ ÷ ν̃ (cm⁻¹). 1,650 cm⁻¹ is 6,061 nm; 20,000 cm⁻¹ is 500 nm. The same formula works in reverse, nm to cm⁻¹.

How do I convert wavenumber to eV?

Multiply by hc = 1.239842 × 10⁻⁴ eV·cm: E (eV) = ν̃ (cm⁻¹) ÷ 8,065.54. A 1,650 cm⁻¹ photon carries 0.2046 eV; 1 eV corresponds to 8,065.54 cm⁻¹.

Why do IR spectra use cm⁻¹ rather than nm?

Wavenumber is proportional to energy and frequency, so equal spacings on the axis mean equal energy steps and overtones fall at simple multiples; wavelength is inversely related, which compresses the high-energy end. The mid-IR range 400-4,000 cm⁻¹ (25 to 2.5 µm) is also a convenient size in cm⁻¹.

Is a wavenumber the same as the k in wave equations?

No. Spectroscopic wavenumber is ν̃ = 1/λ in cm⁻¹; the angular wavenumber is k = 2π/λ in rad/m. Multiply ν̃ (cm⁻¹) by 2π × 100 to get k in rad/m.

How do I get kJ/mol from cm⁻¹?

Multiply by N_A h c = 0.011963 kJ/mol per cm⁻¹: 1,650 cm⁻¹ is 19.74 kJ/mol, and 350 cm⁻¹ (a typical hydrogen-bond energy scale, about 4.2 kJ/mol) is roughly kT × 1.7 at room temperature (kT = 207 cm⁻¹ at 298 K).

Does the medium change the wavenumber?

The vacuum wavenumber does not: energy and frequency are fixed by the source. Spectra are reported in vacuum (or air, within 0.03%) wavenumbers, so convert with vacuum constants as this calculator does.

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