Wavenumber to Wavelength Converter

cm⁻¹

Common Conversions

cm⁻¹	nm
100	100000
250	40000
500	20000
1000	10000
1500	6666.7
2000	5000
2500	4000
3000	3333.3
3500	2857.1
4000	2500
5000	2000
10000	1000

Why this conversion matters in chemistry

IR spectra are almost always plotted against wavenumber, but the wavelength form still matters when you're choosing sample cells or sample-preparation windows — a 1650 cm⁻¹ amide-I band in a protein spectrum corresponds to 6.06 µm, which is well inside the transparency range of CaF₂ windows but outside that of common glass. A C–H stretch at 3000 cm⁻¹ is 3.33 µm; a C=O at 1700 cm⁻¹ is 5.88 µm. Dividing 10⁷ by the wavenumber in cm⁻¹ gives you the wavelength in nm, and shifting the decimal by three converts to µm — the native unit of IR optics specifications.

Formula

nm = 10⁷ ÷ cm⁻¹

Worked Examples

1000 cm⁻¹ = 10000 nm

10 µm — solidly in the mid-IR fingerprint region, where most diagnostic bands live.

3000 cm⁻¹ = 3333.3 nm

3.33 µm. The C–H stretching region — almost any aliphatic organic spectrum shows peaks here.

1700 cm⁻¹ = 5882.4 nm

5.88 µm. The carbonyl C=O region — one of the most diagnostic features of organic IR spectra.

500 cm⁻¹ = 20000 nm

20 µm — far-IR territory, where lattice vibrations and some metal-ligand modes appear.

Frequently Asked Questions

How do I convert wavenumber to wavelength?

λ(nm) = 10⁷ / ν̃(cm⁻¹). So 1000 cm⁻¹ becomes 10,000 nm (10 µm); 3000 cm⁻¹ becomes 3333 nm. The 10⁷ factor comes from the unit mismatch — centimeters to nanometers is a factor of 10⁷.

Why is the relationship inverse?

Wavenumber is 1/wavelength (in cm), so doubling the wavenumber halves the wavelength. Higher wavenumbers correspond to shorter wavelengths and higher photon energies — the same inverse relationship that makes E = hc/λ equivalent to E = hcν̃.

What are the standard wavenumber ranges?

Near-IR covers 4000 to 12,000 cm⁻¹ (around 800 to 2500 nm). Mid-IR runs 400 to 4000 cm⁻¹, which is where most organic spectra are recorded. Far-IR is 10 to 400 cm⁻¹, for lattice vibrations and some inorganic modes. Raman data is usually reported as cm⁻¹ shift from the excitation laser, which puts it on the same scale as IR.