Wavelength to Wavenumber Converter

cm⁻¹

Common Conversions

nm	cm⁻¹
200	50000
250	40000
400	25000
500	20000
700	14286
1000	10000
2500	4000
5000	2000
10000	1000
20000	500
50000	200
100000	100

Why this conversion matters in chemistry

IR spectra get plotted against wavenumber — cm⁻¹ — almost universally, even when the underlying instrument records wavelength. The reason is physical: wavenumber is directly proportional to photon energy (E = hcν̃), so the axis actually corresponds to something molecules care about. A 3.33 µm (3333 nm) C–H stretch shows up at 3000 cm⁻¹; a 10 µm C–O stretch lands at 1000 cm⁻¹; the fingerprint region runs roughly 400 to 1500 cm⁻¹. The conversion is one over the wavelength, expressed in centimeters — divide 10⁷ by your wavelength in nanometers and you're done.

Formula

cm⁻¹ = 10⁷ ÷ nm

Worked Examples

500 nm = 20000 cm⁻¹

Green light, right in the middle of the visible. UV-Vis spectra are usually plotted in nm, but the wavenumber is useful for comparing transitions across spectral regions.

10000 nm = 1000 cm⁻¹

Ten microns — smack in the IR fingerprint region, where most diagnostic vibrational modes live.

250 nm = 40000 cm⁻¹

UV territory. Common for aromatic π→π* transitions and the upper end of DNA absorbance.

1000 nm = 10000 cm⁻¹

A round anchor in the near-IR — well past the red end of the visible range, in the region near-IR spectroscopy and many fiber-optic systems work in.

Frequently Asked Questions

How do I convert wavelength to wavenumber?

ν̃ (in cm⁻¹) = 10⁷ ÷ λ (in nm). So 500 nm becomes 20,000 cm⁻¹; 10,000 nm becomes 1000 cm⁻¹. The 10⁷ factor comes from the unit mismatch — nanometers times centimeters need the conversion baked in.

Why does spectroscopy prefer wavenumber?

Because wavenumber is proportional to photon energy, and photon energy is what molecular transitions actually depend on. E = hcν̃, so a plot in cm⁻¹ is really a plot in arbitrary energy units. Wavelength, which is inversely proportional to energy, makes the spectrum look skewed against the relevant physics. Once you're fluent in cm⁻¹ you can mentally size up bond types and transition regions faster than you can in nm.

What's the visible range in cm⁻¹?

Roughly 14,286 to 25,000 cm⁻¹, corresponding to 700 to 400 nm at the red and violet ends. The UV picks up above that, the near-IR below it. Worth anchoring mentally — once you know those endpoints, you can place any visible or adjacent transition by inspection.