Photon Momentum Calculator
Enter a wavelength in nanometres to get the momentum of a photon in kg·m/s — and see why light carries momentum even though a photon has no mass.
Momentum from wavelength
Enter the wavelength in nanometres and the calculator returns the photon momentum (p = h / λ) in kg·m/s using the Planck constant.
Enter nanometres
Wavelength is entered in nanometres (nm) here — visible light runs from about 380 to 700 nm — and is converted to metres internally before dividing.
What is photon momentum?
The push that light carries
A photon momentum calculator works out how much momentum a single particle of light carries. A photon has no rest mass, yet it still carries momentum: that momentum equals the Planck constant divided by the wavelength, p = h / λ. The shorter the wavelength, the more momentum each photon packs. This calculator turns one measurement — the wavelength in nanometres — into the momentum in kilogram-metres per second, the number behind radiation pressure, solar sails, and the recoil of atoms when they absorb or emit light.
Enter a wavelength in nanometres to get the momentum of a photon in kg·m/s instantly.
The momentum of a photon is the Planck constant divided by its wavelength. The Planck constant is h = 6.62607015 × 10⁻³⁴ J·s, an exact value fixed by the SI definition.
p = h / λBecause the wavelength sits in the denominator, momentum and wavelength are inversely related: halve the wavelength and you double the momentum. Enter the wavelength in nanometres and the calculator converts it to metres before dividing, returning the momentum in kg·m/s.
Suppose you shine green light with a wavelength of 500 nm.
Convert the wavelength to metres
500 nm × 10⁻⁹ = 5 × 10⁻⁷ m — nanometres become metres for the formula.
Divide the Planck constant by the wavelength
6.62607015 × 10⁻³⁴ ÷ 5 × 10⁻⁷ — Planck's constant over the wavelength in metres.
Read the momentum
p ≈ 1.3252 × 10⁻²⁷ kg·m/s — the momentum carried by one 500 nm photon.
The result is tiny — on the order of 10⁻²⁷ kg·m/s for visible light — yet it is real and measurable. Photons carry momentum despite being massless because momentum for light depends on energy and wavelength, not rest mass. The key relationship is inverse: a shorter wavelength means more momentum, so a blue or ultraviolet photon (around 400 nm) carries more momentum than a red one (around 700 nm), and an X-ray photon carries far more still. Add up the momentum of the countless photons in a beam and the effect becomes macroscopic — this is radiation pressure, the gentle but steady push that drives solar sails, nudges comet tails away from the Sun, and lets laser tweezers trap and move microscopic particles. A single photon's momentum is also what an atom recoils against when it absorbs or emits light, the basis of laser cooling.
The formula is exact, but a couple of practical points are worth keeping in mind.
Wavelength in nanometres, vacuum assumed
This calculator takes the wavelength in nanometres and converts it to metres internally before dividing by it, so enter a value in nm — not metres or ångströms. The formula uses the wavelength in vacuum; inside a medium with a refractive index the wavelength shortens, so use the in-medium wavelength if that is what you mean. The energy form p = E / c gives the same momentum when E is the photon energy and c is the speed of light.