Photon Momentum Calculator

A photon's momentum is the Planck constant divided by its wavelength: p = h / λ. At 500 nm that is 1.325214, in units of 10⁻²⁷ kg·m/s — so 1.325214 × 10⁻²⁷ kg·m/s. Halving the wavelength doubles the momentum, which is why X-ray photons push far harder than red light.

Light carries momentum without carrying rest mass, and the amount depends on one thing only: the wavelength. The relation is p = h / λ, where h is the Planck constant. Since the 2019 revision of the SI, h is not a measured quantity at all — NIST lists it as exactly 6.626 070 15 × 10⁻³⁴ J Hz⁻¹, with the uncertainty column reading “exact”, because that value now defines the kilogram. So the arithmetic here has no experimental error in it; the only uncertainty is in the wavelength you type. Enter 500 nanometres, the calculator reads it as 5 × 10⁻⁷ metres, divides, and returns 1.325214 × 10⁻²⁷ kg·m/s. The result is displayed in units of 10⁻²⁷ kg·m/s rather than in kilogram-metres per second, because in base units every visible wavelength would show as 0.000000. Wavelength sits in the denominator, so the relation is inverse: halve the wavelength and the momentum doubles. A 400 nm violet photon carries 1.66 of those units, a 700 nm red one 0.95, and a 1 nm X-ray photon 662.6. That is the whole reason X-rays damage tissue while radio waves do not. The same photon can also be described by its energy, E = hc / λ, and the two views are tied together by p = E / c.