Radiation Pressure Calculator
Light carries momentum as well as energy, so a beam striking a surface pushes on it. This radiation pressure is tiny in everyday terms but real, and it is the propulsion principle behind solar sails, a factor in optical tweezers and a design concern for high-power lasers. This calculator takes the light intensity, the illuminated area and the surface reflectivity, then returns the radiation pressure in pascals and the total force in newtons. A reflectivity of zero models a perfect absorber and a reflectivity of one models a perfect mirror, which feels double the pressure.
Radiation pressure formula
Pressure: P = I * (1 + R) / c
Force: F = P * A
I = intensity (W/m2), R = reflectivity (0 to 1)
c = 299,792,458 m/s, A = area (m2)
For normal incidence, a perfect absorber (R = 0) gives P = I / c and a perfect mirror (R = 1) gives P = 2I / c. The force is the pressure multiplied by the illuminated area.
Radiation pressure context
- The solar constant above Earth's atmosphere is about 1,361 watts per square metre.
- A perfect mirror feels twice the radiation pressure of a perfect absorber.
- The speed of light in vacuum is exactly 299,792,458 metres per second by definition.
- Solar sails such as those tested on IKAROS and LightSail use radiation pressure for propulsion.
- Radiation pressure also helps push the gas tail of a comet away from the Sun.
Radiation pressure: frequently asked questions
What is radiation pressure?
Radiation pressure is the mechanical pressure that electromagnetic radiation exerts on any surface it strikes. Light carries momentum, and when it is absorbed or reflected it transfers that momentum, producing a small but real force. It drives solar sails, shapes comet tails and matters in high-power laser systems.
What is the radiation pressure formula?
For light arriving perpendicular to a surface, radiation pressure is P = I * (1 + R) / c, where I is the intensity in watts per square metre, R is the surface reflectivity from 0 to 1 and c is the speed of light. A perfect absorber has R = 0 and a perfect mirror has R = 1, doubling the pressure.
Why does a mirror feel twice the pressure of a black surface?
An absorbed photon transfers its momentum once. A reflected photon reverses direction, so it transfers momentum twice, once on arrival and once on departure. A perfect mirror therefore experiences double the radiation pressure of a perfect absorber under the same illumination.
What intensity does sunlight provide?
The solar irradiance at Earth's distance, the solar constant, is about 1,361 watts per square metre above the atmosphere. That gives a radiation pressure of only a few micropascals on an absorbing surface, which is why solar sails need very large, very light reflective areas to gather useful thrust.
How do I get the force from the pressure?
Multiply the radiation pressure by the illuminated area: force equals pressure times area. This calculator does that automatically and reports both the pressure in pascals and the total force in newtons for the area you enter, assuming the light hits the surface straight on.
Official sources
- NASA: Solar sailing and LightSail.
- U.S. National Institute of Standards and Technology: Speed of light constant.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.