Solar Flux at Distance Calculator
Sunlight spreads out as it travels, so the energy falling on each square meter weakens with distance from the Sun. The solar constant, about 1,361 watts per square meter, is the flux at Earth's distance of one astronomical unit. Everywhere else, the flux follows the inverse-square law: it drops as the square of the distance grows. This calculator takes the solar constant and any orbital distance in astronomical units and returns the flux there, along with the ratio to Earth, useful for planetary climate, solar panel sizing on probes, and teaching radiation falloff.
Solar flux formula
Flux at distance = solar constant / d^2
Ratio to Earth = 1 / d^2
Percent of Earth = 100 / d^2
The inverse-square law applies because the Sun's output spreads over the surface of an expanding sphere whose area grows as the square of the radius. With d measured in astronomical units, dividing the solar constant by d squared gives the flux at that distance.
Solar flux facts
- The solar constant at one astronomical unit is about 1,361 watts per square meter.
- Mars at 1.524 astronomical units receives roughly 586 watts per square meter.
- Jupiter at about 5.20 astronomical units receives only about 50 watts per square meter.
- Mercury at about 0.39 astronomical units receives nearly seven times Earth's flux.
- The solar constant is user-editable to match the latest measured total solar irradiance.
Solar flux: frequently asked questions
What is the solar constant?
The solar constant is the average solar radiative flux reaching the top of Earth's atmosphere at the mean Earth-Sun distance of one astronomical unit. NASA and NOAA use a total solar irradiance value of about 1,361 watts per square meter. It is the reference flux from which the flux at any other distance is scaled.
How does flux change with distance from the Sun?
Solar flux follows the inverse-square law: it falls as one over the square of the distance from the Sun. At distance d (in astronomical units) the flux equals the solar constant divided by d squared. Twice as far means one quarter the flux; half as far means four times the flux.
What is the formula this calculator uses?
Flux at distance = solar constant / d^2, where d is the distance from the Sun in astronomical units and the solar constant is the irradiance at one astronomical unit. The solar constant is a user-editable input preset to 1,361 watts per square meter, the accepted total solar irradiance value.
What is the flux at Mars or Jupiter?
Mars orbits at about 1.524 astronomical units, so its flux is 1,361 divided by 1.524 squared, roughly 586 watts per square meter. Jupiter at about 5.20 astronomical units receives 1,361 divided by 5.20 squared, only about 50 watts per square meter, which is why outer planets are so cold.
Why is the solar constant a user-editable input?
Total solar irradiance varies slightly over the roughly 11-year solar cycle and is refined as measurements improve. Presetting it to 1,361 watts per square meter follows current NASA and NOAA values, but exposing it as an input lets you enter the latest measured value or model a different star's irradiance at its reference distance.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.