Luminous flux or lumen is the quantity of the energy of the light emitted per second in all directions. Its unit is lumen (lm). It is the quantity of the energy of the light emitted per second in all directions. Given here is the simple luminous flux formula to measure the lumen of any given light source based on the values of maximum luminous intensity and cone full angle. First, find the equivalent solid angle and substitute the value in the lumen formula, to get the flux.
You could also make use of the online calculator given here to verify your answers. Also, this luminous flux formula page shows you the equivalent solid angle formula for the calculation of solid angle in 'ster'.