I am interested in knowing a formula to convert lux into watt.

Hi, manveen!

If you click on the following link, you'll find some ways for conversion.

LINK

Thanks!

One lux is equivalent to 1.46 milliwatts per square meter for light with wavelength 555 nm (which is the middle of the visible spectrum).

If on the other hand you are trying to figure out how many lux a lightbulb produces of a given wattage produces, there is no standard conversion, as the amount of light produced by a light biulb depends on many design factors of the bulb, such as whether it's an incandescent or fluorescent one.