You want to convert from photometric luminous power to radiometric power.
Radiometric power, measured in Watts, emitting from an LED is a measurement total amount of photons emitted.
lm is the photometric equivalent of the radiant power which is the luminous flux or luminous power measured in lumens.
Photometrics is how the human eye perceives the photons emitted from a light source.
In addition to power there is the intensity which is the measure of photon at a single point at a particular angle.
There is also the concentration of photons within a given area within a cone where the cone has a given angle.
Radiometric units Photometric units
Radiant power Watts Luminous flux Lumens (lm)
Radiant intensity Watts per steradian (W/ster) Luminous intensity Candelas (cd)
Irradiance Watts per square meter (W/m2) Illuminance Lux (lx)
The conversion is easier for a narrow band LED (e.g. red green blue) within the visible bandwidth. It is not possible for a narrow band LED outside the visible spectrum (e.g deep & far red, royal blue, UV, or ultraviolet).
It is very difficult to convert a wide band LED like white. The radiometric power of a white LED varies for each wavelength in the visible spectrum for the measured photometric lumens.
You can approximate the radiant power if the LED's datasheet publishes the efficacy in lm/Watt. The conversion is still bandwidth dependent where you have to account for how the human eye perceives each wavelength emitted.
That is just the beginning. The characteristics of an LED as indicated in the LED's datasheet are min, max and typical. No LED is typical. The amount of current, the forward voltage, the junction temperature, and the view angle all come into play. All of these characteristics vary widely even if the LED's dies (chips) were produced on the same wafer.