If you have a PAR meter like an Apogee MQ-500, your measurements will quantify the PAR within the wavelength range of 400-700nm.

But if you want to convert these measurements to the updated ePAR range that includes far-red light (encompassing 400-750nm), your measurements must be converted with conversion factors that are specific to the light source you are measuring. Use this calculator to convert PAR measurements to ePAR measurements.

PAR Measurements for LED light sources will have about the same ePAR value, unless your LED lights contain far-red LEDs. If your lights use far-red LEDs, this calculator will not be an accurate tool since each lighting manufacturer uses different proportions of far-red LEDs. To accurately quantify ePAR in an LED light with far-red LEDs, an ePAR meter is necessary. 

How this calculator works

This calculator is based on conversion factors from Apogee Instruments. For the theoretical ePAR result, the conversion uses a flat response and sharp cutoffs at 400nm and 750nm (see the blue line below). For the measured result, empirical conversion factors are used which are based on a measurement taken with an Apogee SQ-500 sensor (400-700nm) compared to the response of an Apogee SQ-610 sensor (400-750nm, see green line below). 

Is there a formula to convert PAR to ePAR?

No - conversion factors must be used. Since the spectral distribution varies across different light sources, each light source has a unique ratio of PAR and far-red light. The following conversion table can be used for your own calculations:

References

A Sensor to Measure Extended PAR (ePAR): the Sum of Photons from 400 to 750 nm. Apogee Instruments, Inc. & Dept. Of Plants, Soils, and Climate, Utah State University.