Actually there is no factor for the conversion to surface reflectance.
But there is a factor to TOA reflectance. See my post here
And for landsat8 I don’t know, but I don’t think that there is constant factor.
And I think these are also TOA radiance or TOA reflectance. (see http://landsat.usgs.gov/Landsat8_Using_Product.php)
By default SNAP provides the TOA radiance but you can change to TOA reflectance by specifying a property.
To do this lookup the .snap folder in your user directory. Inside this folder create a folder named etc if not already existing. And in this folder create a file named s3tbx.properties.
To this file add the follwoing line:
s3tbx.landsat.readAs=reflectance
Afterwards the values of landsat will be TOA reflectance.
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