The search function will find hundreds of posts on this very topic.

Short version

1) YMMV
2) Without a power meter, any number involves guesswork. a HRM reduces that somewhat.
3) Lots of riders find that they burn somewhere in the neighborhood of 100 calories per 5 km, but that obviously varies with the rider, the bike, the weather, the terrain, and how hard you're riding.
4) If you're trying to lose or maintain weight, trial and error will eventually give you the usable answer for you, but it will take weeks or months to be confident in your number and eliminate other causes of daily weight fluctuation.

My advice - use #3 and work with it a while.

BB