Hey everyone,
I don't know if anything similar has been posted before (I hope it's appropriate in this forum), but after flicking through the DM Basic Rules working out encounter difficulties looked a little fiddly, so I figured I'd write a thing to calculate it:
http://dhmholley.co.uk/encounter-calculator-5th/
Hopefully you find it helpful. Let me know if you have any comments or ideas for improvement.
There is an excel file going around, which includes the xp values of monster in it.
I would also recommend that instead of typing in the xp value of the monster, you have users typ in the CR value, since it's easier to remember the CR than the XP value, and the XP for a given CR is always the same.
Good idea. I'd done it with XP because the basic rules make it clear that CR is only really a proxy for the true values, but I guess many DMs hold the CR of monsters in their heads. When I next update it, I'll add in a toggle to let players enter CR instead. 
Thanks for making this, I'll try it out soon.