Kettlebell exercise - cals burned?

Hi! I don't have a heart rate monitor (yet) and wondered if there was any way to get an approximate number of calories burned during a 40 minute kettlebell session? I realise off course it depends on weight and type of exercise done etc, but just have no idea how much to put down for it. :frown:

This includes strength and cardio, my legs are jelly and I am sweating like a pig at the end of it. :laugh: Seems such a shame to not log it for calories burned! :sad: Any advice would be appreciated!