High intensity interval training

I agree with you, it is hard to pin point the exact calories burned but I usually estimate it. For me I run 4-5 days a week. My Nike app tells me I burn about 700 calories for a 10 k run at a pace of about 7.5mph. My treadmill says it's a bit more but i think the app is closer. I also box/cross train 2-3 days a week for exactly 60 minutes which includes burpees, planking, squats, skipping and so forth. When I cross train I sweat way more than when I go for a run. I am absolutely drenched after cross training but again mfp doesn't really say what the calories burned are. I usually estimate those days at about 600 calories. It could be more but if I'm not sure I'll go with a bit less.

It's very difficult to determine because it's all dependent upon your level of intensity and what exercises you're actually doing and how high you get your heart rate. Things that fitness trackers have a problem with. Typically, you're likely not going to burn more than 700 calories for one hour of HIIT, and that's if you're doing it the way it's designed, until you almost feel like you're going to throw up and you reach max effort during the periods where you're supposed to raise your heart rate. If you're pretty consistent, you can judge more from the weight loss or lack thereof and make adjustments on MFP from there.