Finally I stumbled on this article:
So an average person of average weight burns:
- 0.11 calories per step going up
- 0.05 calories per step going down
Add a bit if you're big, subtract a bit if you're small... but at least it's a good ballpark.
I figured it out for a 10 stories of stairs I climb in my office building, and it was around 20 calories. Doesn't sound like a lot, but I go up at least 3 times a day. That's 60 calories... and if I go down as well, that's another 30, almost 100 per day!
Not bad for virtually no investment in time.