Most adults can expect to burn nearly 200 calories for every 30 minutes shoveling snow by hand, according to the Harvard Heart Letter, a newsletter published by Harvard Medical School. Specifically, a 125-pound person can expect to burn 180 calories, a 155-pound person can expect to burn 223 calories, and a 185-pound person can expect to burn 266 calories per half hour of shoveling.
If you don't fall into those specific weight groups, this calculator can give you a more personalized (albeit rough) estimate of how many calories you've burned. It's also worth noting that compared to other physical activities detailed in the same data set, shoveling snow burns just as many calories in 30 minutes as teaching aerobics, using a start-step machine, and fencing -- so don't feel so bad about skipping the gym during the next blizzard.
For those lucky enough to have access to a snow-blower to get the job done, you're also getting a pretty solid workout. According to the Harvard stats, pushing one of those beasts around for 30 minutes will burn 135, 167, and 200 calories, respectively, in each of the aforementioned weight brackets.
Of course, there are myriad factors that can play into how many calories you're actually burning. For instance, the colder it is outside, the more calories your body will burn just to stay warm. Also, if you're taking frequent breaks, you won't burn as many.
Stay warm out there, and try not to throw your back out.