1 can of coke has 140 calories. A good rule of thumb is 3500 calories is a pound. So if you drink a single can of coke every day, after a year that's worth almost 15lbs.
This observation is what led to me cutting soda out of my diet entirely almost a year ago (barring the rare treat).
And don't be fooled by the promise of diet soda. Lots of studies recently have shown that drinking diet soda doesn't help with weight loss either. Apparently the artificial sweeteners do something to your body by tricking it into thinking it's getting sugar; I'm not really familiar with the details, but studies showed that people who swapped soda for diet soda didn't lose weight, but people who swapped soda for e.g. water did. This advice is what led to me cutting diet soda out of my diet 6 months ago (which I had switched to after cutting out regular soda), so now I drink almost exclusively water.
If you're interested in cutting out soda as well, and don't think you can just switch to water, I have a couple of suggestions:
1. Start with those crystal geyser flavored sparkling waters you can get at Costco. I drank those for a while, because the hint of flavor tricked my mouth into thinking I wasn't just drinking water. I eventually stopped though, in favor of
2. Get a SodaStream and just make your own sparkling water. You can add flavor packets to this if you want, but I never have. Once you get used to it, unflavored sparkling water is pretty great. The downside here is the refill tanks for the SodaStream aren't cheap, but if you're concerned about that, there are homebrew carbonation setups as well (they just take up a lot more space; SodaStream is pretty compact).