If you are riding 13mph on a bike for a hour how much weight would you lose?

For one hour, one time - nothing.

For one hour every day/every 2nd days for weeks, maybe some - but only if you're also watching your diet. With the high calorie foodstuff that we have available today it's so easy to overeat that any attempt at weight lossmust start with controlling your intake. it's virtually impossible to burn enough through exercise to lose any weight if your eating isn't controlled.

And forget about how fast you're riding, it doesn't say anything about how hard you're working, which is the interesting bit when it comes to weight loss and exercise. And if it's 13 mph on a stationary bike it's doubly pointless, as it isn't going anywhere anyhow. The only use you can have of that value is to monitor if you're able to ride at maybe 15 mph for one hour after working out for a couple of weeks.

If you're serious about it, get yourself one of those heart rate monitors that has a chest strap and a display that looks like a wristwatch. Choose one with a calorie counter, then exercise within the recommeded pulse rate limits regularly. That, together with a monitored diet should see quite a bit of weight loss.