The number of calories you burn during exercise is largely determined by several factors. First off, your individual weight plays a large role in your overall calorie burn. Secondly, calorie is determined by exertion: that is, how much work you are putting into the exercise, how far or long you are walking, and how high your heart rate gets.
People who weigh more will typically burn calories at an increased rate than others who weigh less. Therefore, if a 300 pound person walks at exactly the same pace and the same distance as a 150 pound person, the 300 pound person will burn more calories.
Pace, distance, and heart rate factor into calorie burn too. The longer and farther you walk, the more calories you will burn because you will be exerting yourself for a longer period of time. Similarly, the faster you walk, the higher your heart rate will rise and the more calories you will burn. This is why, mile for mile, running does burn more than walking. You have to exert more energy to run a mile as opposed to walking a mile. The same goes for walking rates - power walking will burn more calories than just a leisurely stroll.
To figure out how many calories you burned per mile, a simple equation can be used: multiply your weight in pounds by .57. So, for instance, a 150 pound female probably burns around 85 calories per mile walked. If you don't know your mileage and instead just walked for time, multiply your weight in pounds by .03. This will tell you how many calories you burn per minute. (Using our example, a 150 pound individual burns around 4.5 calories per minute walked.) Know that these formulas are just an estimate though, as they don't take into consideration how fast you walked or how high your heart rate got.
If you are trying to get the most "bang for your buck" and burn the most calories in the least amount of time by walking, then be sure to pick up your pace so that your heart rate starts to rise. Not only does this result in more calories burned during the time you are walking, but studies show that the after-burn effect (i.e. the amount of calories burned after the exercise is complete) will be higher too. Bonus points if you break a sweat!