Mountain biking encompasses a variety of styles of off-road riding. As a result, no firm guidelines for calorie burning exist for mountain biking. However, you can estimate your calorie usage by taking into account a few different factors.
Weight
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According to an article in "Shape" magazine by Amy Goldhammer, if you're a 145-lb. woman, you'll burn about 528 calories per hour mountain biking on terrain of moderate difficulty. This amount increases for higher weights and decreases for lower weights. A heavier or lighter mountain bike can also affect your calorie burning -- a heavier bike is effectively more weight that you are carrying around.
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Terrain
Less technical courses or trails may allow you to build up speed and pedal consistently for higher calorie burning or may feature a lot of downhill coasting that doesn't burn many calories. More technical routes might require speed, balance and cornering adjustments that exercise your upper body and burn more calories than average.
Rate
Riding 15 miles per hour on a flat or slightly inclined road burns 526 calories per hour for a 145-lb. person, about the same as moderate mountain biking. Riding faster enables you to burn more calories in the same amount of time. Stay within your target heart rate so you can exercise for a longer period of time and burn more calories.