Ever wonder how many calories you burn when riding your bike? The answer is pretty complex, and it depends on what kind of bike you’re riding, what kind of resistance there is, and how fast you’re going.

When you use your muscles, they start using the oxygen you breathe in to convert fats and sugars, and sometimes proteins, into adenosine triphosphate, or ATP. This is the basic molecule that supplies energy to cells.

“You pretty much need a constant stream of ATP, even if you're just hanging out. But when you're exercising, you need a lot,” says Rachel DeBusk, CPT, a triathlete coach at Seattle’s Unstill Life.

Depending on how long and intense your workout is, your body might access or make ATP in different ways. “There's some ATP just waiting in your muscles,” says DeBusk. “But when that's used up, you have to make more.”

During short, intense bursts of exercise, your body uses anaerobic metabolism to convert carbohydrates into ATP. During longer, less intense workouts, your body gets ATP from aerobic metabolism, where most of the energy comes from carbs.

If you're biking at a moderate, steady speed and without much resistance, you're mostly using your aerobic metabolism system. This improves how well your heart and lungs work and helps your body use glucose efficiently.

DeBusk cautions that not using glucose efficiently can raise your risk of pre-diabetes or metabolic syndrome. With moderate levels of cycling, your body also improves its ability to mobilize fats stored in muscle.

If you're biking at a higher speed or at greater resistance, you'll rely more on your anaerobic metabolism system. This system isn't one that you can maintain for very long, but cycling harder will help your muscle fibers learn how to adapt to demand.

As a general rule, the faster your speed, the more calories you’re likely to burn, because your body uses more energy to go faster. According to Harvard University, biking at a moderate speed of 12 to 13.9 miles per hour will cause a 155-pound person to burn 298 calories in 30 minutes. At a faster rate of 14 to 15.9 miles per hour, a person of the same weight will burn 372 calories.

The calorie burn may vary slightly between using an indoor stationary bike and biking outside. “You can get a great workout in a fitness studio or outside,” says DeBusk.

However, biking outside is more dynamic: You have to be aware of your surroundings, and there’s more variety of movement as you turn to follow roads and paths. There can also be wind resistance and inclines such as hills, and this may help you burn more calories than when you’re indoor cycling, depending on the spin class you do.

If your work or family schedule makes it hard to exercise unless it's a scheduled activity, spin classes can be a good option.

According to Harvard University, the number of calories burned riding a stationary bicycle at a “moderate” pace varies based on what a person weighs.

A moderate pace is going about 12 to 13.9 miles per hour. For a 30-minute time period, the following are calories burned by average weight:

• 125 pounds: 210 calories

• 155 pounds: 260 calories

A person may burn slightly more calories when bicycling outside. Bicycling at a moderate pace outdoors may burn the following number of calories over a 30-minute time span:

• 125 pounds: 240 calories

• 155 pounds: 298 calories

• 185 pounds: 355 calories

Some people may choose to do BMX or mountain biking. This tends to cause a person to burn more calories because the person may be going up hills and navigating rocky, uneven terrain.

According to Harvard University, a person burns the following number of calories when mountain biking for 30 minutes based on their weight:

• 125 pounds: 255 calories

• 155 pounds: 316 calories

• 185 pounds: 377 calories

It’s important to remember that these calories burned are only estimates. They’re based on metabolic equivalents, or METs. Research surrounding calories burned estimates a person burns about 5 calories per 1 liter of oxygen consumed, according to the American Council on Exercise (ACE).

The more challenging the pace, the more METs a person requires. The calculations of how much a person burns takes into account their weight and the METs.

The average METs of modern effort-cycling are 8.0, while mountain biking with vigorous effort is 14.0 METs, according to ACE. However, people have different levels of calorie expenditure based on their unique metabolic rate. So, it’s important to remember that METs are an estimate.

Biking during pregnancy | Pregnancy

You also might opt for indoor biking if you’re pregnant to reduce risk of an accident. “Cycling is a great form of exercise during pregnancy,” says DeBusk. “Many pregnant women find the non-weight-bearing posture reduces pressure in the lower back.”

The low-impact nature of cycling makes it a great benefit to pregnant women.

As pregnancy progresses, make adjustments to your seat and handlebars to accommodate changing hip angles, or try using a more padded seat. “Always stay well-hydrated, and don’t overheat,” reminds DeBusk.

It’s also important to listen to your body. If cycling starts to cause discomfort due to positioning or the demands of a growing baby, you might consider other forms of exercise or cross-training with yoga or Pilates in addition to bicycling sessions.

Calories burned aren’t the only reason to consider biking as exercise. Biking is a great way to relieve stress as well as build muscles in the legs. Other benefits include:

Low impact

Biking doesn’t require high impact to the knees and joints like running or jumping can.

Varying intensity

You can make your biking session as challenging as you like. Some days, you can alternate short bursts of speed, while on others you can do a slower, steady cycling session.

Transportation

Cycling can be an alternative to driving and get you places in less time.

Improved overall fitness

Cycling is challenging to the cardiovascular and muscular systems. You can enhance your overall fitness level by engaging in regular sessions.

The length of time you ride and the intensity are the main factors in how many calories you burn. If you're starting with little or no activity, biking 15 minutes a day, or 30 minutes a few times a week, is an excellent way to improve health and will likely reduce your weight.

Once you’ve adapted to moderate riding, add some intensity intervals, which are even better for burning calories.