In the beginning, you'll lose weight doing any regular, consistent cardiovascular exercise, especially if you haven't done it in a while, provided you walk or run at a pace that increases your heart rate, and maintain this for at least 20 minutes a day, three times a week.

Studies have shown treadmills are typically better for weight loss than machines where you sit, like exercise bikes. You can burn the same calories on a costly or inexpensive treadmill as long as your workouts are the same intensity and duration. However over time (as your body adapts), you will need to increase the intensity of your workouts (or mix them up) to continue losing weight or maintain it.

However there are big differences in quality and features among treadmills, which can impact your workouts. For example, more expensive machines offer more features to vary the speed and incline, you can run faster on them, and they're smoother (inexpensive machines often have excessive vibration). High-end treadmills also include a wide range of pre-defined workout programs to keep your workouts more interesting and engaging, and simulate real outdoor walking or running. Do not buy any machine that does not at least let you specify an incline.

While a treadmill is a good option for weight loss, especially when you're starting out, it's not a complete substitute for running and walking outdoors, or weight training. It's a machine and it's generally much easier than running outside. There is no wind, there are no uneven surfaces, no up or down hills, and no variation in the surface, and for some people, 20 minutes may seem like an hour. All of these factors impact your workout. Sometimes, due the unchanging consistency, you can also develop injuries.