National guidelines for physical activity recommend at least 150 minutes a week of moderate intensity aerobic exercise. But if you are a walker, for instance, how do you know whether your walking speed is fast enough?

Simply count your steps, says new research. Kinesiology researchers have found that walking 100 steps a minute will give adults ages 21 to 40 the recommended, moderate-intensity exercise. Move a bit faster – at least 130 steps a minute – and you will qualify for a vigorous intensity workout. Exercising at that level cuts the recommended weekly time in half, to 75 minutes a week, according to the most recent physical activity guidelines.

The researchers suggest a simple method to figure out how many steps you take in a minute, or your walking cadence: Count your steps for 15 seconds and then multiply by four. Their research on walking cadence and exercise intensity has been published in the International Journal of Behavioral Nutrition and Physical Activity.

Other ways to determine exercise intensity? Some people rely on how they feel while exercising. Feeling sweaty after just a few minutes vs. having just a light sweat after 10 minutes of exercise, for instance, differentiates between vigorous and moderate intensity. So does whether you can carry on a conversation while exercising (yes for moderate intensity, no for vigorous).

In addition to aerobic activity, the national guidelines also recommend twice-a-week strength-training, such as weightlifting or exercise that uses your own body weight.