Just like many other questions in the fitness world the answer is, “it depends.” Doing cardio before or after your strength training all depends on what your specific goals are. Cardio can take away some of the energy that your body has to do the strength part of a workout. Doing it before can lead to not lifting quite as heavy. So before determining when to do each activity you have to know what your goals are.
If your goal is to increase your endurance, or you are an endurance athlete (runner, swimming) you will want to do your cardio training before hand. The primary focus of your workouts is going to be that cardio part, since that is the goal of your training. Of course strength training is still important for you to do, however, your focus will most likely not be on squatting your max weight.
If your goal is to improve strength then you should do your cardio after your strength training. You do not want to deplete your muscle energy before you are able to do your actual lifts. The goal of your workout is probably to lift heavy and really challenge the weights. If you drain your muscle’s energy storage before even starting the workout then you will not be able to do that.
If your main goal is to lose weight you should actually do your strength training first. Muscle burns more calories at rest, and strength training will build more muscle than cardio. Cardio is a great tool for weight loss, but try doing a few exercises that focuses on building muscle and strength before hopping onto the treadmill.
If you just want to improve your overall fitness then the timing of your cardio session is up to you! The best exercise for just overall fitness is the exercise that you are going to do. If it is easier to get the cardio out of the way first then go for it.
Cardio is much more than just a way to burn calories. It helps condition your heart and lungs. It challenges your heart and lungs to work hard and get strong. Don’t skip it all together, but know when the best time is to get it done.