It is better to do cardio before your weight training under certain circumstances but ideally you should do your weight training before your cardio.
If your only goal is to improve your aerobic and cardiovascular conditioning or lower your blood pressure, it makes sense to put your top priority first. This is goal specific.
You can also get away with doing cardio first if you brought a pre-workout snack with you or intend on visiting the snack bar after you do your cardio. Still, you'll only want to do this periodically to get the best overall results.
Weight training at the intensity necessary to generate results requires adequate energy supply, unlike cardio. While cardio you can do on an empty stomach and still experience best results such as fat burning and endurance and cardiovascular improvement, weight training demands for immediate adequate energy supply to trigger the similar caliber of its' benefits.
If you do your cardio first without eating a good snack in between going to your weight training, it will be like lifting weights on an empty stomach because you will have depleted much of the available sugar, complex carbohydrates, and fat you need for an effective weight lifting session that will trigger the tone, definition, and strength gains you're going for.
The best results from weight training occur because during your workout and sets, you have the available energy your body needs to take the set to a point called overload. This is basically muscle failure with good form, where you do one or two more reps or a pound level of weight you never did before, and this can only be done effectively when complex carbohydrates are available. The reason why is because whether you realize it or not, by the time you approach failure on a set of an exercise, your body's heart rate will pass your "anaerobic threshold", which is approximately 80% of your max. When this happens, your body's energy use system automatically goes from primarily using simple sugars and fat to complex carbohydrates in order to keep going and performing.
Remember these extra reps you need to get to produce the best results can only be reached if you can perform anaerobically, or above that 80% heart rate level. So you must have fresh available complex carbohydrate energy in order to do this. It can be argued that ketosis can occur if you excessively train with weights at an anaerobic level on an empty stomach. However, you definitely will not be able to perform well enough to generate the muscle definition and sculpting or strength as you would with good fresh fuel available.
If you are going to do cardio before weights, be sure to pack a snack. An energy bar or protein bar is a quick easy meal you can use after you do and before moving onto weights. This way you still get the most effective lift possible. But if you're looking for the ideal approach, do your weights before your cardio. Not only is this the most effective way, but it also sets up for ideal fat burning on cardio.
When you've used up all your complex carbs on your weight training, and toned and defined your body to the max as a result by doing weights first, when you move on to cardio, all your body has to use as fuel is stored body fat, which for most people is the true purpose of it.
Subscribe to my page to get more monthly advice as you pursue your health and fitness goals this year.