Let’s get this out of the way first—it’s entirely possible to create a calorie deficit and lose weight without yawn-provoking cardio which often feels like a terrible dream.

That’s exactly what this study found—not only did the resistance training group lost the same amount of weight but it also gained muscle in the process:

Cardio versus wight lifting for fat loss and muscle preservation

In fact, you should keep cardio to a minimum (if done at all) and instead, prioritize resistance training and let your diet do the fat burning part:

Now, just because I prescribe zero formal cardio for my clients (they’d probably cut my reproductive organs if I asked them to), doesn’t mean that you shouldn’t do it. Cardio is fine if:

You’re a petite female who’s already on a low caloric intake:
Reducing food intake even further wouldn’t be realistic for you because it would require a very low caloric intake that would make dietary adherence challenging. The liquid lettuce diet doesn’t sound enticing, does it?
You genuinely enjoy cardio and it contributes positively to your life:
But be careful not to overdo it. There’s a slippery slope to slaving away on the Stairmaster for the sole purpose of burning calories.
You’ve hit a weight loss plateau:
Only if progress stalls should you consider sprinkling in some cardio. Though I’d still adjust resistance training or daily steps before considering the addition of cardio (my reproductive organs, remember?).

There’s a serious challenge to the laws of probability but let’s say you found yourself in one of the above—you need or want to do cardio.

How should you go about adding cardio to your weight loss plan? Should you do cardio before or after weights?

Further reading: Fat Loss Fundamentals (And Shit That Can Just Fuck Off)

The Best Option: Do cardio on rest days

If you lift weights up to four times per week, you should be able to squeeze in one or two cardio sessions on rest days.

That’s your best option because the more cardio you do, the more it interferes with strength and muscle gains from resistance training, as this study found:

Cardio interference with resistance training

And since resistance training is your top priority, it’s better to space cardio and resistance training sessions by placing them on separate days. That way you will recover well from both.

The Second Best Option: Space cardio out as far from resistance training session as possible

If you have a hill of free time (which you probably don’t) and you can train twice a day, you could do one session, say, resistance training, in the morning and one—cardio, in the evening.

You should have at least 6 hours of recovery between the sessions, as this study concluded:

Specific Training Effects of Concurrent Aerobic and Strength Exercises Depend on Recovery Duration

If you’re going to ignore the 6-hour recovery window, keep in mind that there will likely be some sacrifice to strength or muscle gains/preservation while dieting.

The Third Best Option: Do cardio after weights

If you can’t do cardio on rest days or two sessions in one day because you’re busy smoking “suspicious-looking cigarettes” aka weed or something, do cardio after weights. In that particular order.

Doing cardio before resistance training can result in lower intensity and volume, as this study shown:

Effects of Aerobic Exercise on Strength Performance Following Various Periods of Recovery

If you wonder why, try doing cardio when you’re tired. It sucks but nothing that would make you barf up your toenails.

Then, try lifting weights when you’re tired. Now that sucks balls. It’s an inescapable hell. You will sink to your knees and beat your confused head on the squat rack wondering where did your strength go.

For this reason, I would always do cardio after weights if they are done on the same day.

Oh! One last thing—don’t do cardio and resistance training workouts using the same muscle groups because the previous study also noted that:

Effects of Aerobic Exercise on Strength Performance Following Various Periods of Recovery 2

Meaning, training lower body and then doing cardio on a spin bike or Stairmaster just isn’t the best idea. It adds even more fatigue to already fatigued quads which makes recovery an issue.

Application

Cardio should not be a huge part of your weight loss strategy. Your diet, non-exercise activity thermogenesis (calories expended for walking, fidgeting, typing, etc.), and TIME can control the calorie deficit part. So do as little cardio as possible.
If you have to or want to do some cardio, do it on rest days or space it out as far from the RT session as possible.
In the worst-case scenario, do cardio after weights and make sure both involve different muscle groups.