If you're pretty new to the fitness game, or if cardio was never really your thing, you might be pretty clueless when it comes to what is best.
The treadmill or the elliptical?
The rower or the stairmill?
In the morning or in the evening?
Fasted or after meals?
High intensity intervals or steady state?
There is no one size fits all answer.
The best cardio that you can do for weight loss is the cardio that GETS DONE.
At the end of the day, the formula for weight loss is calories burned > calories consumed=weight loss.
The key to achieving weight loss is to put your body in a calorie deficit. YOU HAVE TO BURN MORE THAN YOU TAKE IN.
Any cardio you do works toward that goal; it will burn calories.
So find what fits your life and what you ENJOY (if there is any form you can say you truly enjoy!).
If it is not at all feasible to fit in a half hour sweat session, break it into 10 minute increments. Sneak in a quick 10 minutes before your shower in the morning, maybe 10 before dinner and 10 before you go to bed. However it works for your schedule.
If you'd rather get it all done and over with, get up a little earlier and do all twenty or thirty minutes.
The result will still be the same amount of calories burned whether or not you burned them all at the same time or throughout the day.
Another factor to consider is intensity. You do NOT have to push the duration to the max if your intensity is dialed up.
For example: walking thirty minutes at a brisk pace is one option for days when you have ample time. A day you are pressed for time, you would want to crank up the intensity to make up for the shorter duration; ten minutes of all-out challenging sprints with minimal recovery time between them would be an example of this.
I see A LOT of people wasting their time going at the same pace every day while reading a magazine on the elliptical.
"But, I got in forty-five minutes of cardio every day this week and I don't see ANY difference!"
Right. You might now know more about the Kardashians but you would still get winded chasing after a garbage bag fluttering across a parking lot.
It has to be challenging! It doesn't mean fast, but it means INTENSE.
For example, a day when I do have thirty minutes to spare, I like to do power walking on the treadmill with the incline as high as it goes for all thirty minutes. I also like to watch a show on Amazon on my phone because it makes it go by faster and is frankly, more enjoyable.
But that show does not distract me from pushing the intensity. I am sweating, I am breathing heavily, and I couldn't really carry on a conversation at the rate I am going.
If you are able to flip through a magazine while chatting with your friend next to you, can you honestly expect to see results after a few weeks of the same?
If you have very specific goals or are training for something specifically, you would need a much more targeted training program than the advice above. No, you should not train for a half marathon by doing 10 minutes of burpees 3x a week.
But if your goal is just overall fat loss for a healthier body composition, don't over-complicate it.
Pick something you enjoy or at least tolerate, and get it done when you can.