Fast Weight loss

Tom's right, the proof is in how much weight change you see after a month of doing that.

If you want independent theory from us, you'd need to say how old you are, how tall, current weight, and some idea of activity level (exercise and daily life stuff like job/home both).

Or, take a look at that calculator's output. I ran my numbers, and it says I'd maintain at 2164 calories at very active (not far off on calories based on almost 8 years counting experience experience; maybe a couple of hundred calories low, but I'm weird). It says I'd need to eat 1299 calories to "lose weight fast".

If I subtract 1299 from 2164, I get 865 calories. That would be their projected calorie deficit for me. Using the assumption that 500 calories of deficit roughly equates to a pound a week weight loss on average, the implication is that they think I'd lose 865 divided by 500 calories, or 1.73 pounds a week. Do the equivalent arithmetic with your values from the calculator, and you can see how fast that calculator projects you'd lose at the "fast loss" calorie goal.

Only your month of experience will tell you whether you individually match its estimate. (Compare your weight at the same relative point in at least 2 different menstrual cycles, if you have those.)

I'd lose like a house afire on 1299 gross calories as a 5'5', 133-pound, 67 year old woman - probably faster than that estimate - but I'm mysteriously a good li'l ol' calorie burner for my demographic, as it turns out.

One cautionary comment: All of us want to lose weight fast. (I did too, when I was obese.) But losing a meaningful total amount of weight, like 10+ pounds let alone 50+, is going to be a long term thing, not a short project with a quick end date. It'll take weeks to months, even at the fastest remotely tolerable rate.

When I joined MFP along the way to losing from class 1 obese (183 pounds), MFP suggested 1200 + exercise calories to me, so not much different a recommendation. Personally, I felt great at first, not hungry, good energy . . . then a few weeks out, I suddenly hit a wall. I felt weak and fatigued. It took multiple weeks to recover, after I started eating more. I suspect some hair thinning a few weeks later was another consequence. There can be worse health consequences - I was lucky. Fast weight loss isn't necessarily a great idea.

Sometimes a slower weight loss rate that we can stick to happily for a long time will get us to goal weight in less calendar time than a fast loss rate that includes periodic bouts of compensatory "cheat days", longer breaks in the action, or even giving up altogether. As a bonus, slower loss can help a person experiment, find, and groove in habits that they can use long term happily to stay at a healthy weight permanently.

I'm not saying bad things will for sure happen if you go for a fast rate - the only way to find out is to try it. It's a higher risk path for both health and long-term success, though, IMO.

Wishing you success, however you decide to proceed.