You can get a very good estimate of your BMR from an InBody machine. My fitness studio has one, and lots of drs offices have them. It's not a difficult or expensive thing to do, and it doesn't require extensive testing. You stand on a scale and hold some electrodes. You get a printout with your BMR as well as an estimation of your % body fat and the amount of muscle mass you have. It's not perfect, but it perfectly fine for most people.
Knowing your BMR is half the battle, IMO, because I would gather that most people have absolutely no clue what their caloric intake needs are and end up swinging too far one way or the other (I know just as many women who chronically UNDER eat as who over eat, and both are bad if you really want to change your body composition). The 2000 calories per day guideline (at least for women) is total garbage, unless you want to be overweight.
If you want to lose weight, the advice from actual experts in this area is to eat 10-20% less than your BMR. You get a little bit of wiggle room if you exercise, but most professionals in this area do not take calories burned from exercise as part of your BMR during weight loss. Why? Because most people grossly overestimate how much they exercise and how many calories they burn doing it. Those fitness trackers we all wear are great for tracking trends and habits, but they are not accurate when it comes to calories burned.
Step 2 is to change your eating habits, based on your BMR. I say change because you should start prioritizing protein. There are a few different rule of thumb recommendations on this: 1g for every lb you WANT to weigh (so if you are 150 lbs and want to weigh 130, eat 130 g of protein), at least 100 g per day (for women), or 40-50% if your total caloric intake (closer to 50%, if you are really serious).
The focus on protein is for two reasons: it keeps you from over-eating the other macros and it can help prevent muscle LOSS during the calorie deficit stage. You won't likely gain any muscle in a calorie deficit (there are a few exceptions, if you read that link I posted), but you can keep from losing significant muscle.
For most people, especially women, eating this much protein is a HUGE change in their habits. The remaining macros are split between fat and carbs, but because you're in a calorie deficit, it's going to inherently be low fat and low(ish) carb. Basically once you hit your protein goal, there just aren't a whole lot of calories left for other stuff.
Do this for 6-8 weeks, and you will see progress. The plan I've used steps the deficit down in two week increments. You also get a "double carb day" on day per week (meaning you eat 2x the number of carbs you eat the rest of the week but keep fat and protein the same). Depending on how much weight you want to lose, you may have to do a couple of different cycles.
Once you hit your goal weight, your BMR will have likely changed. If you weigh significantly less, your BMR will be lower. However if you are active, your maintenance window has a little more wiggle room - meaning you can eat more on some days and less on others. In maintenance, you shouldn't have to track fat or carbs at all as long as you are hitting your protein goal and not exceeding your total calories.
Like was said in a previous post you do this long enough and you "learn" what to eat. It is most definitely a lifestyle change. If you do want to gain significant muscle, this plan is not sufficient. You need to be in a calorie surplus to build muscle (in which you gain fat), and then you will need to be on a calorie deficit to lose the fat. Most people seriously committed to this lifestyle understand that building significant muscle takes YEARS and a very consistent presence in the gym. Like being there 5 days a week lifting HEAVY.
I light weights 3 days a week for a total of about 3 hrs (the other days I do cardio). My lower body is very strong (I can hip thrust 305 lbs). But I am by no means "shredded". I do look muscular, but 3 days a week isn't enough to get me quite where I would prefer to be from an aesthetic standpoint.
I will add a couple of things I have noticed - tracking calories is KEY. If I do not pay attention to what I eat (and how much I eat) I WILL overeat. Throw in something like alcohol, and your ability to hit your calorie goals goes out the window. Personally, even after several years, I still measure a lot of my food. It's really not a big deal or as onerous as people think it is. I have a scale in my kitchen and just weigh a portion into whatever container or plate I'm eating from.
Most Americans also don't realize how small the portion size on the nutrition label really is. Until you make this connection in your brain, you will also continue to overeat. A typical serving size of just about any processed/packaged food is 30 g, and that is NOT a lot of good. This will hit home when you decide you want some nuts or peanut butter and go to weigh out the portion.
Also, the maintenance window is probably not as wide as we would all hope. I've been doing this long enough to know that if I eat at the high end of my maintenance window all the time, eventually I will gain weight.
If you have significant weight to lose and keto works for you (because it keeps you from over eating foods that you have a real problem with consuming in moderation), then by all means go keto. The goal is to lose weight, so do whatever it takes.
Where a lack of calorie and protein tracking will become an issue is when you start to approach or hit your goal weight. If you want to gain muscle (and in particular for women, not be "skinny fat"), you have to put in more effort by tracking your protein intake, limiting total calories, and lifting heavy. Keto alone will only get you so far.
I suspect men have more "success" on keto because inherently most men have a higher BMR and higher amount of muscle mass than most women. So they are starting with a bit of an advantage. But they will eventually hit the same plateau as well. Also, as you lose fat, muscles become more visible, mistakenly leading people to believe they "gained" muscle. No, the muscle you already had just isn't covered by fat.
If you lose weight and are cool with that plateau, that's a perfectly great place to be, too. But you still have to be mindful of BMR because, as we age, even keeping all things the same, we lose muscle mass. So, you have to be aware of your BMR and how many calories you are consuming in relation to it if you want to keep the weight off for the LONG TERM (ie, the rest of your life).
I will also say that the information above applies to ALL diets - macro counting, keto, carnivore, vegetarian, and even pharmaceutically-driven weight loss (semaglutide). Don't get me started on how unethical it is to put unhealthy/overweight people on a drug that makes them eat significantly less and not give them information on how to optimize the calories they are eating. Semaglutide alone is a starvation diet. I've watched a good friend swing from one eating disorder (binge eating) to another (grossly under eating). Her body is literally eating itself because she is not consuming enough calories. But, hey, at least she's not obese.
I realize this is a really long post, but I hope it's enough to finally debunk this myth that somehow keto and carnivore diets are "healthier" or "better" for "burning fat and "building muscle". Because they are not. At best, that's misleading, and worst, it's flat out not true.
