How do you know how many calories and fat grams you need to have in one day?

Im not sure on the fat grams part but I belive for calories to maintain current weight you should take your weight x 10 then add about 200 more calories. For example a 180lb person needs about 2000. Now it also depends on your activity level. The same 180lb person could easily use another 500 calories to support their activities. Are you trying to lose, gain, or maintain?

I really just want to maintain my weight. I weigh about 123 or 124, about 5'6, and I exercise regularly.

Maintenance calories is closer to 12 or 15 x bodyweight. If you are active, (cardio and resistance training) closer to 15x bodyweight. If you actually plan on eating exact ratios of protein/carbs/fat, you might try 33/33/33 or 40/30/30. If you have no intention of calculating this every day, just make sure you get at least 1g protein per pound of body weight, a little healthy fat here and there and the rest complex carbs.

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Keep your body lean, your blood clean and your mind sharp. -Rollins

Thanks for all your replies. I had know idea how to figure out how much I needed.