There's a simple formula for finding your ideal weight:
1. Measure your height in cm.
2. Measure your weight in kg.
3. Take measurements of the circumference of your neck, wrists, and ankles.
The next couple of steps take a little bit longer:
4. Gain or lose weight as needed until you are happy with the way you look, the way you feel, and your general health.
5. Weigh yourself again.
6. That's your ideal weight!!!
This question goes around fitness circles every so often - 'what should I weigh?' It's an interesting, but silly question.
Why is it silly? Because it presupposes some perfect weight that a person should reach independent of... everything else.
Imagine you are very healthy, very pleased with your appearance (the right amount of fat and muscle that makes you think you look good, whatever that is). Suppose also that you are happy with your physical performance - you can jump the way you want, move smoothly the way you want, you don't get excessively tired, etc.
Now imagine that someone says, "oh, sorry, you should way 10kg more. Or 10 kg less. You're not at your ideal weight."
What would you do? Starve yourself to lose weight - keep in mind that you'd be moving AWAY from an appearance that you like. Or overeat to bulk up - again, moving away from your ideal?
I think people ask this question most often when they're UNHAPPY with their appearance, health, and/or performance, and want a target to shoot for. They weigh, say, 200 lbs, and want to be able to say something like, "if I can only get to 150 lbs, that would be ideal, so I have exactly 50 lbs to lose."
I can see the appeal in having a target, but it's not really useful. Imagine some expert determines that your ideal weight is 160 lbs. Then you diet and exercise (in a healthy way) and get to 160, and you are still kind of too fat (by your own standards). Let's assume you're not anorexic; maybe you have lighter bones than the 'expert' anticipated. Or your doctor does bloodwork and testing and says you still have too high a bodyfat percentage. What do you do, stop losing weight because you're already at 'ideal'? Of course not. And the opposite is also true - suppose that expert says your ideal is 160, and you hit 170 but you're ripped and feeling great and your doctor says you're in awesome shape. Do you keep trying to lose more weight?
I bet almost everybody already knows if they need to gain, lose, or maintain their weight (I'm assuming you're a regular person, not an actor, model, physique competitor, or competitor in a weight class sport - those guys all have unique considerations and might need expert advice regarding their weight). If you need to lose weight, try to lose weight (as long as you can do it in a healthy way). Reassess every so often (weekly at most). If you're prone to body dismorphia (like anorexia), be super careful to get some objective measures (like bodyfat analysis or the advice of a trusted friend) along the way. Once you are where you want to be, you're good! The same goes for those of you who might need to gain weight.
Don't fall into a paralysis by analysis trap of spending time and energy defining a target while you aren't working to get there.