Each health tool here answers a narrow question, and none of them examines you. They take two or three numbers and apply a formula derived from population studies. That is genuinely useful as a starting point and genuinely useless as a diagnosis, and the distinction matters more here than anywhere else on the site.

BMI compares your weight to your height. It knows nothing about muscle, fat distribution or fitness.
TDEE estimates how many calories you burn in a day, from your resting rate plus an activity multiplier you choose yourself.
Water intake scales a clinical rule of thumb to your body weight.
Ideal weight applies formulas written in the 1970s for drug dosing, not for aesthetics.

The limitation they share

All four were derived from population averages, which means they describe a typical person rather than you. A formula fitted to thousands of people will be wrong for any given individual, sometimes substantially. Muscular builds, very short or very tall frames, pregnancy, childhood, and a range of medical conditions all break the assumptions — and the formula has no way of telling you it has gone wrong. It returns a confident-looking number either way.

Use them for direction and change over time, not for a verdict on a single day. A BMI drifting upward across a year tells you something. A BMI of 25.3 this morning does not.

If you only read one thing

The guide on what BMI gets wrong is the most useful page in this section, because BMI is the number people take most seriously and the one that misleads most often. TDEE explained covers the activity-multiplier trap, which is where nearly every calorie estimate goes astray, and the hydration guide deals with where "eight glasses a day" came from.

None of this is medical advice. If a result concerns you, a professional who can actually examine you is worth more than any calculator.