Ideal Weight, Body Fat & Water Intake: How to Calculate Your Real Body Needs
Three of the most-searched body metrics, each with a real formula behind it and a worked example — no guesswork required.
For a 175 cm (5'9") male: ideal weight (Devine formula) ≈ 70.5 kg; body fat (U.S. Navy method, 85 cm waist / 38 cm neck) ≈ 16.9%; water intake at 75 kg ≈ 2,475 ml/day. Each uses a different, well-established formula.
Enter your own measurements in our ideal weight, body fat & water intake calculator.
Ideal weight, body fat percentage, and daily water needs are three different questions with three different formulas — despite often being lumped together as "body stats." None require special equipment, and all three can be estimated at home in a couple of minutes.
Ideal Weight — the Devine Formula
Originally developed for medication dosing, the Devine formula has since become a widely used general reference point for "ideal" weight based on height alone. It says nothing about frame size or muscle mass, so it's best treated as a reference range, not a target.
Example: A 175 cm (about 68.9 inches) man: 50 + 2.3 × (68.9 − 60) = 50 + 20.5 = 70.5 kg.
Body Fat — the U.S. Navy Method
This method estimates body fat from waist and neck circumference (plus hip for women) and height — the same approach the U.S. Navy uses for body composition screening, since it only needs a tape measure. It's an estimate, typically within a few percentage points of more precise methods like DEXA scans.
| Measurement | Where to measure |
|---|---|
| Waist | Narrowest point |
| Neck | Narrowest point, just below the larynx |
| Hip (women only) | Widest point |
Example: A 175 cm man with an 85 cm waist and a 38 cm neck comes out to roughly 16.9% body fat, landing in the "Fitness" category.
Worth knowing: Measuring at the wrong spot — a few centimeters off on waist or neck — is the most common source of error with this method. Take your time and measure at the exact narrowest/widest points described above.
Water Intake — A Weight-Based Guideline
This is a commonly cited starting point for a sedentary adult in a temperate climate, not a precise medical requirement. A 75 kg person works out to about 2,475 ml/day (roughly 10 cups). Exercise, heat, altitude, and illness all increase actual needs, and some daily water intake also comes from food.
Body Fat Ranges for Reference
| Category | Men | Women |
|---|---|---|
| Essential fat | 2-5% | 10-13% |
| Athletic | 6-13% | 14-20% |
| Fitness | 14-17% | 21-24% |
| Average | 18-24% | 25-31% |
| Above average | 25%+ | 32%+ |
Other Ideal Weight Formulas Compared
Devine isn't the only formula in use — several others were developed for similar clinical purposes and give slightly different results for the same height.
| Formula | Men (base + per inch over 5') | Women (base + per inch over 5') |
|---|---|---|
| Devine (1974) | 50 kg + 2.3 kg | 45.5 kg + 2.3 kg |
| Robinson (1983) | 52 kg + 1.9 kg | 49 kg + 1.7 kg |
| Miller (1983) | 56.2 kg + 1.41 kg | 53.1 kg + 1.36 kg |
| Hamwi (1964) | 48 kg + 2.7 kg | 45.5 kg + 2.2 kg |
For the same 175 cm (68.9 in) man, these four formulas produce results ranging from roughly 68 kg to 73 kg — a reminder that "ideal weight" is a reference range built from population averages, not a single precise number your body is required to hit.
Body Fat Measurement Methods Compared
| Method | Equipment needed | Typical accuracy |
|---|---|---|
| U.S. Navy (tape measure) | Tape measure only | Reasonable estimate, ±3-4% |
| Skinfold calipers | Calipers, trained technician | Good, ±3-5% depending on skill |
| Bioelectrical impedance (BIA, smart scales) | BIA scale or handheld device | Variable, sensitive to hydration level |
| DEXA scan | Clinical/lab equipment | Very accurate — often used as the reference standard |
The Navy method's appeal is that it needs nothing but a tape measure and produces a result close enough to more expensive methods for general tracking purposes — DEXA and hydrostatic weighing are more precise but far less accessible for a quick, at-home check.
When You May Need More Water Than the Baseline
The 33 ml/kg guideline is a starting point for a sedentary adult in a temperate climate — several common situations push actual needs meaningfully higher:
| Situation | Effect on water needs |
|---|---|
| Regular exercise or physical labor | Higher — sweat losses add on top of the baseline |
| Hot or humid climate | Higher — increased sweat rate even at rest |
| High altitude | Higher — increased respiratory water loss |
| Pregnancy or breastfeeding | Higher — increased physiological demand |
| Illness with fever, vomiting, or diarrhea | Higher — active fluid loss |
A simple practical check alongside any calculated target: pale yellow urine generally indicates adequate hydration, while consistently dark yellow suggests increasing intake.
Frequently Asked Questions
What formula calculates ideal body weight?
The Devine formula: 50 kg + 2.3 kg per inch over 5 feet for men, 45.5 kg + 2.3 kg per inch over 5 feet for women. It's a long-standing clinical reference point based on height alone.
How is body fat percentage calculated without special equipment?
The U.S. Navy method estimates body fat from waist and neck circumference (plus hip for women) and height, using only a tape measure — no calipers or scans needed.
How much water should I drink per day?
A commonly cited guideline is about 33 ml of water per kilogram of body weight per day — actual needs increase with exercise, heat, and altitude.
Is the Devine formula the only way to estimate ideal weight?
No — other formulas like Robinson, Miller, and Hamwi exist and give slightly different results. Devine is one of the most widely used, originally developed for medication dosing calculations.
Are these calculations a substitute for a doctor's assessment?
No — all three are general estimates for informational purposes, not medical advice. Body composition and hydration needs vary by individual factors a formula can't capture.
