These four numbers show up constantly in fitness apps and health advice, usually presented as a single confident figure. Each one is actually a formula-based estimate with real limitations — useful as a reference point, not a precise measurement of your body. Here's what each formula actually calculates.
Body fat percentage: the U.S. Navy method
This is the most common method that doesn't require special equipment — it estimates body fat from a few circumference measurements (neck, waist, and for women, hips) plus height.
Worked example (male, height 178cm, neck 38cm, waist 85cm):
| Input | Value |
|---|---|
| Height | 178 cm |
| Neck | 38 cm |
| Waist | 85 cm |
| Estimated body fat | 16.4% |
Limitation: because it relies on circumference rather than directly measuring fat tissue, it can be noticeably less accurate for people who are very lean or very muscular — a highly muscular person can register a higher waist-to-neck ratio without carrying proportionally more fat, skewing the estimate upward.
Ideal weight: the Devine formula
Originally developed for medical dosing calculations, this formula has become a common shorthand for a weight range by height. It uses a base weight plus an addition per inch of height over 5 feet.
| Example | Height | Result |
|---|---|---|
| Male | 178 cm | 73.2 kg |
| Female | 165 cm | 56.9 kg |
Limitation: this produces a single number based on height alone — it doesn't account for frame size, muscle mass, or body composition. A muscular athlete can weigh well above this figure while being lean and healthy; the formula was never designed to be a target for that population.
Protein intake: grams per kilogram of body weight
The factor depends on activity level and goals — sedentary adults typically need less than athletes doing regular strength training.
Worked example: a 70kg person at a moderate-activity factor of 1.6 g/kg:
| Calculation | Result |
|---|---|
| 70 kg × 1.6 g/kg | 112 g of protein/day |
Common reference ranges: roughly 0.8 g/kg for sedentary adults (a general baseline, not a target for anyone trying to build or maintain muscle), 1.2–1.6 g/kg for moderately active people, and up to 2.0–2.2 g/kg for intense strength training — though individual needs vary and this isn't medical advice.
Water intake: body weight plus activity
Worked example: a 70kg person doing 30 minutes of exercise:
| Component | Calculation | Result |
|---|---|---|
| Base (body weight) | 70 × 0.033 | 2.31 L |
| Exercise adjustment | (30 ÷ 30) × 0.35 | 0.35 L |
| Total | 2.31 + 0.35 | 2.66 L |
Limitation: this only estimates fluid from drinking — a meaningful portion of daily water intake normally comes from food, and factors like climate, individual sweat rate, and health conditions all shift real needs beyond what a simple weight-based formula can capture.
The common thread: these are estimates, not diagnoses
Every formula here trades precision for practicality — none of them require lab equipment or a clinical visit, which is exactly why they're useful as a starting reference point. But that same simplicity means none of them account for individual variation in muscle mass, frame, metabolism, or health conditions. Treat the output as a reasonable estimate to track over time, not a precise or diagnostic number.
Try the calculators
Each formula above has a dedicated calculator: the body fat calculator, ideal weight calculator, protein intake calculator, and water intake calculator. For total daily calorie needs, see the calorie calculator and our BMR vs. TDEE guide.
Frequently asked questions
How accurate is the Navy body fat method?
It's a reasonable estimate for most people, typically within a few percentage points of more precise methods like DEXA scans, but it can be less accurate for very lean or very muscular individuals since it relies on circumference measurements rather than directly measuring fat tissue.
Is there really one "ideal" weight for a given height?
No — ideal weight formulas produce a single reference number, but they don't account for muscle mass, frame size, or body composition. A muscular person can weigh well above the formula's estimate while being lean and healthy.
How much protein do I actually need per day?
It depends on activity level and goals. A common range is roughly 0.8 g/kg for sedentary adults, 1.2-1.6 g/kg for moderately active people, and up to 2.0-2.2 g/kg for those doing intense strength training, though individual needs vary.
Is the 8-glasses-a-day water rule accurate?
It's a rough rule of thumb, not a precise target. Actual water needs scale with body weight and activity level, and a meaningful portion of daily water intake also comes from food, so the right amount varies significantly by person.