# Body Fat Calculator

- **URL:** https://codeasystem.com/calculators/health/body-fat-calculator/
- **Category:** health
- **Description:** Estimate body fat percentage with the US Navy circumference method using height, neck and waist measurements (plus hips for women).
- **Primary output:** Estimated body fat (US Navy): 9.8%

## Inputs
- Sex (name: `sex`, type: select)
- Height (name: `height`, type: number, example: 180)
- Neck circumference (name: `neck`, type: number, example: 38)
- Waist circumference (name: `waist`, type: number, example: 85)
- Hip circumference (name: `hip`, type: number, example: 95)

## Outputs
- Estimated body fat (US Navy): 9.8%
- Category: Athlete
- Hip measurement: Ignored; used for the female formula only

## Formula / methodology
```
Men:   %BF = 495 / (1.0324 − 0.19077·log₁₀(waist−neck) + 0.15456·log₁₀(height)) − 450
Women: %BF = 495 / (1.29579 − 0.35004·log₁₀(waist+hip−neck) + 0.221·log₁₀(height)) − 450
(circumferences in inches)
```

The Hodgdon & Beckett (1984) circumference equations, adopted by the US Navy, predict body density from log-transformed tape measurements taken at standardized sites, then convert density to fat percentage via the Siri-equivalent 495/D − 450 relation.

Logically, a bigger waist raises estimated fat while thicker necks (muscular, not fatty tissue) lower it. Measurements must follow the same landmarks each time for trend-tracking to mean anything.

## Assumptions & limitations
- Adults with typical fat distribution; accuracy degrades at extremes.
- Typical error versus DEXA is ±3–4 percentage points.
- Hip field exists for both sexes but enters only the female equation.

## How to use
1. **Measure with a flexible tape**; Neck below the larynx; waist at the navel for men, narrowest point for women; hips at the widest point (women).
2. **Keep the tape snug, not tight**; Parallel to the floor without compressing skin; repeat each measurement twice and average.
3. **Track the trend, not the decimal**; Monthly remeasurements reveal direction reliably even where absolute precision is ±3%.

## Example
A 180 cm man with a 38 cm neck and 85 cm waist measures around 9.8% body fat by the Navy method; the athletic end for his frame.

Result for these inputs:

```
Estimated body fat (US Navy): 9.8%
```

## About this calculator
### Why circumference beats BMI for composition

Because tape measures respond to where fat actually sits, the Navy method distinguishes a muscular waist-thick torso from a soft one far better than weight-for-height ratios ever can. It costs nothing and takes ninety seconds.

### Reading your number honestly

Essential fat sits around 2–5% for men and 10–13% for women; athletic populations commonly measure 6–13% and 14–20%; average healthy ranges extend to roughly 24% and 31% respectively. Lower is not better once essential fat is approached.

### Improving measurement reliability

- Same time of day, ideally mornings before food.
- Same measurer or careful mirror positioning each session.
- Average of two or three takes per site, recorded verbatim.

## FAQs
### Why do men not need the hip measurement?

The male regression uses only waist and neck because male fat pattern concentrates centrally; female equations incorporate hips to capture the gluteofemoral depot. The hip box stays visible so women have everything in one form.

### How close is this to a DEXA scan?

Group-level correlation is good (r ≈ 0.8–0.9) but individuals can differ from DEXA by 3–4 points either way; use it for trends.

### Can I be too lean?

Yes. Sustained measurement near essential-fat levels disrupts hormones, immunity and (in women) menstrual function. Performance suffers before health collapses; reverse course early.

> **Disclaimer:** This calculator is for general educational purposes only and does not constitute professional advice.

## Related calculators
- [BMI Calculator](https://codeasystem.com/calculators/health/bmi-calculator/)
- [Waist-to-Height Ratio Calculator](https://codeasystem.com/calculators/health/waist-to-height-ratio-calculator/)
- [Ideal Weight Calculator](https://codeasystem.com/calculators/health/ideal-weight-calculator/)
- [Weight & Mass Converter](https://codeasystem.com/calculators/conversion/weight-mass-converter/)

---
Last updated: 2026-08-23 · Version: 1.0.0 · [HTML version](https://codeasystem.com/calculators/health/body-fat-calculator/)