What Is Hydrostatic Weighing?
Hydrostatic (underwater) weighing is based on Archimedes' principle: a submerged body displaces an amount of water equal to its volume. Because fat has a lower density than water (0.9 g/cm³), while muscles and bones — a higher (> 1.0 g/cm³), body density directly reflects body composition.
The athlete is weighed dry and underwater (having fully exhaled air). From the weight difference and water density, body volume and density are calculated, and from these — body fat percentage (using the Siri or Brozek equations). Accuracy: ±1-2%.
📊 Siri and Brozek Equations (Body Fat % Calculation)
Once body density is determined (TD = body mass / body volume), body fat percentage is calculated:
| Formula | Equation | Notes |
|---|---|---|
| overview | Body fat % = (4.95 / TD - 4.50) × 100 | Most commonly used |
| overview | Body fat % = (4.57 / TD - 4.142) × 100 | Alternative formula |
| TD example | TD = 1.05 → Siri: 21.4%, Brozek: 21.1% | Person of average activity |
| TD example | TD = 1.08 → Siri: 8.3%, Brozek: 8.1% | Very lean athlete |
How the Measurement Is Performed
Hydrostatic (underwater) weighing has long been considered one of the most accurate methods of body composition. A person is weighed in air and fully submerged underwater, and body volume and fat proportion are calculated from the difference in weights.
The procedure requires maximum exhalation underwater and several attempts, so it is not the most convenient for everyday use. Nevertheless, the method is valued for its good accuracy and long served as a reference standard against which other methods are compared.
How to Interpret the Result
The result shows body fat percentage and is considered fairly accurate, but also evaluate it according to age and sex tables. The method measures whole-body density, so residual air in the lungs or intestines can affect the result.
As always, the most important thing is the change in the result over time, not a single number. Since the procedure requires special equipment, measurements are usually taken less often, so the trend is worth supplementing with other indicators.
Common Mistakes
- Incomplete exhalation — air remaining in the lungs distorts the volume measurement.
- Movement underwater — difficult to weigh accurately.
- Fear or tension — prevents performing the test calmly.
- Eating before measurement — gases in the intestines change the result.
When the Result Is Unreliable
The result is misleading if you did not exhale fully, if measured immediately after a meal, or if the test was performed superficially. This method is often replaced today by more convenient ones, such as Bod Pod or DEXA — you will find the comparison in body composition test.
🙋 Frequently Asked Questions
What makes this method special? It measures whole-body density and has long been the reference standard against which other methods are compared.
Is it difficult to perform? The procedure requires maximum exhalation underwater and may be uncomfortable, so more convenient methods are often chosen today.
How often should you measure? Due to special equipment, measurements are taken less often; supplement the trend with other indicators.
Why is this method used less often now? It is accurate but requires special equipment and is uncomfortable, so in practice faster methods are more often chosen.
Can the result be compared with DEXA? The values may differ slightly, so it is better to track changes using the same method rather than compare different ones.
Although the method is accurate, in practice simpler methods are sufficient for most people — the most important thing is to choose one method and stick to it so that you can correctly compare results over time.
Before the measurement, avoid carbonated drinks and heavy food, because gases in the digestive tract can distort the body volume measurement. If you feel tense, ask the specialist to explain the procedure in advance — calmness helps perform the test more accurately.