What does mmHg mean in compression socks?

Compression sock packaging displays a number: 23-32 mmHg, for example. But what does that actually mean, and how important is it? In this article, we explain what mmHg is, how compression classes work, and why the highest number doesn't automatically indicate the best sock. Clear and without a physics lesson.

mmHg is a unit of pressure

Justin, physiotherapist and co-founder of KINEX, summarizes it simply: "mmHg is the unit in which pressure is expressed. You might know it from a blood pressure measurement. For compression socks, the number indicates how much pressure the sock exerts, usually measured around the ankle." The abbreviation stands for millimeters of mercury; technically, one mmHg is approximately equal to 133 Pascals. So the number indicates pressure, not thickness, material, or how tight a sock feels.

Why the highest number doesn't tell the whole story

Justin warns against the most common misconception: "The higher the number, the firmer the compression usually is. However, the highest number doesn't tell the whole story. The pressure must also decrease in a controlled manner towards the calf. A sock that only feels very tight is not automatically a good compression sock." The pressure at the ankle is the reference point, but the profile above it also determines whether the sock does its job.

Compression classes vary by country

A class is a range, not a fixed number. In the widely used German RAL system, Class I is 18 to 21 mmHg, and Class II is 23 to 32 mmHg. But this classification is not internationally interchangeable: in the French system, for example, Class I is 10 to 15 mmHg, and the classic British Class I is 14 to 17 mmHg. Therefore, just the word "Class 2" is insufficient to know what pressure you're getting. A good manufacturer specifies both the mmHg range and the standard. To learn which class suits which purpose, read about the difference between Class 1 and Class 2 compression socks.

The label is not a personal guarantee

A labeled range is a controlled product characteristic, not a promise that every leg at every point will experience precisely that pressure. The actual pressure depends on your leg circumference, your leg shape, your posture, and how you put on the sock. According to Laplace's law, with the same material, more pressure is generated around a thinner ankle than around a wider calf. Research by Partsch and colleagues (2006, Journal of Vascular Surgery) showed that laboratory pressure and the actual worn pressure are strongly correlated but not identical. This makes the correct size so important; you can read how to determine it in our size guide.

The conclusion

mmHg tells you how much pressure a sock provides around the ankle, and the compression class places that within a range. But the combination of pressure, a controlled distribution, fit, and measurement control determines whether a sock does its job well. So don't just look at a large number on the packaging. If you want to know how that pressure is measured and controlled, read how compression is measured and controlled.

Perseverance wins. Not the highest number, but the right pressure in the right place.

Sources

Partsch et al. (2006), Journal of Vascular Surgery · Mosti & Partsch (2012), European Journal of Vascular and Endovascular Surgery · Rabe et al. (2018), Phlebology