MmHg stands for “millimeters of mercury,” and it’s the unit used to measure how much pressure a compression sock applies to your leg. The number before mmHg tells you how tightly the sock squeezes. A 20 mmHg sock applies less pressure than a 30 mmHg sock, and that difference matters for both comfort and effectiveness. You’ll see this same unit on blood pressure monitors, where it measures the same thing: how high a column of liquid mercury would rise under that amount of pressure.
Why Mercury Is the Unit of Measurement
The measurement dates back to early pressure instruments that used actual columns of mercury in glass tubes. When pressure increased, the mercury rose. The height it reached, measured in millimeters, became the standard way to express pressure in medicine. Blood pressure cuffs, barometers, and compression garments all inherited this convention. Nobody is putting mercury in your socks. It’s simply the scale the medical world settled on, and it stuck.
What the Numbers Mean on the Package
Compression socks are sold in pressure ranges rather than single numbers. You’ll typically see a pair of values like 15-20 mmHg or 20-30 mmHg. The first number is the minimum pressure and the second is the maximum. In graduated compression socks (which is most of what you’ll find), the highest pressure sits at the ankle and gradually decreases as the sock moves up toward the calf. This design pushes blood upward against gravity, helping your veins return it to the heart and preventing pooling in the lower legs.
Compression Levels and Who They’re For
The mmHg range you choose should match what your legs actually need. Higher isn’t automatically better, and too much compression can cause problems if you have certain circulatory conditions.
15-20 mmHg (mild): The lightest medical-grade level. This range works well if you experience occasional leg tiredness after long stretches of sitting or standing, mild swelling during air travel, minor discomfort from early spider veins, or general fatigue in your lower legs by the end of the day. These are available over the counter and don’t require a prescription.
20-30 mmHg (moderate): The most commonly prescribed compression level. Consider this range if you deal with moderate swelling or fluid retention, visible varicose veins that cause heaviness or discomfort, post-surgical recovery where circulation support matters, demanding athletic training, or jobs that keep you on your feet for extended shifts. Many people find this level noticeably firmer than 15-20 and may need a few days to adjust.
30-40 mmHg (firm): A therapeutic level often used for more significant venous conditions or lymphedema. Socks in this range feel considerably tighter and are frequently paired with flat-knit designs for better shape control. Most people at this level are working with a healthcare provider.
40-50 mmHg and above (very firm): Reserved for severe lymphedema, significant tissue changes, or cases where lower levels haven’t worked. These are only used after a clinical assessment.
US vs. European Compression Classes
If you shop internationally or read medical literature, you’ll encounter compression “classes” labeled CCL 1 through CCL 4. These follow standards set by organizations like Germany’s RAL Quality Mark Association. The classes map to specific mmHg ranges:
- Class I: 18-21 mmHg
- Class II: 23-32 mmHg
- Class III: 34-46 mmHg
- Class IV: Above 49 mmHg
In the US, there’s no single standardized scale. Cleveland Clinic groups compression simply as low (under 20 mmHg), medium (20-30 mmHg), and high (above 30 mmHg). The takeaway: always check the actual mmHg number on the packaging rather than relying on words like “medium” or “firm,” which can vary between brands.
How Compression Socks Use That Pressure
The mmHg rating describes how much the sock squeezes your tissue, and that squeeze does real mechanical work. By compressing the veins in your lower legs, the sock narrows them slightly. This speeds up blood flow back toward the heart, reduces the backward leaking that happens with weakened vein valves, and limits the fluid buildup that causes swelling. Compression also helps move lymphatic fluid along, which is why it’s a standard treatment for lymphedema alongside venous conditions like chronic venous insufficiency and varicose veins.
Getting the Right Fit
The mmHg printed on the package only delivers its intended pressure if the sock fits your leg correctly. A sock that’s too large won’t compress enough. One that’s too small can compress unevenly or create uncomfortable pressure points.
To measure, you need three numbers. First, wrap a tape measure around your heel and ankle bend and record the circumference. Next, measure around the widest part of your foot at the ball. Finally, measure from the back of your heel to that widest point. For knee-high socks, you’ll also need your calf circumference at its widest and the distance from the floor to the bend of your knee. Take these measurements first thing in the morning before any swelling develops, since leg size naturally increases throughout the day.
When Compression Socks Lose Their Pressure
Compression socks don’t last forever. The elastic fibers that create the rated mmHg pressure gradually break down with wear and washing. If you wear your socks daily, expect them to maintain their therapeutic pressure for roughly three to six months. After that point, they start to loosen, and the mmHg you’re actually getting drops below what the label says. The simplest test: if the socks slide down more easily than they used to, or your legs feel the way they did before you started wearing compression, it’s time for a new pair.
A Note on Peripheral Artery Disease and Diabetes
Compression socks are not safe for everyone at every pressure level. People with peripheral artery disease (PAD) already have reduced blood flow to their legs, and adding external pressure can make that worse. Research published in BMJ Open Diabetes Research & Care found that mild to moderate compression (up to about 32 mmHg) can be used in some patients with diabetes or PAD, but only under clinical supervision with specific screening. If you have diabetes, PAD, or any condition that affects circulation in your legs, get fitted and monitored by a provider rather than choosing compression on your own.