Compression socks are rated in millimeters of mercury (mmHg), the same unit used to measure blood pressure. This number tells you how much pressure the sock squeezes against your ankle, and it’s the single most important spec when choosing a pair. Ratings typically range from under 15 mmHg for light support up to 40 mmHg or higher for medical-grade compression.
What mmHg Actually Means
The mmHg rating on a package refers to the pressure the sock exerts at the ankle, which is where compression is strongest. Graduated compression socks are designed so that pressure decreases as the fabric moves up toward the calf and knee. This gradient is what helps push blood back up toward your heart rather than letting it pool in your lower legs.
Manufacturers determine these pressure values through laboratory testing on standardized leg models. The rating you see on the label reflects the amount of squeeze at the narrowest part of the ankle. A sock rated 15-20 mmHg, for example, delivers between 15 and 20 mmHg of pressure at the ankle and progressively less as it rises up the leg.
Common Compression Levels
Compression socks fall into a few broad categories based on their mmHg range. Here’s what each level is designed for:
- 8-15 mmHg (light compression): The gentlest option, often sold as “support hosiery.” These are popular for minor leg fatigue, long days of standing, or casual travel. No prescription needed.
- 15-20 mmHg (moderate compression): The most widely purchased over-the-counter level. Commonly used for mild swelling, leg discomfort during pregnancy, long flights, and everyday wear by people on their feet all day.
- 20-30 mmHg (firm compression): Considered the first medical-grade level. This range is frequently recommended for moderate swelling, varicose veins, recovery after minor vein procedures, and prevention of blood clots during travel or after surgery. Many retailers sell these without a prescription, though some insurance plans require one for reimbursement.
- 30-40 mmHg (extra firm compression): Used for more serious venous conditions, significant swelling, and active venous ulcers. Compression above 30 mmHg is recommended for ulcer healing. This level typically requires a prescription or medical guidance.
- 40-50 mmHg and above: Reserved for severe venous disease, severe lymphedema, or post-thrombotic syndrome. Always prescribed and fitted by a healthcare provider.
Why Ratings Vary by Country
There is no single worldwide standard for compression classes, which can cause real confusion if you’re comparing products from different countries. European nations each maintain their own classification systems, and the class numbers don’t line up with each other. In the French system, Class I covers light compression from 10 to 15 mmHg. In the English system, Class I starts at 14 to 17 mmHg. The German system sets Class I even higher, at 18 to 21 mmHg.
This means a “Class II” sock from France delivers roughly the same pressure as a “Class I” sock from Germany. If you’re shopping internationally or reading European product labels, ignore the class number and look at the actual mmHg range printed on the package. That number is consistent regardless of which country’s classification system the manufacturer follows. In the United States, most products skip the class system entirely and just list the mmHg range directly.
How Sizing Affects the Rating
A compression sock can only deliver its rated pressure if it fits your leg correctly. Too large, and you’ll get less compression than advertised. Too small, and you may get more pressure than intended, which can restrict blood flow.
For knee-length socks (the most common style), you’ll need two measurements: the circumference of your calf at its widest point, and the length from the back of your heel to the bend of your knee. For thigh-high stockings, add the circumference of your upper thigh at the buttock fold and the length from heel to buttock fold. Take these measurements first thing in the morning before any swelling sets in, since that gives you the most accurate baseline.
Each manufacturer publishes a sizing chart that maps your measurements to a specific size. Don’t assume your size in one brand matches another. A medium in one product line can easily correspond to a large in another.
How Long the Rating Lasts
Compression socks lose their elasticity over time, which means the actual pressure they deliver gradually drops below the rated level. The elastic fibers in the fabric stretch out with repeated wear and washing, and heat from dryers accelerates the breakdown. Most sources recommend replacing your compression socks every 3 to 6 months if you’re wearing them daily. If you rotate between two or more pairs, each pair will last longer, but the elastic degradation still happens with time even if the socks look fine on the outside.
Washing in cool water and air drying rather than machine drying helps preserve the compression. If a sock that once felt snug starts sliding down your leg or feels noticeably looser at the ankle, it’s no longer delivering its rated pressure.
When Higher Ratings Can Be Harmful
More compression isn’t always better. For most people, moderate levels (15-20 mmHg) are safe and comfortable. But higher ratings can be dangerous for people with certain conditions. Severe peripheral artery disease is the most important contraindication, particularly when blood flow to the legs is already compromised. People with severe heart failure, significant diabetic neuropathy with loss of sensation, or allergies to the compression material should also avoid standard compression therapy or use it only under close medical supervision.
The risk with high compression in someone who has poor arterial circulation is that the external pressure can further reduce blood flow to the foot and toes, potentially causing tissue damage. If you have any known circulation problems in your legs, get a vascular assessment before using socks rated above 15-20 mmHg.