What does mmHg mean on compression socks?

mmHg stands for millimetres of mercury, the same unit used for blood pressure. On a compression sock it describes how much pressure the fabric applies at the ankle, where graduated compression is strongest. A sock rated 15-20 mmHg applies between 15 and 20 millimetres of mercury of pressure at the ankle and progressively less as it moves up the calf.

Is 20-30 mmHg better than 15-20 mmHg?

Not better, just stronger, and stronger is only an advantage if you need it. 15-20 mmHg is the right level for most people who stand or sit for long periods without a diagnosed condition. Jumping to 20-30 mmHg without a reason usually means a sock that is harder to put on and more likely to be abandoned in a drawer, which helps nobody.

Can compression socks be too tight?

Yes. Signs that a sock is too strong or badly fitted include numbness, tingling, colour changes in the toes, or a deep indentation that is still visible long after you take them off. A correctly fitted sock feels snug and supportive but never pinching. A tight band at the top that leaves a red trench usually means the calf circumference is wrong rather than the mmHg.

How do I measure for compression socks?

Measure first thing in the morning before any swelling develops. You need ankle circumference at the narrowest point just above the ankle bone, calf circumference at the widest point, and for knee-highs the length from the floor to the bend of your knee. Buy on those measurements, not on your shoe size, because calf circumference is the single most common reason compression socks do not work out.

How long should I wear compression socks each day?

For everyday support during standing or travel, putting them on in the morning and taking them off at bedtime is the usual pattern. Compression socks are generally not worn overnight unless a clinician has specifically advised it, because the circulatory benefit is tied to being upright and gravity working against you.

Do compression socks wear out?

Yes, and this is the most commonly missed point. The elastane that generates the compression degrades with washing and stretching. Most manufacturers put the useful life at somewhere around three to six months of regular wear. A two year old pair may still look fine and be delivering a fraction of its stated pressure.

Compression Sock mmHg Levels Explained: 15-20 vs 20-30 vs 30-40

Every compression sock is sold with a number on it: 15-20, 20-30, sometimes 8-15 or 30-40. That number does more to determine whether the sock helps you than the brand, the material or the price. It is also the thing most buyers guess at.

This guide explains what the number means, which level fits which situation, and why picking a higher one “to be safe” is usually the wrong move.

What mmHg actually measures

mmHg means millimetres of mercury. It is the same unit used for blood pressure, and it describes the pressure the fabric exerts against your leg.

The important detail is where it is measured. Compression socks worth buying are graduated, which means the pressure is highest at the ankle and decreases as the sock goes up your calf. The number on the box always refers to the ankle. A 20-30 mmHg sock applies 20 to 30 mmHg at the ankle and perhaps half that below the knee.

That gradient is the whole mechanism. Blood returning from your feet has to travel up against gravity, and it relies largely on your calf muscles squeezing veins as you move. Stand still for hours and that pump barely operates, so fluid pools in your lower legs. A graduated sock creates a pressure gradient that pushes in the direction you want the blood to go: tightest at the bottom, easing as it rises.

This is also why a sock without a stated mmHg range is a warning sign. “Graduated compression” as a marketing phrase with no number behind it usually means the manufacturer has not tested it, and an untested sock may be applying its tightest band at the top of the calf, which is precisely backwards and acts like a tourniquet.

The levels, and who each one is for

8-15 mmHg: very light

Barely compression in a meaningful sense. You will see this on drugstore “energising” socks. It is fine for mild end-of-day heaviness and for people who find firmer socks uncomfortable, but it is not doing much for genuine swelling. If you are buying compression socks because a problem exists, start higher.

15-20 mmHg: the default

This is the level most people should buy, and it covers the large majority of reasons anyone reaches for compression socks:

Standing or walking all day at work
Long flights and long drives
Mild ankle swelling by evening
Sore, heavy legs after a shift
Pregnancy-related swelling, though this is worth raising with a midwife or doctor first
Recovery after running or the gym

It is strong enough to make a noticeable difference to end-of-day swelling and heaviness, and gentle enough that you can get it on without a struggle and wear it for twelve hours without resenting it. That second half matters more than people expect. The best compression sock is the one you actually wear.

20-30 mmHg: firm, and the first properly medical tier

This is where compression stops being a comfort product. 20-30 mmHg is the level commonly associated with:

Visible varicose veins
More persistent swelling that does not resolve overnight
Post-surgical recovery, where it is often specifically prescribed
Diagnosed venous insufficiency

It is meaningfully harder to put on. Many people need a donning aid or rubber gloves, and the first few wears feel aggressive. If you are buying at this level because of a diagnosed condition rather than tired legs, the level should come from your clinician rather than from a product page, including ours.

30-40 mmHg and above: prescription territory

At this point you should not be choosing based on an article. These are used for serious venous disease, lymphoedema and post-thrombotic syndrome, and they are fitted rather than simply bought. Wearing 30-40 mmHg without an indication is not a bonus, it is uncomfortable and potentially counterproductive.

Why higher is not better

The intuition that more compression means more benefit is wrong in two specific ways.

Adherence. A 20-30 mmHg sock that lives in a drawer because it takes five minutes of wrestling to put on delivers exactly zero mmHg. A 15-20 mmHg sock you wear every day delivers 15-20. For anyone without a diagnosed condition, the lighter sock is very often the one that produces the better real-world outcome.

Circulation risk. Compression works by resisting the outward pressure of blood in your veins. If your arterial supply is already compromised, external pressure can make the problem worse rather than better. This is why peripheral arterial disease and diabetic neuropathy are situations where compression should be selected with a clinician, not self-prescribed. It is also why “just get the strongest ones” is bad advice specifically for the people most likely to be searching for compression socks.

Fit matters more than the number

If a pair of compression socks disappoints, the odds are good that fit rather than compression level was the problem.

Calf circumference is the usual culprit. Compression socks are sized on leg measurements, not shoe size, though many brands print a shoe size range because it sells better. If your calves are muscular or fuller than average, a standard sock will apply too much pressure at the widest part of the calf, produce a red trench at the top, and feel like it is cutting in even at 15-20 mmHg. The fix is a wide-calf model rather than a lower compression level.

Measure in the morning. Your legs are at their smallest before you have been upright for hours. Measure ankle circumference just above the bone, calf circumference at the widest point, and floor-to-knee-bend length for knee-highs.

Length. A knee-high should sit roughly two fingers below the bend of the knee. Too long and it bunches behind the knee, which creates a pressure point in exactly the wrong place.

They expire, and nobody tells you

Compression comes from elastane, and elastane degrades. Every wash and every stretch takes a little of it away. Manufacturers generally put the useful life of a compression sock at three to six months of regular wear.

This is the most commonly missed thing about compression socks. A pair that is two years old and looks fine may be delivering a fraction of its printed pressure, and the owner concludes that compression socks “stopped working” when in fact the socks simply wore out. If you rely on them, rotate two or three pairs and replace them on a schedule.

Wash them in cool or warm water, avoid fabric softener, which coats and degrades the fibres, and air dry them. Tumble drying on heat is the fastest way to kill a pair.

A note on what this guide is

We are independent reviewers, not clinicians. Everything above describes how compression socks are specified and sold, and the conventions around which levels are typically used for which situations. It is not medical advice and it is not a substitute for an assessment.

If you have diabetes, peripheral arterial disease, lymphoedema, a history of blood clots, or any diagnosed venous condition, the compression level should be decided with a clinician. The consequences of getting it wrong in those situations are real, not theoretical.

Where to go next

If you have landed on 15-20 mmHg and want specific picks, our roundup of the best compression socks compares brands on the same five criteria.

If you stand all day for work, the compression level is only half the answer. Blood pooling is one problem and the surface under your feet is another, which is what anti-fatigue mats address.

And if you have been unsure whether to buy Sockwell or Vim & Vigr, we put them head to head rather than reviewing each in isolation.