Which compression level should I buy?

If nobody has told you a number, start at 15-20 mmHg. Move to 20-30 mmHg if your legs still swell noticeably by evening after a week. 30-40 mmHg is prescription strength and should only be worn if a doctor has specified it. Going firmer than you need is not safer.

When are compression socks not safe to wear?

Compression assumes arterial blood is arriving normally. Speak to a doctor first if you have peripheral arterial disease, reduced or absent sensation in your feet, uncontrolled diabetes, an open wound or ulcer on the leg, or untreated heart failure. A well-fitted sock feels firm and supportive. If it feels numbing, or your toes change colour, take it off.

What do the mmHg numbers on compression socks mean?

Q: What do the mmHg numbers on compression socks mean?

mmHg means millimetres of mercury and measures how hard the sock squeezes at the ankle. Compression socks show a range because the pressure is graduated: firmest at the ankle and easing up the leg. That gradient works with your calf muscle to move blood back toward the heart.

mmHg is a pressure measurement — millimetres of mercury — and it tells you how hard the sock squeezes at the ankle. The higher the number, the firmer the compression. Every compression sock shows a range rather than a single figure, because the pressure is graduated: firmest at the ankle, easing as it goes up the leg.

That gradient is the entire mechanism. Your calf muscle acts as a pump pushing blood back toward the heart, and graduated compression works with it. A sock that squeezed evenly, or tighter at the top, would trap blood in your lower leg instead of moving it.

The four levels

8-15 mmHg — light

Barely more than a firm sock. Mild tiredness, occasional desk-bound days. Most people who need compression need more than this.

15-20 mmHg — moderate

The everyday level, and the right starting point for most people. Legs that ache and swell by evening, long flights, standing shifts, pregnancy, mild varicose veins. Available without a prescription and easy enough to put on unassisted.

20-30 mmHg — firm

The most commonly recommended medical grade. Visible varicose veins, swelling that has not settled by morning, twelve-hour shifts on hard floors, recovery after vein procedures, and long-haul flights where DVT risk is a concern. Noticeably harder to put on than 15-20.

30-40 mmHg — prescription strength

This is specified by a doctor after examining your legs. Advanced venous disease, significant lymphoedema, after a deep vein thrombosis. It is not a level to self-select into because you want the strongest option.

Which should you buy?

If nobody has told you a number, start at 15-20 mmHg. If your legs still swell noticeably by evening after a week of wearing them, move to 20-30.

Going firmer than you need is not safer — it is harder to get on, less comfortable, and more likely to end up unworn in a drawer.

When firmer compression is not safe

Compression works by squeezing veins to help blood return to the heart. It assumes arterial blood is arriving in the first place.

If arterial circulation into your leg is already restricted, adding external pressure can reduce blood supply rather than improve drainage. Speak to a doctor before wearing compression at any level if you have:

Peripheral arterial disease
Reduced or absent sensation in your feet — you will not feel a problem developing
Uncontrolled diabetes
An open wound, skin infection or ulcer on the leg
Untreated heart failure

A well-fitted compression sock feels firm and supportive. If it feels numbing, if your toes change colour, or if you get pins and needles, take it off.