Compression Socks for Flying: Who Needs Them and How to Choose

Short answer: On long flights, compression socks reduce the chance of a symptomless blood clot in the legs and cut ankle swelling. In a Cochrane review of 12 trials on flights over five hours, 3 people wearing stockings developed a symptomless clot, compared with 47 who didn't. [S03] Guidelines don't recommend them for everyone. They suggest them mainly for travelers at higher risk, which includes people with active cancer, recent cancer treatment or recent surgery. [S01] [S02] [S04] If that's you, ask your care team before a trip of four hours or more whether stockings, a blood thinner or neither makes sense. [S01] [S02]

Traveling for treatment, a second opinion or a family visit can mean hours in a cramped seat. This guide explains how clot risk works on long trips, what compression socks do, who should consider them and how to choose a pair.

Why long flights raise clot risk

Sitting still in a confined space slows blood flow in the legs, and a clot can form in a deep vein. This is called deep vein thrombosis (DVT). [S01] If part of the clot breaks off and travels to the lungs, it's a pulmonary embolism (PE), which can be life-threatening. [S01] [S02]

For most people, the risk is low. CDC calls it generally very small, and the CDC Yellow Book cites studies estimating about 1 clot per 4,656 to 6,000 flights lasting more than four hours. [S01] [S02] The risk isn't unique to flying: car, bus and train trips carry a similar increase, which points to immobility as the main cause. [S02]

The risk rises when long travel combines with other risk factors. In some studies, 75% to 99% of people who developed a travel-related clot had at least one. [S02] These include: [S01] [S02]

• Active cancer or recent cancer treatment
• Recent surgery, injury or hospital stay
• A previous blood clot, or a family history or inherited clotting disorder
• Estrogen-containing birth control or hormone therapy
• Pregnancy or the months after giving birth
• Older age (risk rises after 40) and obesity
• Limited mobility, such as a leg cast, or a catheter in a large vein

Most travel-related clots happen within the first one to two weeks after the trip, and risk returns to normal by about eight weeks. [S02]

What compression socks do

Graduated compression socks or stockings apply pressure to the lower leg, rated by the pressure at the ankle. [S02] The idea is to support blood flow in leg veins during long periods of sitting, which is when clots tend to form. [S01]

What the trials show

The best summary is a 2021 Cochrane review of 12 randomized trials with 2,918 passengers on flights lasting more than five hours. [S03]

• Symptomless clots: 50 travelers developed a DVT without symptoms. Only 3 of them wore stockings; 47 didn't. The reviewers rated this high-certainty evidence. [S03]
• Superficial vein clots: Possibly fewer with stockings, but the result wasn't clear (moderate-certainty evidence). [S03]
• Swelling: Stockings reduced leg and ankle swelling, though the certainty was low because studies measured it differently. [S03]
• Serious clots: No deaths, PEs or clots with symptoms happened in any group, so the trials can't show whether stockings prevent them. [S03]
• Side effects: None of significance were reported. [S03]

The CDC Yellow Book reaches the same view: compression stockings appear to reduce symptomless clots in travelers and are generally well tolerated. [S02]

Who should wear compression socks on a flight

This is where the guidelines are more cautious than the marketing.

• Travelers without risk factors: The American Society of Hematology (ASH) advises against compression stockings, blood thinners or aspirin to prevent clots during travel. [S02] The American College of Chest Physicians (ACCP) also doesn't recommend stockings for this group. [S02]
• Travelers at substantially increased risk: For trips over four hours, ASH suggests compression stockings or a preventive dose of a blood-thinning injection (low-molecular-weight heparin). [S02] [S04] Its examples of higher risk include recent surgery, a previous clot, the period after giving birth, active cancer, or two or more risk factors combined. [S02]

These are conditional recommendations based on limited evidence, so they're meant to guide a conversation with your clinician rather than replace it. [S02] [S04]

If you're in cancer treatment, you're likely in the higher-risk group. [S01] [S02] Ask your oncology team well before departure, because the right answer depends on your treatment and any medicines you take. If you already take a blood thinner, follow your doctor's instructions on doses during travel. [S01] Our Appointment Logistics Planner has space to note questions like this for your next visit.

How to choose compression socks for flying

• Pressure: The ACCP guidance summarized by CDC describes properly fitted below-the-knee stockings that give 15 to 30 mmHg of pressure at the ankle. [S02]
• Length: Below-the-knee was the style in that guidance. [S02]
• Fit: "Properly fitted" matters. [S02] Use the brand's size chart, or ask a pharmacist or medical supply store to measure you. A sock that's the wrong size can dig in or slip down.
• When to wear them: Put them on before you board and wear them for the whole flight.
• Comfort: If they feel painful or too tight, take them off and ask your clinician.

If you've been prescribed stockings with a specific pressure for another reason, wear those rather than buying a different grade.

Other ways to lower clot risk on long trips

Compression socks are one part of a plan. CDC and NHS also recommend: [S01] [S05]

• Move often. Walk the aisle when it's safe, and take breaks on long car, bus or train trips. [S02] NHS suggests getting up and moving around every hour or so. [S05]
• Work your calves while seated. Extend your legs, flex your ankles to pull your toes toward you, and some airlines suggest pulling each knee toward your chest for 15 seconds, up to 10 times. [S01] Our guide to staying active on the road has more ideas.
• Choose an aisle seat if you can. One study found window seats doubled clot risk compared with aisle seats, likely because people move less. [S02]
• Drink fluids. Staying hydrated is reasonable, though it hasn't been proven to prevent travel clots. [S02] [S05] See our hydration habits guide, and plan snacks with our hotel-room nutrition checklist.
• Pack medicines in your carry-on. If you're prescribed a blood thinner, keep it with you. Our guide to packing medications for travel covers the rules.

Know the warning signs

About half of people with DVT have no symptoms. [S01] When symptoms happen, they usually affect one leg or arm: [S01]

• Swelling
• Pain or tenderness you can't explain
• Skin that feels warm
• Redness or discoloration

Talk with your doctor promptly if you notice these during or after a trip. Most travel-related clots show up within one to two weeks. [S01] [S02] Get emergency help right away for trouble breathing, chest pain that worsens with a deep breath or cough, a fast or irregular heartbeat, coughing up blood, or fainting. These can be signs of PE. [S01] [S05]

If you've already had a clot, CDC notes that airline medical guidance allows flying once you're stable on blood thinners, while British guidance suggests waiting two weeks after diagnosis. [S02] NHS advises delaying flights until at least two weeks after you start blood-thinning medicine. [S05] Ask the clinician managing your clot.