The firmness of flight socks, also known as compression socks, is measured in mmHg (millimeters of mercury), and for most healthy travelers in 2026, the ideal "firmness" is between 15 and 20 mmHg. This level is classified as "mild to moderate" compression and is the standard "over-the-counter" grade for preventing Deep Vein Thrombosis (DVT) and reducing leg swelling during long-haul flights. If you have a history of circulatory issues or have recently had surgery, a doctor might prescribe a firmer grade of 20–30 mmHg. The socks should feel snug but not painful; they should apply the most pressure at the ankle, with the pressure gradually decreasing as the sock goes up the leg—a feature known as "graduated compression." If the socks are so firm that they cause tingling, numbness, or leave deep indentations in your skin after you take them off, they are likely too tight. It is also a common mistake to choose socks that are too loose (under 15 mmHg), as they may not provide enough pressure to effectively push blood back toward the heart during long periods of immobility in a pressurized cabin.