Leg swelling happens when excess fluid builds up in the tissues below your knees, and the causes range from something as simple as sitting too long to serious conditions involving your heart, kidneys, or blood vessels. The underlying mechanism is always the same: fluid that normally stays inside your blood vessels or gets recycled by your lymphatic system instead leaks out and accumulates in the spaces between cells.
Understanding which type of swelling you’re dealing with matters, because some causes resolve on their own while others need prompt medical attention.
How Fluid Ends Up in Your Legs
Your body’s water is divided into two main compartments. About two-thirds sits inside your cells, and the remaining third circulates in your blood plasma and the spaces between tissues. A careful balance of pressures keeps fluid moving in and out of your blood vessels at the right rate. When that balance tips, fluid pools in the tissues and you see swelling.
Five things can throw off that balance: increased pressure inside your veins (pushing fluid out), low protein levels in your blood (which normally pull fluid back in), leaky blood vessel walls, elevated pressure in the surrounding tissues drawing fluid toward them, or a sluggish lymphatic system that can’t drain fluid away fast enough. Most causes of leg swelling trace back to one or more of these mechanisms.
Venous Insufficiency
The most common vascular cause of chronic leg swelling is a problem with the veins themselves. Your leg veins contain one-way valves that push blood upward toward the heart against gravity. When those valves weaken or fail, blood flows backward and pools in the lower legs, raising pressure inside the veins and forcing fluid into the surrounding tissue.
Valve failure can happen for several reasons. Some people have a pre-existing weakness in their vein walls or valve leaflets. Others develop it after a blood clot damages the deep veins, after an episode of vein inflammation, or from prolonged vein stretching caused by hormonal changes or sustained high pressure. The deep veins, superficial veins, and the connecting veins between them can all be involved. When the connecting veins fail, the powerful pumping action of your calf muscles actually forces blood backward into the superficial system, making swelling worse with activity rather than better.
Venous insufficiency typically causes swelling in both legs, worsens throughout the day, and improves overnight. Over time, you may notice skin discoloration, varicose veins, or a heavy, aching feeling in your calves.
Blood Clots
A deep vein thrombosis (DVT) is a blood clot that forms in one of the deep veins of the leg. Unlike most other causes of swelling, DVT almost always affects just one leg. That sudden, one-sided swelling is actually the most specific physical sign of a clot. Other signs include redness, warmth, and tenderness along the affected area.
DVT is a medical emergency because the clot can break free and travel to the lungs. Risk factors include recent surgery, prolonged immobility (long flights, bed rest), cancer, pregnancy, and a personal or family history of clotting disorders. If one leg swells noticeably more than the other over hours or days, especially with pain or warmth, get it evaluated quickly.
Heart Failure
When the heart’s pumping ability weakens, blood backs up in the veins leading to the heart. Since your leg veins are the lowest point in the system, they feel the pressure first. Fluid gets pushed out of the capillaries and into the tissue, producing swelling that typically affects both legs and ankles symmetrically.
Heart-related swelling tends to worsen over the course of the day and improve after a night of sleep. It often comes with other signs: shortness of breath (especially when lying flat), fatigue, and sometimes abdominal swelling from fluid backing up into the liver. The swelling in heart failure is usually pitting, meaning if you press your thumb into the skin and hold it, a temporary dent remains.
Kidney Disease
Your kidneys regulate how much fluid and salt stay in your blood. When they’re not working well, sodium and water accumulate, increasing blood volume and raising pressure throughout your vascular system. The result is swelling, often in the legs and around the eyes.
A specific type of kidney damage called nephrotic syndrome causes the kidneys to leak large amounts of protein into the urine. Since blood proteins act like sponges that hold fluid inside your vessels, losing them lets fluid seep into tissues more easily. This produces a particularly stubborn form of edema that can be widespread.
Lymphedema
Your lymphatic system acts as a secondary drainage network, collecting fluid and proteins that leak out of blood vessels and returning them to the bloodstream. When part of this system is damaged or blocked, fluid accumulates in the affected area. This is lymphedema, and it behaves differently from other types of swelling.
Standard edema is a low-protein swelling. Lymphedema is a high-protein swelling. That distinction matters because the protein-rich fluid triggers changes in the tissue over time: the skin and tissue beneath it gradually harden, the area becomes more prone to infections, and the swelling increases if left untreated. Unlike regular edema, lymphedema doesn’t resolve on its own once it develops.
You can tell lymphedema apart from regular swelling by a few features. Early on, pressing into the skin may leave a dent (pitting), but as the tissue hardens over months or years, it stops pitting. Lymphedema also tends to affect one limb more than the other, especially when it results from surgery, radiation, infection, or injury to the lymph nodes.
Medications That Cause Swelling
Several common medications list leg swelling as a side effect, but one class stands out. Calcium channel blockers, a widely prescribed group of blood pressure medications, cause ankle swelling in 1 to 15% of patients at standard doses. At higher doses taken long-term, that number can exceed 80%. The swelling happens because these drugs relax blood vessel walls, which increases pressure in the small capillaries of the legs and lets fluid leak out.
Combining a calcium channel blocker with another type of blood pressure medication (one that acts on the body’s salt-regulating hormones) reduces swelling by about 38%. In one trial, patients on the combination had an edema rate of 7.6% compared to 18.7% for those on the calcium channel blocker alone.
Other medications that commonly cause leg swelling include certain diabetes drugs, anti-inflammatory painkillers, steroids, and some hormonal therapies. If you notice new swelling after starting a medication, it’s worth bringing up with your prescriber rather than just tolerating it.
Sodium and Diet
Eating a lot of salt causes your body to retain water to keep sodium concentrations balanced. For most healthy people, this produces temporary, mild puffiness. But for people with heart failure or kidney disease, sodium intake has a much larger effect on fluid retention.
Heart failure guidelines from the American Heart Association recommend keeping sodium under 2,000 to 3,000 milligrams per day, with stricter limits (under 2,000 mg) for people with moderate to severe symptoms. For context, a single fast-food meal can easily contain 2,000 mg or more.
Prolonged Sitting or Standing
Gravity pulls fluid downward, and your calf muscles act as a pump to push venous blood back up toward your heart. When you sit or stand for hours without moving, that pump stops working, and fluid gradually pools in your feet and ankles. This is one of the most common and least dangerous causes of leg swelling, familiar to anyone who’s taken a long flight or worked a desk job.
The swelling is usually symmetrical, painless, and resolves after you move around or elevate your legs. It becomes more common with age, excess weight, and pregnancy.
Pregnancy
Some leg swelling during pregnancy is normal, especially in the third trimester. The growing uterus compresses the veins that return blood from the legs, and hormonal changes relax blood vessel walls, both of which increase fluid leakage into tissues.
What’s not normal is sudden, severe swelling, particularly if it comes with headaches, vision changes, or upper abdominal pain. These can be signs of preeclampsia, a serious blood pressure condition that develops during pregnancy and requires immediate care.
How to Tell Mild From Serious Swelling
Doctors assess swelling using a simple pitting test: pressing a thumb into the swollen area for a few seconds and measuring the depth of the dent and how long it takes to bounce back. A shallow 2 mm dent that rebounds immediately is grade 1. An 8 mm dent that takes two to three minutes to fill back in is grade 4 and suggests significant fluid overload.
Several patterns should prompt faster evaluation. One-sided swelling with pain, warmth, or redness raises concern for a blood clot. Swelling that comes with shortness of breath could point to heart failure or a clot that has traveled to the lungs. Swelling around the eyes alongside leg swelling suggests a kidney problem. And any swelling that progresses quickly over days, doesn’t improve with elevation, or is accompanied by skin changes or fever warrants a closer look.
Reducing Everyday Swelling
For swelling caused by gravity, inactivity, or mild fluid retention, a few strategies help. Elevating your legs above heart level for about 15 minutes, three to four times a day, lets gravity work in your favor and moves fluid back into circulation. Compression socks or stockings provide steady external pressure that helps your veins push blood upward. Regular movement, even just flexing your ankles or taking short walks, activates the calf muscle pump. And reducing sodium intake limits the amount of fluid your body holds onto in the first place.
These measures work well for positional or lifestyle-related swelling. But they’re supporting strategies, not solutions, when the swelling stems from heart, kidney, liver, or lymphatic disease. In those cases, the swelling is a signal from a deeper problem that needs its own treatment.