Legs and feet swell when fluid that normally stays inside your blood vessels leaks out into the surrounding tissue and gets trapped there. This happens because of a pressure imbalance: either too much force is pushing fluid out of the vessels, not enough force is pulling it back in, or the drainage system that normally returns excess fluid to the bloodstream isn’t working properly. The causes range from sitting too long on a flight to serious conditions like heart failure, and knowing the difference matters.
How Fluid Ends Up Trapped in Your Tissues
Your capillaries, the smallest blood vessels, constantly exchange fluid with the tissue around them. Two opposing forces keep this in check. Hydrostatic pressure pushes fluid out through the vessel walls, while proteins in your blood (mainly albumin) act like a sponge, pulling fluid back in. When something tips this balance, fluid accumulates faster than your body can reabsorb it.
There are essentially four ways this goes wrong. The pressure inside the vessels can increase, which forces more fluid out. The protein levels in your blood can drop, which weakens the “sponge” effect. The vessel walls can become leakier than normal, letting fluid pass through too easily. Or the lymphatic system, a separate network of channels that drains excess tissue fluid, can become blocked or damaged. Most causes of leg swelling involve one or more of these mechanisms.
Vein Problems Are the Most Common Cause
Your leg veins contain one-way valves that keep blood moving upward against gravity. When those valves weaken or fail, blood pools in the lower legs and increases pressure inside the veins. That extra pressure pushes fluid into the surrounding tissue, and your ankles and feet swell. This condition, called chronic venous insufficiency, is the single most common reason for persistent leg swelling in adults.
It’s also remarkably common. A large screening study in Edinburgh using ultrasound found venous insufficiency in about 9% of men and 7% of women overall, but those numbers climbed sharply with age: 21% of men and 12% of women over 50 had it. The swelling typically starts around the ankles and works its way up the leg as fluid accumulates throughout the day. It’s usually worst in the evening and improves overnight when you’re lying flat.
Varicose veins are a visible sign that venous valves aren’t functioning well, but you can have valve failure in deeper veins without any visible surface changes. Prolonged standing, obesity, and a history of blood clots all increase the risk.
Heart, Kidney, and Liver Disease
When the heart can’t pump blood efficiently, blood backs up in the veins leading to the heart. That backup raises pressure in the leg veins, and fluid leaks into the tissue. Heart failure is one of the most important systemic causes of leg swelling, and it typically affects both legs equally. You might also notice shortness of breath, fatigue, or waking up at night needing to catch your breath.
Kidney disease causes swelling through a different route. Damaged kidneys can leak protein into the urine, which lowers protein levels in the blood. With less protein to pull fluid back into the vessels, it seeps out and collects in the legs, feet, and sometimes the face. Liver disease works similarly: the liver produces albumin, so when it’s failing, blood protein levels drop and fluid retention follows.
In all three cases, the body also retains extra sodium and water, which increases blood volume and makes the swelling worse. This is why sodium restriction (generally under 2,000 mg per day for people with these conditions) is a cornerstone of managing the fluid buildup.
Medications That Cause Swelling
Several widely prescribed drugs cause leg and foot swelling as a side effect. Calcium channel blockers, commonly used for high blood pressure, are among the most frequent culprits. They relax blood vessel walls, which can increase the pressure that pushes fluid out of capillaries. Some people notice swollen ankles within days of starting these medications.
Other medications that commonly cause fluid retention include certain diabetes drugs, steroids like prednisone, nonsteroidal anti-inflammatory drugs (NSAIDs like ibuprofen), and some antidepressants. Hormone therapies, including estrogen-containing birth control, can also contribute. If your swelling started shortly after beginning a new medication, that’s worth mentioning to whoever prescribed it.
Lymphedema: When Drainage Fails
Your lymphatic system acts as a secondary drainage network, collecting excess fluid from tissues and returning it to the bloodstream. When lymphatic vessels are damaged or removed, fluid has nowhere to go and accumulates in the affected limb. This type of swelling, called lymphedema, feels different from other causes. The tissue often feels firm or spongy rather than soft, and pressing on it may not leave a visible dent.
Lymphedema can result from surgery (particularly when lymph nodes are removed during cancer treatment), radiation therapy, infection, or, less commonly, a genetic condition present from birth. Unlike venous swelling, it tends to worsen progressively over time and doesn’t resolve as easily with elevation. One quick clue: if you can’t pinch the skin at the base of the second toe into a fold, that suggests lymphatic involvement rather than simple fluid retention.
Pregnancy and Preeclampsia
Some swelling during pregnancy is normal, especially in the third trimester. The growing uterus compresses veins in the pelvis, slowing blood return from the legs, and hormonal changes cause the body to retain extra fluid. Mild ankle swelling that comes and goes is expected.
What’s not normal is sudden, severe swelling after the 20th week of pregnancy, particularly if it’s accompanied by headaches, blurred vision, dark spots in your vision, or pain in the upper right abdomen. These are warning signs of preeclampsia, a serious condition marked by high blood pressure (above 140/90) and protein in the urine. Preeclampsia can progress to seizures and organ damage. Sudden facial swelling or rapid weight gain over a few days also warrants urgent evaluation.
When Swelling Affects Only One Leg
Swelling in both legs usually points to a systemic cause: heart failure, kidney disease, medication effects, or prolonged sitting. Swelling in just one leg is a different situation and raises concern for a blood clot.
Deep vein thrombosis (DVT) occurs when a clot forms in a deep vein, usually in the calf or thigh. The affected leg swells because the clot blocks blood from draining properly. Along with swelling, you may notice pain or cramping (often starting in the calf), warmth in the leg, and a change in skin color to red or purple. DVT is dangerous not because of the leg swelling itself, but because the clot can break free and travel to the lungs. A pulmonary embolism causes sudden shortness of breath, chest pain that worsens with breathing, rapid heartbeat, dizziness, or coughing up blood. This is a medical emergency.
Other causes of one-sided swelling include a localized infection (cellulitis), a cyst behind the knee that ruptures, or an injury. But any unexplained swelling in a single leg, especially with pain or skin changes, needs prompt medical evaluation to rule out a clot.
How Severity Is Measured
If you’ve ever had a doctor press a thumb into your swollen ankle, they were checking for “pitting,” the hallmark of fluid-based swelling. When you press on swollen tissue and a dent stays behind, that’s pitting edema, and the depth and recovery time tell you how severe it is. A shallow 2 mm dent that bounces back immediately is grade 1, the mildest form. A deep 8 mm pit that takes two to three minutes to refill is grade 4, indicating significant fluid accumulation. Grades 2 and 3 fall in between, with recovery times of 15 seconds to one minute.
Swelling that doesn’t pit when you press on it suggests a different mechanism, often lymphedema or a condition where the tissue itself has changed rather than simply filling with fluid.
Reducing Everyday Swelling
For swelling caused by gravity, inactivity, or mild venous insufficiency, several strategies help. Elevating your legs above heart level for 20 to 30 minutes a few times a day lets gravity work in your favor. Compression stockings apply gentle, graduated pressure that helps push fluid back into the veins. Walking activates the calf muscles, which act as a pump to squeeze blood upward through the leg veins.
Cutting back on sodium makes a measurable difference. Most guidelines recommend staying under 2,000 mg per day if you’re dealing with fluid retention. For context, the average American consumes over 3,400 mg daily, and most of it comes from processed and restaurant food rather than the salt shaker. Reading nutrition labels and cooking more meals at home are the most practical ways to get sodium under control.
Staying hydrated seems counterintuitive when your body is retaining fluid, but dehydration actually triggers your kidneys to hold onto more sodium and water. Regular movement throughout the day, even just flexing your feet or taking a short walk every hour, prevents the blood pooling that drives gravity-related swelling. If you sit at a desk all day or take long flights, these small movements matter more than you might expect.