Fluid builds up in the legs when the balance between what pushes fluid out of your blood vessels and what pulls it back in gets disrupted. The medical term is peripheral edema, and the causes range from sitting too long on a flight to serious conditions like heart failure or kidney disease. Understanding what’s behind the swelling helps you figure out whether it’s something that resolves on its own or something that needs attention.
How Fluid Normally Moves In and Out of Tissues
Your smallest blood vessels, the capillaries, constantly filter fluid into the surrounding tissue and then reabsorb most of it. Two opposing forces control this exchange. Blood pressure inside the capillary pushes fluid out into the tissue. Proteins dissolved in your blood (mainly albumin) pull fluid back in. At the arterial end of a capillary, the outward push is stronger, so fluid seeps into the tissue. At the venous end, the inward pull dominates, and fluid returns to the bloodstream. Whatever small amount is left behind gets picked up by the lymphatic system and routed back into circulation.
Leg swelling happens when something tips this balance. Higher pressure inside the veins pushes more fluid out. Lower protein levels in the blood mean less pulling force to bring it back. Damaged lymph vessels can’t drain the excess. Or the kidneys hold onto too much salt and water, expanding your overall fluid volume. Nearly every cause of leg swelling traces back to one of these four mechanisms.
Heart Failure
Heart failure is one of the most common causes of fluid in both legs. When the heart’s pumping ability weakens, blood backs up in the veins that return it from the lower body. That backup raises pressure inside the leg veins and capillaries, forcing more fluid into the surrounding tissue than can be reabsorbed. The swelling typically worsens over the course of the day and improves overnight when you’re lying flat. It often comes with other signs like shortness of breath, fatigue, or swelling in the abdomen.
Venous Insufficiency
Your leg veins contain one-way valves that keep blood moving upward toward the heart. When those valves weaken or fail, blood flows backward and pools in the lower legs, a condition called chronic venous insufficiency. The pooling raises pressure in the veins, which gets transmitted all the way down to the capillaries. Over time, capillary walls become more permeable, letting fluid, proteins, and even red blood cells leak into the tissue.
The most common reason those valves fail is a previous blood clot (deep vein thrombosis) that damaged them. But valves can also weaken with age, obesity, or prolonged standing. The swelling tends to be worse later in the day, and over months to years the skin on the lower legs may darken, thicken, or develop sores that heal slowly. Varicose veins are often visible. Unlike heart failure, venous insufficiency can affect just one leg, though both legs are frequently involved.
Kidney and Liver Disease
Your kidneys regulate how much salt and water stay in your bloodstream. When kidney function declines, the body retains both, expanding overall fluid volume and raising pressure in the blood vessels. The result is swelling in the legs and sometimes puffiness around the eyes. Kidney disease can also cause protein to leak into the urine, lowering blood protein levels and reducing the pulling force that normally keeps fluid inside the capillaries.
Liver cirrhosis works through a different path. A scarred liver produces less albumin, the protein responsible for holding fluid in the bloodstream. With less albumin, fluid escapes more easily into the tissues. Cirrhosis also raises pressure in the veins draining the abdominal organs, which causes fluid to accumulate in the belly (ascites) and back up into the legs.
Medications That Cause Swelling
Several widely used medications cause the body to retain salt and water, leading to noticeable leg swelling. Blood pressure medications, particularly calcium channel blockers like amlodipine, are among the most frequent culprits. Corticosteroids (such as prednisone), NSAIDs like ibuprofen, and hormonal medications including birth control pills can all contribute. The swelling usually affects both legs equally and may improve if the medication is adjusted, though you should never stop a prescribed medication without talking to your prescriber first.
Sodium and Prolonged Sitting
Eating a lot of salty food triggers your kidneys to hold onto extra water to keep sodium concentrations balanced. Research shows this causes a rapid, though typically temporary, increase in the volume of fluid outside your cells. In most healthy people, the body compensates within a couple of weeks, but while you’re eating a consistently high-sodium diet, you may notice your legs and ankles feel puffy, especially by evening.
Gravity plays a role too. Sitting or standing in one position for hours, whether on a long flight or at a desk, lets fluid pool in the lowest parts of your body. Your calf muscles normally act as a pump, squeezing veins with each step to push blood upward. Without that pumping action, venous pressure in the legs climbs and fluid accumulates. This is why walking around periodically or flexing your calves can make a noticeable difference.
Pregnancy
Some degree of leg swelling is normal during pregnancy, especially in the third trimester. Pregnancy hormones cause the body to retain more sodium and water. At the same time, the growing uterus presses on the large vein (inferior vena cava) that carries blood back from the legs, partially obstructing return flow. The combination of increased fluid volume and impaired drainage makes swelling in the feet and ankles very common. It tends to be worse when lying on your back and late in the day. Sudden or severe swelling, particularly if accompanied by high blood pressure or headaches, is a different situation that warrants prompt evaluation.
Lymphedema
The lymphatic system acts as a secondary drainage network, picking up fluid and proteins that the veins don’t fully reabsorb. When lymph vessels are damaged or blocked, that drainage fails and protein-rich fluid accumulates in the tissue. This is lymphedema, and it has a distinct character compared to other types of swelling.
Early lymphedema can actually pit when you press on it, just like other forms of edema. But over time, the trapped proteins trigger inflammation and scarring in the tissue, making the skin thick, firm, and resistant to indentation. A useful sign: if you can’t pinch and lift the skin over the top of the second toe (called the Stemmer sign), lymphedema is likely. The swelling is typically painless but feels heavy. It often affects the top of the foot, giving the toes a squared-off appearance. Causes include surgery that removed lymph nodes, radiation therapy, infections, and in some cases inherited abnormalities of the lymphatic system.
One Leg vs. Both Legs
Whether swelling affects one leg or both is a practical clue to the cause. Swelling in both legs generally points to something systemic: heart failure, kidney disease, liver disease, medications, or a diet-related issue. The body is retaining fluid overall, and gravity pulls it to the lowest point.
Sudden swelling in just one leg is a different concern. The most urgent possibility is a deep vein thrombosis (DVT), a blood clot in one of the deep veins of the leg. DVT often comes with pain, warmth, and redness in the affected leg. It requires prompt treatment because the clot can break loose and travel to the lungs. One-sided swelling can also result from a localized infection, an injury, or venous insufficiency that’s worse on one side. If one leg swells rapidly and is painful, red, or warm, that combination warrants urgent medical evaluation.
How Swelling Severity Is Assessed
Doctors often check for “pitting” by pressing a finger firmly into the swollen area for several seconds and watching what happens. The depth of the dent and how long it takes to bounce back indicate severity on a four-point scale:
- Grade 1: A shallow 2 mm dent that rebounds immediately.
- Grade 2: A 3 to 4 mm dent that rebounds in under 15 seconds.
- Grade 3: A 5 to 6 mm dent that takes 15 to 60 seconds to refill.
- Grade 4: An 8 mm dent that persists for two to three minutes.
You can do a rough version of this at home. Press your thumb into the skin above your ankle for about 10 seconds. If a visible dent remains for more than a few seconds, that’s pitting edema. It won’t tell you the cause, but it helps you communicate what you’re seeing to a doctor.
Managing Mild Swelling
For everyday fluid retention that isn’t tied to a serious medical condition, a few strategies reliably help. Elevating your legs above heart level for 20 to 30 minutes several times a day lets gravity work in your favor, moving fluid back toward the trunk. Reducing sodium intake lowers the amount of water your body holds onto. Regular walking activates the calf muscle pump that drives blood upward through the veins.
Compression stockings apply graduated pressure that’s strongest at the ankle and decreases toward the knee, helping veins push blood upward. For mild swelling, stockings rated at 15 to 20 mmHg provide light support and are available without a prescription. Moderate swelling often responds to 20 to 30 mmHg compression. People with more significant edema, venous disease, or lymphedema may need 30 to 40 mmHg or higher, typically fitted with guidance from a clinician. Compression is not appropriate for everyone, particularly people with arterial circulation problems, so it’s worth checking before going to higher pressure levels.