Ankles swell when fluid leaks out of small blood vessels and accumulates in the surrounding tissue faster than your body can drain it away. This can happen for reasons as simple as sitting too long or eating a salty meal, or it can signal something more serious like heart failure, a blood clot, or kidney disease. Whether the swelling affects one ankle or both, how quickly it appeared, and what other symptoms accompany it all point toward different causes.
How Fluid Ends Up in Your Ankles
Your capillaries, the tiniest blood vessels, constantly filter small amounts of fluid into the tissue around them. Normally about 1% of plasma seeps out, and most of it gets pulled back in or drained away by your lymphatic system. Swelling happens when that balance tips: either too much fluid is being pushed out, not enough is being pulled back in, or the drainage system can’t keep up.
Four things can break this balance. First, pressure inside the veins can rise, forcing more fluid out. This is what happens with heart failure, prolonged standing, or a blood clot. Second, your blood can lose the proteins (mainly albumin) that act like sponges to hold fluid inside vessels. Kidney disease and malnutrition can cause this. Third, inflammation or injury can make capillary walls leaky, letting fluid pour out faster than normal. Fourth, the lymphatic drainage channels themselves can become blocked or damaged, so fluid has nowhere to go. Any of these mechanisms, alone or in combination, will cause your ankles to puff up.
Ankles and feet are especially vulnerable because gravity pulls fluid downward all day long. That’s why swelling often worsens by evening and improves overnight when you’re lying flat.
Prolonged Sitting and Standing
The most common and least worrisome cause is simply being on your feet or sitting in one position for hours. Gravity increases the pressure in your leg veins, and without the pumping action of your calf muscles during walking, fluid pools in the lowest point of your body. Long flights, desk jobs, and road trips are classic triggers. Moving around, elevating your legs, or wearing compression socks usually resolves this kind of swelling within hours.
Too Much Sodium
A high-salt meal or a pattern of eating salty foods increases the total amount of sodium in your body. That extra sodium raises the concentration of your blood, which triggers your kidneys to hold on to water to dilute it back to normal. When you retain more than roughly 2.5 liters of extra fluid, visible swelling develops. Your ankles and feet are typically the first place you’ll notice it. Cutting back on processed foods and restaurant meals, where most dietary sodium hides, can make a noticeable difference within a few days.
Chronic Venous Insufficiency
Your leg veins have one-way valves that push blood upward toward the heart. When those valves weaken or fail, blood pools in the lower legs, raising pressure inside the veins and forcing fluid into surrounding tissue. This condition, called chronic venous insufficiency, is the most common cause of persistent bilateral leg swelling.
It tends to develop gradually. Early on you might just notice tired, achy legs. Over time, visible spider veins or varicose veins appear, followed by ankle swelling that’s worse after standing and better in the morning. In advanced stages, the skin around the ankles can darken, thicken, and eventually break down into ulcers. Compression stockings, regular walking, and leg elevation are the cornerstones of management. In more severe cases, procedures to close or remove damaged veins can help.
Heart Failure
When the right side of the heart weakens, it can’t pump blood efficiently to the lungs. Blood backs up in the veins returning from the body, raising venous pressure throughout. That pressure pushes fluid out of the veins and into surrounding tissue, causing swelling in the feet, ankles, and legs. If fluid also accumulates in the abdomen, you may notice bloating or a feeling of fullness.
Heart failure swelling tends to affect both legs equally and worsens over the course of the day. It’s often accompanied by other symptoms: shortness of breath (especially when lying down or during activity), fatigue, and rapid weight gain from fluid retention. Gaining several pounds over just a few days is a telltale sign that fluid is building up. Treatment focuses on reducing that excess fluid and improving heart function.
Blood Clots
A deep vein thrombosis (DVT) is a blood clot that forms in one of the deep veins of the leg. It blocks blood flow, causing a rapid increase in pressure behind the clot and forcing fluid into surrounding tissue. The hallmark of DVT is sudden swelling in one leg, not both. The affected leg often feels warm, looks red or discolored, and may be painful or tender, especially in the calf.
DVT is a medical emergency because the clot can break loose and travel to the lungs, causing a pulmonary embolism. If you develop sudden one-sided leg swelling, especially after surgery, a long period of immobility, or a long flight, it warrants urgent evaluation. Diagnosis typically involves an ultrasound of the leg veins.
Kidney Disease
Healthy kidneys filter your blood while keeping important proteins, especially albumin, from leaking into your urine. Albumin acts like a sponge inside your bloodstream, holding fluid in your vessels through osmotic pressure. When kidney damage allows albumin to spill into the urine (a condition called nephrotic syndrome), blood protein levels drop and fluid seeps into tissues throughout the body. Swelling around the eyes, in the ankles, and in the feet is characteristic. The urine may appear foamy due to excess protein.
Medications That Cause Swelling
Several common medications list ankle swelling as a side effect. The most well-known culprits are calcium channel blockers, a class of blood pressure drugs. These medications relax the walls of small arteries, which increases blood flow into the capillaries and raises the pressure that pushes fluid outward. At standard doses, anywhere from 1% to 15% of people taking these drugs develop ankle swelling. At higher doses taken long-term, that number can exceed 80%.
Other medications that can cause fluid retention include certain diabetes drugs, steroids, hormone therapies (including estrogen and testosterone), and some anti-inflammatory painkillers. If you notice new ankle swelling after starting a medication, your prescriber may be able to adjust the dose or switch to an alternative. In one study, adding a specific type of companion blood pressure drug to a calcium channel blocker cut the rate of swelling from about 19% to under 8%.
Pregnancy-Related Swelling
Mild ankle swelling is extremely common during pregnancy, especially in the third trimester, as the growing uterus compresses pelvic veins and your blood volume increases. This is generally harmless. However, sudden or severe swelling after 20 weeks of pregnancy can be a warning sign of preeclampsia, a serious condition involving high blood pressure and organ stress.
Preeclampsia is diagnosed when blood pressure reaches 140/90 or higher along with signs like protein in the urine, persistent headaches that don’t respond to pain medication, vision changes, or upper abdominal pain. It requires prompt medical treatment to protect both mother and baby.
How Doctors Assess Ankle Swelling
One of the first things a clinician does is press a finger firmly against the swollen area for several seconds, then release. If the finger leaves a visible dent that takes time to refill, that’s called pitting edema, and it’s graded on a scale from 1+ to 4+. At the mild end (1+), there’s slight pitting over the shin. At the severe end (4+), the swelling extends above the knees and the tissue is so waterlogged that the bone underneath can’t even be felt.
Whether the swelling is in one leg or both matters enormously. Sudden swelling in one leg raises concern for a blood clot, an injury, or a localized infection. Swelling in both legs points toward systemic causes: heart failure, kidney disease, venous insufficiency, medications, or excess sodium. Your doctor will also ask about timing. Swelling that’s been creeping up over months suggests a chronic condition, while swelling that appeared overnight or in hours requires more urgent attention.
Warning Signs That Need Immediate Attention
Most ankle swelling is not an emergency, but certain combinations of symptoms require urgent care. Swelling paired with chest pain, difficulty breathing, shortness of breath when lying flat, dizziness, or coughing up blood can indicate a blood clot in the lungs or a serious cardiac event.
Sudden swelling in one leg that’s painful, pale, or cool to the touch also warrants immediate evaluation for a blood clot. And any swelling that follows a physical injury, such as a fall, sports accident, or car crash, should be assessed promptly to rule out fractures, torn ligaments, or compartment syndrome.