A swollen foot happens when fluid builds up in the tissues of your foot, ankle, or lower leg. The causes range from something as simple as sitting too long on a flight to serious conditions like heart failure or a blood clot. Whether one foot is swollen or both, and whether the swelling came on suddenly or gradually, tells you a lot about what’s behind it.
How Fluid Ends Up in Your Feet
Your body constantly moves fluid between your bloodstream and the tissues around it. Two opposing forces keep this exchange in balance: the pressure inside your blood vessels pushing fluid out, and proteins in your blood pulling fluid back in. When either force shifts, fluid leaks into surrounding tissue faster than your lymphatic system can drain it away, and swelling results.
Gravity makes the feet and ankles especially vulnerable. Blood has to travel the longest distance from your feet back to your heart, fighting gravity the entire way. Anything that raises pressure in your leg veins, lowers protein levels in your blood, or damages the tiny vessels that regulate fluid exchange can tip the balance toward swelling.
Damaged Vein Valves
Chronic venous insufficiency (CVI) is one of the most common causes of persistent foot and ankle swelling. Your leg veins contain one-way valves that keep blood moving upward toward your heart. When those valves weaken or fail, gravity pulls blood back down, and it pools in your lower legs. The increased pressure forces fluid out of your veins and into surrounding tissue.
CVI swelling is typically worst at the end of the day or after long periods of standing. Along with puffiness in the ankles and feet, you may notice aching or heavy legs, a burning or tingling sensation, nighttime leg cramps, and skin that gradually turns reddish-brown around the ankles. Over time, the constant high pressure can burst tiny capillaries near the skin’s surface, making the skin fragile and prone to slow-healing open sores called venous ulcers.
Heart, Kidney, and Liver Disease
When a major organ struggles, the effects often show up in your feet first. Each organ disrupts fluid balance in a different way, but all three tend to cause swelling in both legs rather than just one.
- Heart failure. When the heart can’t pump blood efficiently, blood backs up in the veins of your legs, ankles, and feet. The rising pressure pushes fluid into surrounding tissue. Swelling that worsens through the day and improves overnight (when you’re lying flat) is a hallmark pattern.
- Kidney disease. Your kidneys regulate how much fluid and salt stay in your blood. When they’re damaged, excess sodium and water accumulate, causing swelling in the legs and often around the eyes. A specific type of kidney damage called nephrotic syndrome also lets protein leak out of your blood, reducing the pulling force that keeps fluid inside your vessels.
- Liver disease. Cirrhosis impairs your liver’s ability to produce albumin, the main protein responsible for holding fluid in your bloodstream. Low albumin levels allow fluid to seep into tissues, causing swelling in both the legs and the abdomen.
Blood Clots
A deep vein thrombosis (DVT) forms when blood clots in one of the large veins of your leg. It’s a medical emergency because the clot can break free and travel to your lungs. The key distinguishing feature is that DVT almost always affects one leg, not both. Symptoms include sudden swelling in one leg or foot, pain or tenderness that may only appear when you stand or walk, warmth over the swollen area, and skin that looks red or discolored.
If you notice sudden one-sided leg swelling, especially after a long period of immobility like a hospital stay, surgery, or long flight, seek emergency care. Don’t wait to see if it resolves on its own.
Infections
Cellulitis is a bacterial skin infection that commonly affects the feet and lower legs, causing rapid swelling along with warmth, pain, and reddened skin. The bacteria typically enter through a small crack, cut, or patch of dry skin. People with athlete’s foot, diabetes, or poor circulation are especially vulnerable because small skin breaks are more common and harder to notice.
Unlike a sprain, which swells around a specific joint and follows a known injury, cellulitis spreads outward from the infection site and often comes with fever, chills, or a general feeling of being unwell. A swollen rash that’s expanding rapidly, particularly with fever, needs immediate medical attention.
Injuries: Sprains and Fractures
Traumatic swelling is usually the easiest to identify because you know what happened. A sprained ankle or a foot fracture triggers an inflammatory response: blood vessels in the injured area dilate, and extra fluid rushes in to begin the repair process. The swelling is localized to the injury site, often accompanied by bruising, and typically peaks within the first 24 to 72 hours.
Stress fractures are the exception. These tiny cracks in bone develop gradually from repetitive impact, and the swelling can appear without a clear injury event. The top of the foot is a common location. If you have persistent swelling and pain that worsens with activity but you can’t recall a specific injury, a stress fracture is worth considering.
Lymphedema
Your lymphatic system acts as a drainage network, collecting excess fluid from tissues and returning it to your bloodstream. When lymph vessels are damaged or blocked, fluid accumulates and causes a distinct type of swelling called lymphedema. This can result from surgery (particularly procedures that remove lymph nodes), radiation therapy, infection, or in some cases, genetics.
Lymphedema looks and behaves differently from other types of swelling. In early stages, pressing a finger into the swollen area leaves a temporary dent, and elevating the limb helps. As it progresses, the tissue becomes firm and fibrotic, elevation no longer reduces swelling, and the skin develops a thickened, pebbled texture. A classic sign is the inability to pinch and lift the skin at the base of the second toe, known as a positive Stemmer sign. You may also notice exaggerated skin creases at the base of the toes and a rounded hump on top of the foot.
Lymphedema progresses through stages, from a latent phase with no visible swelling to advanced stages with significant skin changes and tissue hardening. Early treatment with compression and specialized massage is far more effective than waiting.
Pregnancy Swelling and Warning Signs
Some degree of foot and ankle swelling is normal during pregnancy, particularly in the third trimester, as blood volume increases and the growing uterus puts pressure on pelvic veins. But sudden swelling of the feet, hands, or face can signal preeclampsia, a serious pregnancy complication involving high blood pressure and protein in the urine.
Warning signs that distinguish preeclampsia from routine pregnancy swelling include a severe headache that doesn’t respond to simple pain relievers, vision changes like blurring or flashing lights, pain just below the ribs, vomiting, and persistent heartburn that doesn’t improve with antacids. Any of these symptoms alongside swelling need urgent evaluation.
Sodium, Medications, and Daily Habits
Not every swollen foot points to a serious condition. Several everyday factors can tip your fluid balance enough to cause noticeable puffiness.
High sodium intake is a major contributor. Salt causes your body to retain water, and the effects accumulate in the lowest parts of your body. The American Heart Association recommends staying under 1,500 mg of sodium per day. For context, a single fast-food meal can easily exceed 2,000 mg. People with heart failure are typically advised to stay below 2,000 mg daily, with even stricter limits for moderate to severe cases.
Prolonged sitting or standing keeps blood pooled in your lower legs. Long flights, desk jobs, and jobs that require standing in one spot for hours are common culprits. Simply flexing your ankles, taking short walks, or elevating your feet periodically can make a noticeable difference. Several common medications also cause fluid retention as a side effect, including certain blood pressure drugs, hormones (including birth control pills), steroids, and some diabetes medications.
One Foot vs. Both Feet
This distinction is one of the most useful clues when figuring out what’s causing swelling. Swelling in both feet and ankles usually points to a systemic issue: heart, kidney, or liver problems, medication side effects, or too much sodium. The underlying cause is affecting your entire circulatory system, and gravity sends the excess fluid to the lowest point.
Swelling in just one foot is more likely a local problem: a blood clot, injury, infection, or lymphatic blockage on that side. One-sided swelling that appears suddenly, with pain and warmth, is the pattern that most urgently needs evaluation, since both DVT and cellulitis can become dangerous quickly if untreated.
How Swelling Severity Is Measured
If you visit a doctor for foot swelling, they’ll likely press a finger into the swollen area for several seconds to check for “pitting,” the temporary dent left behind. Pitting edema is graded on a four-point scale based on how deep the dent is and how long it takes to bounce back. A barely detectable impression is grade 1. A slight indentation that takes about 15 seconds to refill is grade 2. A deeper dent lasting around 30 seconds is grade 3, and anything that takes more than 30 seconds to rebound is grade 4.
Not all swelling pits. Advanced lymphedema, for example, becomes too firm and fibrous to leave a dent. The absence of pitting in a chronically swollen foot actually provides useful diagnostic information, pointing away from heart or kidney causes and toward lymphatic problems.