A leg that swells from the knee down usually points to a problem with blood flow, fluid drainage, or infection in that limb. The swelling can develop in hours or build gradually over weeks, and the timeline matters because it helps narrow the cause. Some possibilities are harmless and manageable at home, while others need same-day medical attention.
Causes That Need Urgent Attention
The most serious explanation for sudden swelling in one lower leg is a deep vein thrombosis, or blood clot. A clot blocks blood from returning to the heart, so fluid backs up below the blockage. The leg typically feels warm, tender along the inner calf or thigh, and may look slightly red or discolored. Swelling that appears over hours to a couple of days, especially if only one leg is affected, should be evaluated promptly. Doctors use a scoring system called the Wells criteria to estimate clot risk: factors like recent surgery, immobilization, active cancer, a calf that measures more than 3 cm larger than the other leg, and pitting edema each raise the probability. A person with three or more of those risk factors has roughly a 53% chance of actually having a clot.
The danger of an untreated clot is that a piece can break off and travel to the lungs. If you develop sudden shortness of breath, a rapid heartbeat, chest pain, or pale and clammy skin alongside a swollen leg, that combination suggests the clot may have migrated. This is a medical emergency.
Cellulitis, a bacterial skin infection, is the other common cause of rapid-onset swelling. It produces redness, warmth, tenderness, and sometimes fever. The redness spreads with smooth, indistinct borders and is almost always limited to one leg. A history of a cut, insect bite, or cracked skin gives bacteria an entry point. People with diabetes or weakened immune systems are at higher risk. Cellulitis progresses over days rather than weeks, and it typically responds well to antibiotics when caught early.
Chronic Venous Insufficiency
If your leg swelling has been building for months or comes and goes depending on how long you’ve been on your feet, chronic venous insufficiency (CVI) is the most likely culprit. The veins in your legs contain one-way valves that push blood back up toward the heart. When those valves weaken or fail, blood pools in the lower leg, and the increased pressure forces fluid into the surrounding tissue. Valve failure can happen on its own due to inherited weakness in the vein walls, or it can follow a previous blood clot that damaged the valves. An estimated 2.5 million people in the United States have CVI, and about 20% of them eventually develop skin ulcers from it.
The hallmark pattern is swelling that worsens after standing or sitting for long stretches and improves overnight. Over time, CVI produces visible changes: varicose veins, brownish discoloration around the ankles, dry and itchy skin (stasis dermatitis), and in advanced cases a hardening of the tissue that gives the lower leg a narrowed, “inverted champagne bottle” shape. Stasis dermatitis is often mistaken for cellulitis, but there are clear differences. Stasis changes tend to affect both legs, develop over years, and the skin is typically not tender. Cellulitis, by contrast, is almost always one-sided, progresses in days, and hurts.
Lymphedema
Your body has a second drainage network alongside your veins called the lymphatic system, which clears excess fluid and immune cells from your tissues. When this system is damaged or overwhelmed, fluid accumulates and causes swelling that feels different from the puffy, water-balloon quality of venous edema. Lymphedema progresses through distinct stages:
- Stage 0: No visible swelling, but you notice heaviness, tightness, or tingling in the leg. Fluid changes are happening below the surface.
- Stage 1: Swelling comes and goes. Pressing on the skin leaves a dent (pitting), and elevating the leg brings relief.
- Stage 2: The swelling no longer goes away with elevation. The tissue feels firmer because scar tissue (fibrosis) has formed from prolonged fluid buildup. Pitting becomes less obvious.
- Stage 3: The tissue is very hard, the skin thickens, and the limb becomes prone to repeated infections because stagnant lymph fluid is an ideal breeding ground for bacteria.
One reliable sign of lymphedema is the Stemmer sign: if you can’t pinch a fold of skin on top of the second toe, lymphedema is very likely. Common causes include surgery or radiation that removed lymph nodes (often during cancer treatment), repeated infections, and obesity. In some people, the lymphatic system is simply underdeveloped from birth.
Medication-Related Swelling
Certain blood pressure medications are a surprisingly common cause of lower leg swelling. Calcium channel blockers, particularly the type called dihydropyridines (amlodipine is the most widely prescribed), relax blood vessel walls in a way that lets more fluid leak into surrounding tissue. The effect is dose-dependent: at lower doses, ankle swelling occurs in 1 to 15% of people, but at high doses taken long-term the rate can exceed 80%. This swelling typically affects both legs and ankles, worsens as the day goes on, and isn’t dangerous, but it can be uncomfortable enough to warrant a medication change. Other drugs that commonly cause leg swelling include certain diabetes medications, anti-inflammatory painkillers, and steroids.
How to Read Your Swelling
You can learn a lot by pressing a finger firmly into the swollen area for about ten seconds. If it leaves a visible dent that takes time to fill back in, that’s called pitting edema, and it suggests the swelling is fluid-based, as seen with blood clots, venous insufficiency, or medication side effects. Doctors grade this on a four-point scale: a shallow 2 mm dent that rebounds immediately is grade 1, while a deep 8 mm pit that takes two to three minutes to fill is grade 4.
If pressing the skin doesn’t leave a dent and the tissue feels firm or rubbery, that points toward later-stage lymphedema, where scar tissue has replaced the fluid component. Skin that looks shiny, brownish, or feels thickened and leathery suggests a long-standing process like chronic venous insufficiency rather than something new.
What Helps Reduce the Swelling
For most causes of knee-down swelling, elevating your legs above heart level for about 15 minutes, three to four times a day, provides meaningful relief. You can do this by lying on your back with pillows stacked under your calves so your feet are higher than your chest. This uses gravity to help fluid drain back toward the heart.
Compression stockings are the mainstay treatment for both CVI and lymphedema. They apply graduated pressure, tightest at the ankle and looser up the calf, to support the veins and lymphatic vessels in moving fluid upward. For CVI, compression can slow the progression of skin changes and reduce the risk of ulcers. For lymphedema, compression is combined with specialized massage techniques that manually redirect fluid through working lymphatic channels.
Staying active matters more than any single intervention. Walking engages the calf muscles, which act as a pump that squeezes blood back up through the veins. Prolonged sitting or standing without movement is one of the biggest contributors to lower leg swelling regardless of the underlying cause. If your job keeps you in a chair or on your feet all day, brief walking breaks every 30 to 60 minutes can make a noticeable difference.
Reducing salt intake helps because sodium causes the body to retain water, which worsens any edema. This is especially true for people whose swelling is related to venous insufficiency or medication side effects.
Patterns That Point to the Cause
A few questions can help you and your doctor zero in on what’s happening. Is the swelling in one leg or both? One-sided swelling raises concern for a blood clot, infection, or lymphatic damage, while bilateral swelling more often points to venous insufficiency, medication effects, or systemic issues like heart or kidney problems. Did it come on suddenly or gradually? Rapid onset over hours to days suggests a clot, infection, or injury. Slow onset over weeks to months points to CVI or lymphedema. Does the swelling improve when you lie down overnight? If yes, that favors a venous cause. If the swelling barely changes with position, lymphedema or tissue scarring is more likely.
Left-sided leg swelling with a blood clot has a specific anatomic explanation worth knowing about. In a condition called May-Thurner syndrome, the main vein draining the left leg gets compressed by an overlying artery, which increases venous pressure and clot risk on that side. It’s an underdiagnosed cause of left leg DVT, particularly in younger women.