Swollen feet happen when fluid builds up in the tissue beneath your skin, a condition called edema. The causes range from something as simple as sitting too long on a flight to serious conditions like heart failure or blood clots. Understanding what’s behind the swelling, whether it affects one foot or both, and how quickly it came on can help you figure out what you’re dealing with.
How Fluid Ends Up in Your Feet
Your smallest blood vessels, capillaries, constantly exchange fluid with the surrounding tissue. Under normal conditions, the pressure inside the capillary pushes fluid out, while proteins in your blood pull fluid back in. Your lymphatic system acts as a cleanup crew, carrying away any excess. Swelling develops when this balance tips: too much fluid gets pushed out, not enough gets pulled back in, or the lymphatic system can’t keep up.
Gravity makes your feet the most common landing spot. Blood returning from your legs has to travel upward against gravity, and any weakness in that system lets fluid settle at the lowest point. That’s why your feet swell more at the end of the day and why elevating them helps.
Prolonged Standing, Sitting, and Salt Intake
The most common cause of foot swelling is simply being on your feet (or off them in a chair) for extended periods. When your legs stay in one position, the calf muscles that normally squeeze blood back toward the heart aren’t doing their job. Fluid pools in the lower legs. Long flights, desk jobs, and road trips are frequent culprits.
High sodium intake plays a direct role, too. Salt causes your body to retain water, increasing the volume of fluid your blood vessels have to manage. For people already prone to swelling, Georgetown University’s nephrology division recommends keeping daily sodium between 1,375 and 1,800 milligrams, well below the 3,400 milligrams most Americans consume. Cutting back on processed food is often the single most effective dietary change.
Venous Insufficiency and Varicose Veins
Your leg veins contain one-way valves that open to let blood flow toward the heart and close to stop it from falling back toward the feet. When those valves weaken or fail, blood flows backward (called reflux), and pressure in the lower veins rises. That increased pressure forces fluid out of the blood vessels and into the surrounding tissue.
This problem, chronic venous insufficiency, is remarkably common. More than 25 million adults in the United States have varicose veins, and over 6 million have more advanced venous disease. Prevalence rises significantly with age: roughly 21% of men and 12% of women over 50 show signs of it. Valve damage from a previous blood clot is one of the most frequent causes, though valves can also weaken simply with age, obesity, or prolonged standing over many years.
Early signs include swelling that worsens throughout the day and improves overnight, visible varicose veins, and a heavy or aching sensation in the legs. Over time, the skin near the ankles can darken or become leathery. About 20% of people with chronic venous insufficiency eventually develop venous ulcers, slow-healing sores near the ankle.
Heart Failure
When the heart can’t pump blood efficiently, blood lingers in the veins instead of circulating forward. That backup raises pressure inside the veins, which forces fluid into the surrounding tissue. This chain reaction is why swollen feet and ankles are one of the hallmark signs of congestive heart failure.
Heart-related swelling typically affects both feet and ankles equally, worsens over the course of the day, and may be accompanied by shortness of breath (especially when lying flat), fatigue, or rapid weight gain from fluid retention. If your shoes suddenly feel tight and you’re also getting winded doing things that used to be easy, those symptoms together warrant prompt medical attention.
Kidney Disease
Your kidneys filter waste and excess fluid from the blood. When they’re not working properly, sodium and water accumulate instead of being excreted, leading to swelling throughout the body. In a condition called nephrotic syndrome, the kidney’s filtering units become inflamed and allow too much protein to leak into the urine. Losing that protein, especially albumin, reduces the blood’s ability to pull fluid back from the tissues.
The NIDDK notes that symptoms of nephrotic syndrome include puffy eyelids, swelling in the legs, ankles, feet, and lower abdomen, and unexplained weight gain from fluid retention. Treatment typically involves reducing salt and fluid intake alongside medications that help the kidneys remove excess fluid.
Lymphedema
When the lymphatic system itself is blocked or damaged, fluid accumulates because there’s no drainage pathway. This is lymphedema, and it causes a distinctive type of swelling that doesn’t always respond to elevation the way other types do.
Lymphedema can be inherited (primary) or caused by surgery, radiation therapy, infection, or injury to the lymph nodes (secondary). One useful physical sign: try to gently pinch the skin on the top of your foot near the base of the second toe. If the skin is too thick or swollen to pinch, that’s a positive Stemmer sign, a reliable indicator of lymphedema. Unlike most other causes of foot swelling, lymphedema tends to feel firm rather than squishy, and pressing on it may not leave an indent.
Medications That Cause Swelling
Several common medications list foot and ankle swelling as a side effect. The most well-known culprits are calcium channel blockers, a class of blood pressure medications. These drugs relax blood vessel walls, which can allow more fluid to leak into surrounding tissue. The swelling is dose-related: at lower doses, it affects 1 to 15% of patients, but at higher long-term doses the rate can exceed 80%.
Other medication classes that commonly cause foot swelling include:
- Anti-inflammatory painkillers (NSAIDs) like ibuprofen and naproxen, which cause sodium and fluid retention
- Diabetes medications in certain classes that promote fluid retention
- Steroids like prednisone, especially at higher doses or with prolonged use
- Hormone therapies including estrogen-containing birth control and hormone replacement
If you notice swelling that started around the same time as a new prescription, that’s worth mentioning to your prescriber. Stopping or switching medications often resolves the problem completely.
Pregnancy-Related Swelling
Some foot swelling during pregnancy is normal, particularly in the third trimester. The growing uterus puts pressure on the veins returning blood from the legs, and hormonal changes cause the body to retain more fluid. Mild, symmetrical swelling that comes and goes is generally expected.
What’s not normal is sudden, severe swelling, especially if it’s accompanied by headaches, vision changes, or upper abdominal pain. These can be signs of preeclampsia, a pregnancy complication defined by new-onset high blood pressure (140/90 or above) after 20 weeks of gestation, along with protein in the urine or signs of organ stress. Preeclampsia requires immediate medical evaluation because it can progress rapidly.
When Swelling Affects Only One Foot
Bilateral swelling (both feet) usually points to a systemic cause: heart, kidney, liver, or medication-related. Swelling in just one foot or leg tells a different story and raises the concern for a deep vein thrombosis, a blood clot in one of the deep veins of the leg.
A DVT typically causes swelling, warmth, redness, and pain in one leg. It can develop after surgery, prolonged immobility, or in people with clotting disorders. Clinicians use a scoring system called the Wells score to estimate the likelihood, but the key point for you is this: new, unexplained swelling in one leg, particularly if it’s painful or warm, needs same-day medical evaluation. An untreated clot can break loose and travel to the lungs.
Other one-sided causes include an injury you might not remember (a stress fracture or sprain), an infection, or lymphedema affecting one limb.
How Swelling Is Assessed
When a doctor examines swollen feet, one of the first things they do is press a finger firmly into the swollen area for several seconds and then release. If the pressure leaves an indent that slowly fills back in, that’s called pitting edema, and it’s graded on a scale:
- Grade 1: A shallow 2 mm pit that rebounds immediately
- Grade 2: A 3 to 4 mm pit that fills back in under 15 seconds
- Grade 3: A 5 to 6 mm pit that takes 15 to 60 seconds to rebound
- Grade 4: An 8 mm pit that takes 2 to 3 minutes to fill back in
Higher grades suggest more significant fluid accumulation and generally prompt more urgent investigation into the underlying cause. Non-pitting edema, where pressing doesn’t leave an indent, is more characteristic of lymphedema or thyroid-related swelling and points the evaluation in a different direction.