A knee with excess fluid typically looks puffy or swollen compared to your other knee, feels stiff when you try to bend it fully, and may ache with a dull, pressure-like sensation rather than a sharp pain. The swelling usually develops around and just above the kneecap, giving the joint a rounded, “pillowy” appearance that obscures the normal bony contours you can usually see and feel.
Visual Signs to Look For
The easiest way to check is to compare both knees side by side. Sit on a flat surface with both legs extended and relaxed. Look at the shape of each knee from the front. A knee with fluid inside the joint will look fuller, especially in the hollows on either side of the kneecap and in the area just above it (the suprapatellar pouch). Those natural dips and indentations that you can see on a normal knee get filled in and disappear when fluid accumulates.
The swelling from joint fluid tends to be diffuse and spread evenly around the entire knee, rather than concentrated in one spot. This is one way to distinguish it from bursitis, where swelling sits in a more localized, well-defined pocket right over a specific area, often directly on top of the kneecap. If the swelling looks like a distinct, egg-shaped lump sitting on the surface of your knee rather than a general puffiness throughout the joint, bursitis is more likely.
How It Feels
Fluid inside the joint creates a sensation of tightness and pressure, especially when you bend the knee past a certain point. You might notice that you can’t fully bend or fully straighten the knee without discomfort. Many people describe it as feeling like the knee is “full” or like something is preventing normal movement.
The knee may also feel warm to the touch. Place one hand on the affected knee and the other on the healthy one and compare. Warmth suggests some degree of inflammation inside the joint. If the knee is hot, red, and extremely painful, especially with a fever, that combination raises the possibility of a joint infection, which needs urgent medical attention.
Two Simple Physical Tests
Clinicians use a couple of hands-on tests to confirm fluid in the knee, and you can try simplified versions at home to get a rough sense of what’s going on.
The Stroke (Wipe) Test
This test works best for detecting small amounts of fluid. Lie on your back with the affected leg flat and relaxed. Using the palm of your hand, stroke upward along the inner (medial) side of your knee two or three times, pushing from the joint line toward the thigh. This moves any fluid from the inner side of the knee up into the pouch above the kneecap. Then, with one smooth stroke, sweep downward along the outer (lateral) side of the thigh, just above and to the outside of the kneecap.
If fluid is present, you’ll see a small wave or bulge appear on the inner side of the knee within a second or two of the downward stroke. A tiny ripple suggests a trace amount of fluid. A larger, obvious bulge means more. If the fluid immediately flows back to the inner side before you even do the downward stroke, there’s a significant amount. And if the knee is so swollen that you can’t push the fluid away from the inner side at all, that indicates a large effusion.
The Patellar Tap Test
This one detects moderate to large amounts of fluid. With your leg extended and relaxed, place one hand above your kneecap and gently squeeze downward to push any fluid from the thigh into the space behind the kneecap. Then, with your other hand, press the kneecap straight down toward the bone behind it using a quick, firm tap with your index finger. In a normal knee, the kneecap sits snugly against the bone beneath it. If there’s fluid, the kneecap will feel like it’s floating, and you’ll feel it tap or “bounce” against the bone when you push it down. That bouncing sensation is a positive sign of fluid.
Common Causes
The three most common reasons people develop knee fluid in a primary care setting are osteoarthritis, trauma, and gout. But the full list is broad.
Injuries that cause fluid buildup include torn ligaments (particularly the ACL), meniscus tears, fractures, and general overuse irritation. These tend to cause swelling that develops within hours of the injury, and the fluid may contain blood if a ligament or bone was damaged.
Chronic and inflammatory conditions are the other major category. Osteoarthritis causes the joint to produce excess fluid as cartilage wears down. Rheumatoid arthritis, gout, and pseudogout all trigger inflammation that leads to fluid production. Less commonly, infections, cysts, and tumors can also be responsible. If your knee swells without any obvious injury or overuse, an underlying inflammatory or metabolic condition is worth investigating.
When Swelling Signals Something Serious
Most knee effusions are uncomfortable but not dangerous. A joint infection (septic arthritis) is the exception. It typically causes severe pain that comes on fast, makes it nearly impossible to use the joint, and is often accompanied by fever, significant warmth, and skin color changes over the knee. This is a situation that requires same-day medical evaluation, because untreated joint infections can cause permanent damage quickly.
Other reasons to seek prompt evaluation: swelling after a significant injury (like a fall, twist, or impact), inability to bear weight, or a knee that locks and won’t straighten.
How Doctors Confirm and Evaluate Fluid
If a physical exam suggests fluid, imaging can confirm it and help identify the cause. Ultrasound is often the first step because it’s quick, widely available, and has no radiation. Compared to MRI, ultrasound correctly identifies knee effusions about 81% of the time and is especially accurate at detecting fluid along the sides of the joint and behind the knee, where it picks up fluid with over 95% sensitivity. MRI remains the gold standard for seeing exactly how much fluid is present and evaluating surrounding structures like ligaments and cartilage.
For cases where the cause is unclear, especially when infection, gout, or inflammatory arthritis is suspected, doctors may use a needle to draw out a sample of the fluid. The appearance of the fluid provides immediate clues: normal joint fluid is clear and pale yellow, while cloudy or opaque fluid suggests inflammation or infection, and bloody fluid points to a traumatic injury. The sample can also be tested for crystals (which confirm gout or pseudogout) and bacteria (which confirm infection).
Fluid From Injury vs. Arthritis vs. Infection
The timeline and character of your symptoms can help narrow down the cause before you ever see a doctor. Fluid from an acute injury usually appears within hours, is often associated with a specific event you can point to, and may cause the knee to feel unstable. Fluid from osteoarthritis tends to build gradually, worsens with activity, and improves somewhat with rest. It often comes and goes over weeks or months.
Fluid from gout comes on suddenly and intensely, often overnight, with the knee becoming red, hot, and exquisitely tender. Infected joints also present with rapid onset of severe pain and warmth, but typically with fever and a general feeling of being unwell. If you’ve recently had a skin wound near the knee, a medical procedure on the joint, or a weakened immune system, infection should be higher on your radar.