You have two kidneys, one on each side of your spine. They sit toward the back of your body, tucked behind your other organs in the area between your lowest ribs and your hips. Each one is roughly the size of your fist, about 11 centimeters (4.3 inches) long. If you’re feeling pain on one side and wondering which kidney it might be, the short answer is: whichever side hurts has a kidney there.
Where Exactly Your Kidneys Sit
Your kidneys are positioned deeper than most people expect. They’re not down near your waistline where you might picture them. Instead, they sit high in the back of your abdomen, roughly between the bottom of your rib cage and the top of your lower back. In anatomical terms, they span from about the level of your lowest rib down to the middle of your lower back.
They’re “retroperitoneal,” which means they sit behind the membrane that lines your abdominal cavity, pressed against the muscles of your back. That’s why kidney pain feels deep and internal rather than surface-level.
One important detail: your two kidneys aren’t perfectly level with each other. The right kidney sits slightly lower than the left because the liver, which is a large organ on your right side, pushes it down. The difference is small but consistent across most people. The left kidney is also slightly larger on average, with a median length of 11.2 cm compared to 10.9 cm on the right.
How to Locate the Area From the Outside
If you place your hands on your back with your thumbs pointing upward and your fingers wrapping around your sides just below your ribs, your thumbs are sitting right over your kidneys. The spot where your lowest rib meets your spine on each side is the key landmark doctors use when checking for kidney tenderness. Pain or sensitivity when pressing on that area often points to a kidney issue rather than a muscle problem.
Kidney Pain vs. Back Pain
Because the kidneys are nestled against the back muscles, it’s easy to confuse kidney pain with a pulled muscle or a sore back. The two feel quite different once you know what to look for.
Muscle-related back pain tends to change with movement. It gets worse when you bend, twist, or shift into certain positions, and it often improves when you find a comfortable way to sit or lie down. Kidney pain behaves differently. It stays constant regardless of how you move. You’ll feel it in the flank area, which is the space on either side of your spine below your ribs and above your hips. It doesn’t respond to stretching or repositioning the way a sore muscle would.
Kidney pain also tends to radiate. A dull, steady ache in the flank can spread forward into the lower abdomen. If the pain involves a kidney stone moving through the tube that connects the kidney to the bladder, it often becomes sharp and colicky, sometimes radiating into the groin or inner thigh. That radiation pattern is a strong signal that the pain is urinary tract-related, not muscular.
Common Causes of Pain on One Side
Pain that’s clearly centered over one kidney usually narrows the possibilities. The most common culprits are kidney stones and kidney infections.
- Kidney stones form when minerals and salts in your urine clump together into hard deposits. Small ones may pass without much trouble, but larger stones can get stuck in the urinary tract and cause intense, wave-like pain that comes and goes.
- Kidney infections happen when bacteria travel up from the bladder. They typically cause a steady, deep ache on the affected side along with fever, nausea, and pain when urinating.
- Blockages where the ureter connects to the kidney can cause swelling and a persistent ache on that side. Narrowing of the ureter has a similar effect.
- Polycystic kidney disease is a genetic condition where fluid-filled cysts grow on the kidneys, gradually enlarging them and causing pain as they expand.
Kidney tumors can also cause one-sided pain, though they rarely produce symptoms in early stages. When they do, blood in the urine is often the first noticeable sign.
Symptoms That Point to a Kidney Problem
Flank pain on its own could be many things. What makes it more likely to be kidney-related is the company it keeps. Fever alongside flank pain suggests infection. Blood in your urine, whether visible or discovered on a test, points toward stones, infection, or less commonly a growth. Nausea and vomiting are common with both stones and infections because the nerves serving the kidneys overlap with those serving the gut. Pain or burning during urination alongside flank pain is another strong indicator that something in the urinary tract needs attention.
If your pain is only on one side, stays constant regardless of body position, and comes with any of those additional symptoms, you’re dealing with something different from a typical backache.