Yes, the radius is on the thumb side of your forearm. It runs from your elbow down to your wrist on the same side as your thumb, making it the outer bone of your forearm when your palms face forward. The other forearm bone, the ulna, sits on the pinky side.
How to Tell Which Bone Is the Radius
In anatomy, the thumb side of the forearm is called the “lateral” side, and the pinky side is called the “medial” side. The radius is the lateral bone. An easy way to confirm this on your own body: feel the bony bump on the outside of your wrist, just below the base of your thumb. That’s the radial styloid process, the very end of the radius bone. It sticks out further than the corresponding bump on the pinky side (the ulna), which is why the thumb side of your wrist feels more prominent.
Another reliable landmark is your pulse. When a nurse checks your pulse at the wrist, they press two fingers into the groove on the thumb side of your inner wrist, compressing the radial artery against the radius bone underneath. The artery is named after the bone it runs alongside.
Why the Radius Matters More Than You’d Think
The radius is actually the larger and more functionally important of the two forearm bones, even though many people assume the ulna (which forms the point of the elbow) is the main one. The radius carries the majority of the load between your hand and elbow, and it connects directly to the wrist joint. The ulna, by contrast, plays a bigger role at the elbow.
What makes the radius especially interesting is how it moves. Your radius and your hand function as a single unit that rotates around the ulna. When you flip your palm face-up (like holding a bowl of soup), the radius and ulna run parallel to each other. When you turn your palm face-down, the radius crosses over the ulna, forming an X shape. This crossing motion is what allows you to rotate your wrist and turn doorknobs, use a screwdriver, or pour from a bottle. Three separate joints between the radius and ulna make this rotation possible, and a tough membrane between the two bones keeps everything stable while they move.
The Most Common Forearm Fracture
Because the radius bears so much force at the wrist, it’s the most commonly broken forearm bone. A distal radius fracture (a break near the wrist end) typically happens when you fall and catch yourself with an outstretched hand. The impact travels up through the palm and snaps the radius just above the wrist.
The most frequent version of this injury is called a Colles fracture, where the broken piece of bone tilts backward. This creates a visible deformity at the wrist that looks like the back of a fork. These fractures are especially common in older adults with reduced bone density and in younger people during high-energy falls from bikes or during sports. Recovery typically involves a cast for several weeks, though more severe breaks may need surgical repair with a plate and screws to hold the bone in alignment while it heals.
Quick Reference: Radius vs. Ulna
- Radius: Thumb side, larger at the wrist, smaller at the elbow, rotates during forearm movement
- Ulna: Pinky side, forms the point of the elbow, stays relatively fixed during rotation
A simple memory trick: “radius” and “thumb” don’t share any letters, but the radius points toward the thumb when your palm faces forward. Some people remember it by thinking of a clock’s radius sweeping in a circle, just like the radius bone sweeps across the ulna when you rotate your forearm.