You often cannot tell without an X-ray: bad sprains bruise like fractures, and small fractures can still be walked on. Break-leaning signs: deformity, a snap heard or felt, no weight-bearing at all, pinpoint bone-deep pain, numbness, or immediate ballooning swelling.
Sprain-leaning: you can hobble, the tenderness is diffuse and on soft tissue rather than a specific bone point. The practical clinician rule for ankles: inability to take four steps or bony tenderness at key points earns an X-ray. Same-day evaluation regardless: deformity, numbness or cold pale digits below the injury, an open wound, or zero improvement over 48 hours. First aid is the same either way: rest, ice 15 to 20 minutes with a cloth barrier, snug compression, elevation above the heart, acetaminophen or ibuprofen. Two warnings: never walk it off on a suspected break, and never skip rehab on a bad sprain, because unrehabilitated sprains are the ankles that keep re-rolling.
What to do
Common questions
Is it a sprain or a break?
The honest answer is that you often cannot tell without an X-ray, because bad sprains swell and bruise as dramatically as fractures, and small fractures sometimes let you walk. The signs that push toward a break, per MedlinePlus sprain and fracture guidance: a visible deformity or a limb that looks wrong; a snap or crack heard or felt at the injury; inability to bear any weight or use the limb at all; pain that is pinpoint and bone-deep rather than diffuse; numbness or tingling below the injury; and swelling that balloons immediately. Signs leaning sprain (stretched or torn ligament): you can hobble on it, pain is more diffuse around the joint, and it is tender on the soft tissue rather than on a specific point of bone. The rules clinicians use for the ankle specifically (the Ottawa rules) say an X-ray is warranted if you cannot take four steps, or there is bony tenderness at specific points, which captures the practical bottom line: cannot walk on it at all, get it imaged. The same-day-evaluation list regardless of suspicion: deformity, numbness or cold pale toes or fingers below the injury, an open wound over the injury, or pain that is not improving at all over 48 hours. First aid is identical for both while you decide: RICE, rest from using it, ice 15 to 20 minutes at a time with a cloth barrier, compression wrap (snug, not tourniquet), and elevation above heart level, plus acetaminophen or ibuprofen. Do not "walk it off" on a suspected break, and do not let a bad sprain go unrehabbed either: sprains that skip the strengthening and balance work are the ones that keep re-rolling.