Mild swelling in both ankles after a long day or hot weather is common and usually harmless. The concerning pattern is swelling, pain, warmth, redness, or tenderness in one leg, particularly the calf, which can indicate a deep vein thrombosis (a clot in a deep leg vein). This needs urgent assessment because the clot can travel to the lungs.
Risk is higher after long-haul travel, recent surgery, prolonged immobility, pregnancy, certain hormone medications, or with a personal or family history of clots. If leg symptoms are joined by sudden breathlessness, chest pain that worsens with breathing, coughing up blood, or a racing heart, this may be a pulmonary embolism, a clot in the lungs, which is a medical emergency: call your local emergency number.
Swelling of both legs that builds gradually can relate to heart, kidney, or liver conditions and should be reviewed by a doctor, though it is usually less acutely urgent than one-sided clot symptoms. When in doubt about a possible clot, seek prompt in-person care.