The four main reasons for calf pain are:
Muscular pain is typically on a specific point when you press it. You know when you did something to it and it hurts to go up to tip toes, to hop on or stretch the calf. See the calf muscle page for more information.
Achilles Tendon pain is lower in the calf in the Achilles tendon. See the Achilles pages for more information.
Deep Vein thrombosis feels like a tight muscle, is tender to press and is hot and swollen. If in doubt check here.
Referred pain from the spine is not tender to press or to hop or stretch but does feel tight and achy. It can also have burning and pins and needles alongside. You usually have some symptoms in the thigh, buttock and back as well. See lower back pages for more detail.