First, what kind of ham do you have?

The label tells you, and it changes everything about how you cook it.

Fully cooked ("cooked", "ready to eat", and nearly all spiral-cut hams) is safe to eat cold. You're only warming it, to 140 °F.
Cook-before-eating hams are cured and smoked but not fully cooked. They must reach 145 °F, then rest for at least 3 minutes.
Fresh ham is an uncured leg of pork. It cooks like a pork roast, to 145 °F with a 3-minute rest, and tastes like roast pork rather than ham.
Country ham is dry-cured and salty. It needs soaking and simmering, so follow the package directions (the USDA suggests soaking 4 to 12 hours, then simmering 20 to 25 minutes per pound).

USDA ham cooking timetable (oven at 325 °F)

Ham	Weight	Minutes per lb	Heat to
Smoked, cook-before-eating
Whole, bone-in	10–14 lb	18–20	145 °F + 3 min rest
Half, bone-in	5–7 lb	22–25	145 °F + 3 min rest
Shank or butt portion, bone-in	3–4 lb	35–40	145 °F + 3 min rest
Arm picnic shoulder, boneless	5–8 lb	30–35	145 °F + 3 min rest
Shoulder roll (butt), boneless	2–4 lb	35–40	145 °F + 3 min rest
Smoked, fully cooked (reheat)
Whole, bone-in	10–14 lb	15–18	140 °F
Half, bone-in	5–7 lb	18–24	140 °F
Arm picnic shoulder, boneless	5–8 lb	25–30	140 °F
Canned, boneless	3–10 lb	15–20	140 °F
Vacuum-packed, boneless	6–12 lb	10–15	140 °F
Spiral-cut, whole or half	7–9 lb	10–18	140 °F
Fresh ham, uncooked
Whole leg, bone-in	12–16 lb	22–26	145 °F + 3 min rest
Whole leg, boneless	10–14 lb	24–28	145 °F + 3 min rest
Half, bone-in	5–8 lb	35–40	145 °F + 3 min rest

Source: USDA Food Safety and Inspection Service, "Hams and Food Safety." The 140 °F reheating temperature applies to cooked hams packaged in USDA-inspected plants. Cooked ham that has been repackaged anywhere else (for example, sliced and wrapped at a deli counter) should be reheated to 165 °F.

How much ham to buy

The USDA suggests ¼ to ⅓ pound per serving of boneless ham and ⅓ to ½ pound per serving of bone-in ham, since the bone accounts for part of the weight. Buy toward the top of the range if you want leftovers or ham is the only main dish.

Tips for a juicy ham

Cover it. Put the ham cut-side down in a roasting pan, add a little water, and cover tightly with foil so it doesn't dry out while it warms through. This matters most for spiral-cut hams.
Glaze at the end. Sugary glazes burn if they go on too early. Brush the glaze on during the last 15–20 minutes, uncovered.
Use a thermometer. Check the thickest part without touching the bone. The minutes-per-pound figures are approximate, and hams straight from the fridge vary.
Leftovers: refrigerate within 2 hours and eat within 3 to 5 days, or freeze for 1 to 2 months.

Frequently asked questions

How long do I heat a spiral ham?

At 325 °F, about 10 to 18 minutes per pound until it reaches 140 °F. An 8-pound spiral ham takes roughly 1⅓ to 2½ hours. Keep it tightly covered so the slices stay moist.

Can I eat a fully cooked ham cold?

Yes. Fully cooked hams packaged in USDA-inspected plants are safe to eat straight from the package. Heating is for flavor and texture.

How long does a 10-pound ham take?

A fully cooked, bone-in whole ham takes about 15 to 18 minutes per pound, or 2½ to 3 hours at 325 °F. A cook-before-eating whole ham takes 18 to 20 minutes per pound, or 3 to 3⅓ hours.

How big a ham for 12 people?

A bone-in ham of about 4 to 6 pounds, or a boneless ham of 3 to 4 pounds. Go bigger if you want leftovers for sandwiches and soup.