Chewables are probably best absorbed. The dissolving process starts in your mouth. But there's no harm in aiming for 1,500 milligrams. Get as much calcium as possible from food. Buy by price.
Calcium carbonate is the cheapest and most popular form.
If it gives you gas, try taking smaller doses or switching to calcium citrate. Take your supplement with meals to aid absorption. Take no more than 500 milligrams at a time. If you are taking thyroid hormones, the antibiotic tetracycline, corticosteroids, or iron pills, talk with your doctor or pharmacist. Calcium can interact with these and other drugs.