Calcium carbonate can be taken with food, making it easier for people to remember to take it. Also, calcium carbonate provides twice as much elemental calcium than calcium citrate. Below is another question with more detailed info.
What is the time frame for taking calcium and vitamins and what form of calcium?
You should take your multivitamin with a meal in order to maximize the digesting effects of stomach acids and help control the release of pill’s contents over a longer period of time. If you are also taking a calcium supplement in a carbonate form (calcium carbonate), you can take it at the same time, which is what we recommend for best results including ease of compliance. Also, take only 500mgs of calcium at a time because that’s about all the body can use effectively from one dose. Therefore, if your health professional tells you that you need to add 1000 mg/day of calcium in the form of a supplement, take calcium carbonate in 2-doses: One 500 mg tablet with a morning meal and one tablet with an evening meal. Only if for some reason you need to take calcium on an empty stomach, should you use calcium citrate because you don’t need stomach acid (which is released when you eat) to help it break it down. Otherwise both are absorbed equally. However - calcium carbonate gives you more for your money because 500 mg of calcium carbonate contains twice as much elemental calcium in the same amount of calcium citrate (40% by weight versus 21%, respectively).