@Shadrach has stated something very important, and I want to stress that there are different forms of calcium, and they work differently as well as containing different amounts of actual calcium.
Calcium citrate has half the calcium as calcium carbonate, but the citrate works twice as fast. Due to these different properties, the citrate form is what we want to use in an emergency because of its speed in absorption. The carbonate has much higher calcium content so it's what we want to give regularly to keep calcium levels at optimum levels in our laying hens.