Calcium salt ↔ elemental calcium

Convert between the weight of a calcium salt and the elemental calcium it provides.

What is this for?

Calcium supplements are labelled by salt weight, but requirements and recommendations are expressed as elemental calcium. Carbonate is 40% elemental calcium and citrate 21%, so the two are not interchangeable gram for gram — a citrate product needs almost twice the salt weight for the same elemental dose.

How to use it

Select the salt form of the product.
Choose the direction — from a labelled salt weight to elemental calcium, or from an elemental target back to the salt weight needed.
Use the comparison table to see how much of each salt form would be needed for the same elemental dose.

Worked example

A calcium carbonate tablet is labelled 1250 mg. How much elemental calcium does it provide?

Answer: 1250 mg × 0.40 = 500 mg of elemental calcium. This is why 1250 mg carbonate tablets are commonly described as "500 mg calcium".

Clinical pearls & pitfalls

Calcium carbonate requires gastric acid for dissolution and should be taken with food. Calcium citrate does not, which makes it the preferred form for patients on proton pump inhibitors or with achlorhydria, and after bariatric surgery.
Because citrate is only 21% elemental, a citrate product needs nearly twice the salt weight of a carbonate product for the same elemental dose — which is why citrate tablets are larger and more numerous.
Absorption saturates at around 500 mg of elemental calcium per dose. A 1200 mg daily requirement should be split into two or three administrations.
Calcium binds levothyroxine, tetracyclines, fluoroquinolones, and bisphosphonates. Separate administration by at least two to four hours depending on the interacting drug.
Product labelling is inconsistent about whether the stated figure is salt or elemental. Check before counselling — this is a frequent source of confusion for patients.

Assumptions & limitations

Covers oral carbonate and citrate only. Parenteral calcium chloride and calcium gluconate have different elemental contents and are handled by the milliequivalents calculator.
Elemental percentages are for the anhydrous salts. Hydrated forms contain proportionally less calcium per unit weight.
Does not account for the fraction actually absorbed, which depends on vitamin D status, gastric acidity, dose size, and concurrent food.

References

Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington DC: National Academies Press; 2011.
Straub DA. Calcium supplementation in clinical practice: a review of forms, doses, and indications. Nutr Clin Pract. 2007;22(3):286-296.
Product labelling for calcium carbonate and calcium citrate oral products.