Which Form of Calcium Is Best?
Our pick for most people is Kirkman Calcium Bisglycinate Chelate: bisglycinate is the form ChatGPT ranks first in all 8 of its answers, and each capsule gives 200 mg of calcium. Want more calcium per pill? NOW Calcium Hydroxyapatite gives 500 mg in 2 capsules for $0.17. Want a tested one? Kirkland’s citrate is USP Verified.
By Erin Rose Updated November 2026 How we score
1. Our pick: bisglycinate, the form ChatGPT ranks first
Kirkman Calcium Bisglycinate Chelate 200 mg (120 capsules)
- Calcium
- 200 mg per capsuleFrom calcium bisglycinate
- A day
- 1 to 3 capsulesUp to 600 mg of calcium
- Inside
- Calcium onlyNot on a testing list
- Cost
- $0.81 per 600 mg$0.27 a capsule; $32.50 for 120
Why we pick it Bisglycinate is calcium bound to glycine, an amino acid. ChatGPT ranks it first in all 8 of its saved answers to this question, and the label lists calcium bisglycinate as the only calcium form. Each capsule gives 200 mg of calcium; a NOW hydroxyapatite capsule gives 250 mg and a Mason oyster shell tablet 500 mg. Three Kirkman capsules give 600 mg for $0.81; NOW costs $0.20, Mason $0.05 and Nature Made’s USP Verified carbonate $0.07 per 600 mg. For readers who also want vitamin D, Kirkman sells the same 200 mg capsule with 2 mcg of vitamin D3 for $0.61 per 600 mg.
One product in each form, in ChatGPT’s order
2. Hydroxyapatite: NOW, the form of calcium in bone
Hydroxyapatite is calcium with phosphate, the form NIH says holds more than 99% of the calcium in your body. ChatGPT ranks it second in 5 of its 8 answers. NOW’s comes from Australian pasture-fed cattle: 2 capsules give 500 mg of calcium and 200 mg of phosphorus. It costs $0.20 per 600 mg, the lowest of the 3 hydroxyapatites here.
NOW Calcium Hydroxyapatite
$0.33 a day at 4 capsules · $9.99 for 120
3. Oyster shell: Mason Natural, the lowest cost per 600 mg here
The label says its calcium is from oyster shell and calcium carbonate. One tablet gives 500 mg of calcium and 400 IU of vitamin D3, taken once a day with food. It costs $0.05 per 600 mg, the lowest of the 10 here.
Mason Natural Oyster Shell Calcium
$0.04 a day at 1 tablet · $10.03 for 250
4. Coral: Nutricost, 700 mg of calcium in 2 capsules
Coral calcium comes from coral, a sea animal. Two capsules give 700 mg of calcium from 2,000 mg of coral calcium powder.
Nutricost Coral Calcium
$0.15 a day at 2 capsules · $18.50 for 240
5. Algae: PlantFusion, calcium from red seaweed
Three PlantFusion tablets give 1,000 mg of calcium from Aquamin red algae and 800 IU of vitamin D3 from lichen, and the bottle says vegan. More in best vegan calcium.
PlantFusion Vegan Plant-Based Calcium
$0.92 a day at 3 tablets · $27.58 for 90
6. Citrate: Kirkland, on the USP Verified list
ChatGPT names citrate in 7 of its 8 answers, and NIH says it can be taken without food. Two Kirkland tablets give 500 mg of calcium, 800 IU of vitamin D3, 80 mg of magnesium and 10 mg of zinc.
Kirkland Signature Calcium Citrate, Magnesium and Zinc
$0.08 a day at 2 tablets · $18.85 for 500
Choose Kirkman for the form ChatGPT ranks first, NOW for hydroxyapatite at $0.20 per 600 mg, Mason for the lowest cost per 600 mg of the 10 here, Nutricost for coral, PlantFusion for a vegan algae calcium, and Kirkland for a citrate on a testing list.
ChatGPT ranks bisglycinate first, then hydroxyapatite or algae
In its 8 saved answers to this question, ChatGPT ranks bisglycinate first in all 8, then hydroxyapatite in 5 and algae in 3; oyster shell and coral come after them. It names calcium citrate, which the question leaves out, in 7 of the 8. It names forms, not products.
Its tips in most answers: read the elemental calcium on the label, take 500 mg or less at a time, get enough vitamin D, and look for outside testing.
We checked 37 calciums by form; five are on a testing list
We checked 37 calcium supplements sold on Amazon by the form of calcium on the label: 13 algae, 8 hydroxyapatite, 3 coral, 3 bisglycinate, 2 oyster shell, 2 citrate, 2 malate, 2 mixes of forms, 1 eggshell and 1 liquid calcium chloride.
Five of the 37 are on a third-party testing list: Thorne and Momentous (malate), Klean Athlete and Theralogix (citrate) and ReCalcia (liquid).
Here are the 10 with a buy button: 8 of the 37, plus Nature Made’s carbonate and Kirkland’s citrate, both USP Verified. Our pick comes first, then the three on a testing list, then the rest. Each group runs from lowest cost per 600 mg of calcium.
| Supplement | A day | Cost | Buy |
|---|---|---|---|
| Our pick | |||
 Kirkman Calcium Bisglycinate ChelateOur pick120 capsulesBisglycinate; calcium only | 3 capsules: 600 mg calcium | $0.81 per 600 mg$0.81 a day | See on Amazon |
| On a testing list (USP Verified or NSF/ANSI 173) | |||
 Nature Made Calcium 600 mg with D3220 tabletsCarbonate; USP Verified; adds vitamin D3 | 2 tablets: 1,200 mg calcium | $0.07 per 600 mg$0.13 a day | See on Amazon |
| Kirkland Signature Calcium Citrate, Magnesium and Zinc500 tabletsCitrate; USP Verified; adds vitamin D3, magnesium and zinc | 2 tablets: 500 mg calcium | $0.09 per 600 mg$0.08 a day | See on Amazon |
 Thorne Calcium120 capsulesMalate (DimaCal); NSF/ANSI 173 | 2 capsules: 500 mg calcium | $0.53 per 600 mg$0.44 a day | See on Amazon |
| Not on a testing list | |||
| Mason Natural Oyster Shell Calcium250 tabletsOyster shell; adds vitamin D3 | 1 tablet: 500 mg calcium | $0.05 per 600 mg$0.04 a day | See on Amazon |
| Nutricost Coral Calcium240 capsulesCoral | 2 capsules: 700 mg calcium | $0.13 per 600 mg$0.15 a day | See on Amazon |
| NOW Calcium Hydroxyapatite120 capsulesHydroxyapatite from cattle bone; adds phosphorus | 4 capsules: 1,000 mg calcium | $0.20 per 600 mg$0.33 a day | See on Amazon |
 Jarrow Formulas BoneUp360 capsulesHydroxyapatite; adds vitamin D3, K2 and magnesium | 6 capsules: 1,000 mg calcium | $0.35 per 600 mg$0.58 a day | See on Amazon |
| PlantFusion Vegan Plant-Based Calcium90 tabletsAlgae; adds vitamin D3 from lichen | 3 tablets: 1,000 mg calcium | $0.55 per 600 mg$0.92 a day | See on Amazon |
 Pure Encapsulations Calcium MCHA180 capsulesHydroxyapatite; one capsule a day | 1 capsule: 250 mg calcium | $0.89 per 600 mg$0.37 a day | See on Amazon |
Compare the calcium on the label, and take 500 mg or less at a time
NIH says each form holds a different share of calcium: carbonate is 40% calcium by weight and citrate 21%. The Supplement Facts panel lists the calcium itself, called elemental calcium. Adults need 1,000 to 1,200 mg a day from food and supplements together.
NIH says absorption from supplements is highest at 500 mg or less at a time, and better with food. Each Kirkman capsule is 200 mg, so one at a time, up to 3 a day, stays under that. More in our citrate vs carbonate guide and calcium dosage guide.
The labels: 200 mg of calcium in a Kirkman capsule, 500 mg in 2 NOW capsules
The Kirkman and NOW labels
Questions people ask
How many Kirkman capsules should I take a day?
The label says one capsule one to three times a day: 200 to 600 mg of calcium. Count the calcium in your food first; NIH says adults need 1,000 to 1,200 mg a day from food and supplements together.
Which form gives the most calcium per pill?
Of the 10 here, Nature Made’s carbonate tablet gives the most: 600 mg. Mason’s oyster shell tablet gives 500 mg and a NOW hydroxyapatite capsule 250 mg. Kirkman’s bisglycinate capsule gives 200 mg.
How we picked
On November 9, 2026 we read the Amazon listing of each of the 37 and the form of calcium on its label, read the label photos of the 10 with a buy button, and searched five testing lists for each: NSF/ANSI 173, NSF Certified for Sport, Informed Choice, Informed Sport and USP Verified. Prices are Amazon’s, read November 9, 2026.
The other 29 we checked: Momentous Calcium and Klean Athlete Klean Calcium Citrate (both NSF/ANSI 173 and NSF Certified for Sport); Theralogix TheraCal D4000 and ReCalcia liquid calcium (both NSF/ANSI 173); hydroxyapatite from Nutricost, Douglas Laboratories, Country Life Triple Action Bone Solid, Metagenics Bone Builder and NOW Bone Strength; algae calcium from NEUMINA, NatureWise, New Chapter (Take Care and Tiny Tabs), Garden of Life (mykind Organics and Vitamin Code), NaturesPlus, AlgaeCal (Plant Based and Plus), Pure Nutrition, Future Kind and Supplements Studio; Horbaach and Bob Barefoot coral calcium; MAJOR oyster shell calcium; Kirkman Calcium 200 mg with Vitamin D3 and Bluebonnet Albion Calcium Magnesium Chelate (bisglycinate); Swanson Eggshell Calcium; and the mixes Life Extension Bone Restore and DEVA Vegan Cal-Mag.
Sources
- Amazon listings and label photos, prices read November 9, 2026.
- Testing lists, read November 9, 2026: NSF/ANSI 173 list; NSF Certified for Sport; Informed Choice; Informed Sport; USP Verified.
- NIH Office of Dietary Supplements, Calcium fact sheet for health professionals, read November 9, 2026: Calcium.