Calcium
The most abundant mineral in the body. Bone is where 99% of it lives.
About 99% of the body's calcium lives in bone and teeth, where it acts as a structural reserve and is slowly drawn upon when dietary intake falls short. The remaining 1% circulates in blood and is tightly regulated regardless of how much you take in. Calcium from food is generally better absorbed than from supplements, and spreading supplemental intake across the day improves absorption. D3 and K2 both affect how well calcium does its job, which is why the three are often considered together.
Reference values
Daily reference values from official nutrition authorities. These are population reference figures, not personal recommendations.
| Group | RDA | Upper limit | Source |
|---|---|---|---|
| General adult | 700 mg | 2500 mg | UK SACN DRVs (1991); EFSA UL 2500 mg (2018) |
Forms and bioavailability
Not all forms are absorbed equally. The form on your supplement label affects how much your body can actually use.
| Form | Absorption | With food | Timing | Evidence |
|---|---|---|---|---|
| calcium carbonate | Typical | With food | Either | Strong |
| calcium citrate | Higher | Either | Either | Strong |
How it pairs with other supplements
Things to watch for
High-dose calcium supplementation (>500 mg per dose) modestly reduces magnesium absorption. Space by 2+ hours when both are taken as standalone supplements at supplement-tier doses.
Calcium reduces absorption of non-haem iron at gram-scale doses. Space supplements by 2+ hours when both are in the stack.
High-dose calcium (>600 mg per meal) modestly reduces zinc absorption. Lower-dose calcium intake from food does not show a meaningful effect.
Stack Almanac flags your total calcium intake across your whole stack and highlights when you are approaching the tolerable upper intake level. This matters more for calcium than for most other minerals because excess supplemental calcium is associated with cardiovascular risk in some population studies.
Track your Calcium in Stack Almanac
Last reviewed: 2026-05-21. Reference values are adult-general RDAs from UK. Individual needs may vary. This page is for informational purposes only and is not a substitute for professional medical advice.