What is the normal calcium level in the blood?

Normal Calcium Levels in Blood

Normal serum total calcium in adults ranges from 8.6 to 10.3 mg/dL (2.15 to 2.57 mmol/L), while ionized calcium ranges from 4.65 to 5.28 mg/dL (1.16 to 1.32 mmol/L). 1

Total Calcium Reference Range

• The standard adult range for total serum calcium is 8.6 to 10.3 mg/dL according to the American College of Clinical Chemistry 1
• This represents the most widely accepted reference range for clinical practice 1

Ionized Calcium Reference Range

• Normal ionized (free) calcium levels range from 4.65 to 5.28 mg/dL (1.16 to 1.32 mmol/L) 1
• Ionized calcium represents approximately 45-50% of total calcium and is the physiologically active form 1
• This fraction is what actually interacts with calcium sensors on cells and drives physiologic effects 2

Critical Context for Interpretation

Total calcium measurements must be corrected for albumin levels when serum albumin is abnormal, as approximately 40% of total calcium is protein-bound 1. The correction formula is:

• Corrected total calcium (mg/dL) = Total calcium + 0.8 [4 - Serum albumin (g/dL)] 1, 3

When to Measure Ionized Calcium Directly

Direct measurement of ionized calcium is superior to correction formulas in these situations 3:

• Critically ill patients 3
• Abnormal albumin levels 3
• Acid-base disturbances (pH changes affect ionized calcium independently) 3
• Massive transfusion scenarios (citrate binding affects calcium availability) 3

A common pitfall is relying solely on total calcium without albumin correction in hypoalbuminemic patients, which can falsely suggest hypocalcemia when ionized calcium is actually normal 1, 3.

Special Population Considerations

Chronic Kidney Disease Patients

• Target serum calcium should be maintained toward the lower end of normal at 8.4 to 9.5 mg/dL (2.10 to 2.37 mmol/L) 4, 1
• The calcium-phosphorus product must be kept <55 mg²/dL² to prevent soft tissue calcification 4, 1

Neonates

• Newborn serum calcium ranges from 2.06 to 2.73 mmol/L (8.24 to 10.92 mg/dL), which is significantly wider than the adult range 5
• This broader range reflects the physiologic calcium decline after birth due to interruption of maternal-fetal calcium transfer 5

Clinical Thresholds for Action

Hypercalcemia Definitions

• Mild hypercalcemia: Total calcium <12 mg/dL (<3 mmol/L) or ionized calcium 5.6 to 8.0 mg/dL (1.4-2 mmol/L) 6
• Severe hypercalcemia: Total calcium ≥14 mg/dL (≥3.5 mmol/L) or ionized calcium ≥10 mg/dL (≥2.5 mmol/L) 6
• Intervention threshold: Total calcium >10.2 mg/dL (2.54 mmol/L) requires adjustment of calcium-raising therapies 4

Hypocalcemia Definitions

• Hypocalcemia: Total calcium <8.4 mg/dL (2.10 mmol/L) 7
• Critical threshold: Ionized calcium <0.8 mmol/L is associated with cardiac dysrhythmias and requires prompt correction 7, 3
• Treatment indicated when ionized calcium falls below 0.9 mmol/L, especially in symptomatic patients 7

Safety Limits for Calcium Intake

Total elemental calcium intake (dietary plus supplements) should not exceed 2,000 mg/day 4, 1, 8. Exceeding this limit may cause inadvertent mineral deficiencies and increase risk of complications such as milk-alkali syndrome, particularly in thiazide users and patients with renal failure 8.