Calcium-Phosphate Product Calculator
Calcium-Phosphate Product Calculator
Calculate the calcium-phosphate product, a legacy marker of vascular calcification risk in chronic kidney disease.
Calcium-Phosphate Product
Ca × PO4Calcium 9.6 mg/dL, phosphate 6.2 mg/dL
Formula
- calcium
- total serum calcium, mg/dL — not corrected for albumin in the conventional formula
- phosphate
- serum phosphate, mg/dL
- SI conversion
- multiply the mg²/dL² result by 0.0808 to obtain mmol²/L²
Worked example
Calcium 9.6 mg/dL, phosphate 6.2 mg/dL
9.6 × 6.2 = 59.52 mg²/dL², rounded to 59.5 mg²/dL²
59.5 × 0.0808 ≈ 4.8 mmol²/L²
Conventional interpretation
| Ca × PO4 product | In mmol²/L² | Interpretation |
|---|---|---|
| < 55 mg²/dL² | < 4.4 | Below the conventional target ceiling |
| 55 – 70 mg²/dL² | 4.4 – 5.6 | Above target — review phosphate control |
| > 70 mg²/dL² | > 5.6 | Historically associated with higher calcification risk |
A legacy metric, still in wide use
The calcium-phosphate product has long been used as a marker of vascular and soft-tissue calcification risk in chronic kidney disease, with 55 mg²/dL² established as the conventional target ceiling and 70 mg²/dL² as a threshold flagging substantially higher risk. It remains widely charted and quoted in dialysis units and nephrology notes.
It should be presented candidly as a legacy metric rather than as current best practice. KDIGO has moved away from the product in its chronic kidney disease-mineral and bone disorder guidance, recommending that phosphate and calcium be targeted individually rather than combined into a single number. The reason is a real failure mode of the product itself: it can appear reassuringly normal while phosphate is dangerously high and calcium is low, because the two errors partially cancel in the multiplication. Phosphate alone, measured and trended on its own, predicts cardiovascular and mortality outcomes better than the combined product does.
The calculator is offered because the product remains in wide clinical use and continues to appear in older records, trial criteria and some institutional protocols, not because it is the preferred way to assess mineral bone disease today. Where it is used, it should sit alongside — not instead of — the individual calcium and phosphate values.
To convert the result to SI units, multiply the mg²/dL² value by 0.0808 to obtain mmol²/L².
Frequently asked questions
What is a normal calcium-phosphate product?
Conventionally below 55 mg²/dL² (about 4.4 mmol²/L²), with 70 mg²/dL² (about 5.6 mmol²/L²) historically flagged as a higher-risk threshold in chronic kidney disease.
Why has KDIGO moved away from this metric?
Because the product can look normal while phosphate is dangerously high and calcium is low — the two values partially cancel when multiplied — and phosphate measured on its own predicts outcomes better than the combined product.
How do I convert the result to SI units?
Multiply the mg²/dL² value by 0.0808 to get mmol²/L². A product of 59.5 mg²/dL² is approximately 4.8 mmol²/L².
What causes a high calcium-phosphate product in chronic kidney disease?
Reduced renal phosphate excretion raising serum phosphate, often combined with vitamin D or calcium supplementation, or calcium-based phosphate binders raising calcium. Both push the product upward independently.
Related calculators
References
- KDIGO 2017 Clinical Practice Guideline Update for the Diagnosis, Evaluation, Prevention, and Treatment of Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD).
- Block GA, Hulbert-Shearon TE, Levin NW, Port FK. Association of serum phosphorus and calcium × phosphate product with mortality risk in chronic hemodialysis patients. Am J Kidney Dis. 1998;31(4):607–617.