Calcium-Phosphate Product Calculator (Ca × P) with Corrected Calcium
Calcium × phosphate product in mg²/dL² and mmol²/L² for chronic kidney disease and dialysis, from mg/dL or mmol/L results: the KDOQI threshold of 55, the high-risk level of 70, albumin-corrected calcium, calcium and phosphate against their normal ranges, the phosphate that keeps the product under 55, and what KDIGO 2017 recommends instead.
How it works
Ca × P product (mg²/dL²) = total calcium (mg/dL) × phosphate (mg/dL) · mmol/L → mg/dL: calcium × 4.008, phosphate × 3.097 · mmol²/L² = mg²/dL² ÷ 12.41 · corrected calcium (mmol/L) = Ca + 0.02 × (40 − albumin g/L) [mg/dL: Ca + 0.8 × (4 − albumin g/dL)] · KDOQI target < 55 mg²/dL² (< 4.4 mmol²/L²)
In advanced kidney disease phosphate builds up, and when calcium and phosphate are both high their product can exceed the point where calcium phosphate precipitates in arteries, heart valves, skin and joints. The 2003 KDOQI bone and mineral guideline set a product below 55 mg²/dL² as a goal for CKD stages 3–5 and dialysis, and values of 70 or more are linked to calciphylaxis. The calculator converts the results to both unit systems, multiplies them, and also gives the product with albumin-corrected calcium, because low albumin makes the total calcium look lower than the free calcium really is. The 2017 KDIGO CKD-MBD update moved away from the product: it suggests evaluating calcium and phosphate individually and together, lowering raised phosphate toward the normal range, avoiding hypercalcaemia and restricting calcium-based binders. The product is still widely reported as a quick risk marker, so the calculator shows it next to each value's own range and the phosphate level that would keep the product under 55. Ranges vary slightly between laboratories.
Worked example
Calcium 2.45 mmol/L and phosphate 2.1 mmol/L, albumin 36 g/L. In mg/dL that is 2.45 × 4.008 = 9.82 and 2.1 × 3.097 = 6.50, so the product is 9.82 × 6.50 = 63.9 mg²/dL² (2.45 × 2.1 = 5.15 mmol²/L²): elevated, above the KDOQI target of 55. Corrected calcium is 2.45 + 0.02 × (40 − 36) = 2.53 mmol/L (10.14 mg/dL), giving a corrected product of 65.9 mg²/dL². Phosphate is high (normal up to 1.45 mmol/L) and is the main driver; to keep the product under 55 at this calcium, phosphate would need to be below 5.4 mg/dL (1.75 mmol/L).
Sources: KDIGO 2017 CKD-MBD guideline update, executive summary (recommendations 3.1.5, 4.1.2, 4.1.3, 4.1.6) · KDIGO CKD-MBD guideline page · Calcium, magnesium and phosphate: normal ranges (NCBI Bookshelf, Endotext)
Frequently asked questions
What is a normal calcium-phosphate product?
In adults with normal kidneys it is usually around 30–40 mg²/dL² (2.4–3.2 mmol²/L²). In chronic kidney disease the older KDOQI guideline aimed for below 55 mg²/dL² (4.4 mmol²/L²); values of 70 or more carry a high risk of calcification and calciphylaxis.
How do I convert calcium and phosphate units?
Calcium: mmol/L × 4.008 = mg/dL. Phosphate (reported as phosphorus): mmol/L × 3.097 = mg/dL. A product in mg²/dL² divided by 12.41 gives mmol²/L².
Should I use corrected or measured calcium?
The KDOQI target was set with measured total calcium, so the main result uses that. When albumin is low, the corrected calcium better reflects free calcium, so the calculator also shows the product with corrected calcium. Measure ionised calcium when accuracy matters.
Is the calcium-phosphate product still used?
Less than before. KDIGO 2017 suggests evaluating calcium and phosphate separately and together rather than the product, because the product is dominated by phosphate and hides which value is abnormal. Many reports and clinicians still quote it as a quick risk marker.
How can a high product be lowered?
Mainly by lowering phosphate: dietary phosphate restriction (especially additives), phosphate binders, and adequate dialysis. If calcium is high, non-calcium binders (sevelamer, lanthanum, iron-based binders) are preferred and calcium-based binders and active vitamin D are reduced.
Related calculators
Clinical decision tool for education and for use by trained clinicians alongside judgement, not a diagnosis. Treatment of CKD mineral and bone disorder should follow serial calcium, phosphate and PTH results and current KDIGO guidance.