Calcium-Phosphate Product Calculator
Multiply serum calcium by phosphate to get the Ca x P product and compare it with the 55 mg2/dL2 target.
What it does
The calcium-phosphate product is a single number built from two routine lab values. Multiply serum calcium by serum phosphate and you get a product that rises when either mineral climbs. In chronic kidney disease the number became popular because calcium and phosphate can both run high, and the product was used as a rough marker of the risk that calcium phosphate might deposit in soft tissue.
The arithmetic is simple, but the units are not. US labs report calcium and phosphate in mg/dL, so the product carries the awkward unit mg2/dL2. Most of the world reports both in mmol/L, which gives mmol2/L2. The two products are not interchangeable. One mg2/dL2 equals about 0.08056 mmol2/L2, so the familiar 55 mg2/dL2 ceiling becomes 4.43 mmol2/L2.
KDOQI set the target below 55 mg2/dL2 in its bone and mineral guideline. The same guideline points out that phosphate is usually the driver, because phosphate can double in advanced kidney disease while calcium moves much less. That is why phosphate control, through diet and binders, does most of the work when the product is high.
The 2017 KDIGO guideline took a different view. It recommends using individual calcium and phosphate values, evaluated together, and following their trends rather than chasing a mathematical product. The product is still useful as a quick summary and as a teaching number, but it is not a treatment target on its own. A product below 55 does not prove that vessels or bone are safe.
This page computes the product in both unit systems and shows how far the result sits from the target. It is a lab arithmetic tool, not a risk score. Decisions about binders, vitamin D, dialysis, or diet belong with a clinician who can see the whole picture, including PTH, alkaline phosphatase, kidney function, and the trend over several months.
How to use
- Choose US units if your lab reports mg/dL, or metric if it reports mmol/L.
- Enter total serum calcium from the same draw. If the lab reports an albumin-corrected calcium, use that value and note it.
- Enter serum phosphate from the same panel.
- Read the product and the distance from the target. The band tells you whether the result is at or below 55 mg2/dL2, or above it.
- Compare the number with earlier results from the same lab. A single draw is less useful than a six month trend.
- Review phosphate intake, binders, vitamin D, and PTH with the care team before changing treatment.
Formula
- Ca
- Serum total calcium, in mg/dL or mmol/L.
- P
- Serum phosphate, in mg/dL or mmol/L.
- Ca x P
- The product, in mg2/dL2 or mmol2/L2.
- 55
- KDOQI target ceiling in mg2/dL2.
- 4.43
- The same ceiling in mmol2/L2, rounded to two decimals.
Convert calcium from mg/dL to mmol/L by multiplying by 0.2495. Convert phosphate from mg/dL to mmol/L by multiplying by 0.3229. The product conversion is the product of those two factors, about 0.08056. The target is a ceiling, not a diagnosis: KDOQI wrote below 55 mg2/dL2, while KDIGO 2017 recommends against using the product as the main treatment target. Assay differences between labs can move both inputs, so compare results from the same method.
Worked example
Example: calcium 9.6 mg/dL and phosphate 5.8 mg/dL
A patient with stage 4 chronic kidney disease has a total calcium of 9.6 mg/dL and a phosphate of 5.8 mg/dL on the same draw.
- Multiply calcium by phosphate: 9.6 x 5.8 = 55.68 mg2/dL2.
- The target ceiling is 55, so the product is 0.68 mg2/dL2 above it.
- Convert calcium to SI: 9.6 x 0.2495 = 2.395 mmol/L.
- Convert phosphate to SI: 5.8 x 0.3229 = 1.873 mmol/L.
- Multiply the SI values: 2.395 x 1.873 = 4.486 mmol2/L2, against the 4.43 ceiling.
- The band is Above target in both unit systems.
- The metric form opens with 2.40 and 1.87, which gives 4.488 mmol2/L2, displayed as 4.49, also above target.
Reference table
| Calcium | Phosphate | Product | Band |
|---|---|---|---|
| 9.2 mg/dL (2.30 mmol/L) | 3.8 mg/dL (1.23 mmol/L) | 34.96 mg2/dL2 (2.82 mmol2/L2) | At or below target |
| 9.6 mg/dL (2.40 mmol/L) | 5.8 mg/dL (1.87 mmol/L) | 55.68 mg2/dL2 (4.49 mmol2/L2) | Above target |
| 10.2 mg/dL (2.54 mmol/L) | 6.5 mg/dL (2.10 mmol/L) | 66.30 mg2/dL2 (5.34 mmol2/L2) | Above target |
| 8.8 mg/dL (2.20 mmol/L) | 4.0 mg/dL (1.29 mmol/L) | 35.20 mg2/dL2 (2.84 mmol2/L2) | At or below target |
Limits
- The product is not a measure of ionized calcium, and it does not show whether calcification is actually present.
- KDIGO 2017 recommends using calcium and phosphate values and trends rather than the product as a treatment target.
- Assay differences between labs can shift both inputs, so results from different methods are not directly comparable.
- A single draw says little. The trend over months, together with PTH and kidney function, carries more information.
Common mistakes
- Multiplying mg/dL calcium by mmol/L phosphate and labeling the result mg2/dL2.
- Using ionized calcium in place of total calcium without recording which one was used.
- Comparing a US product with the SI 4.43 threshold, or the other way around.
- Treating a product below 55 as proof that bone and vessels are safe.
- Mixing values from different draws or different labs in one calculation.
Try it yourself
Quick checks. Answers sit tucked under each question.
Calcium 9.2 mg/dL and phosphate 3.8 mg/dL. What is the product?
9.2 x 3.8 = 34.96 mg2/dL2, below the 55 target. In SI, 2.30 x 1.23 = 2.82 mmol2/L2.
Calcium 10.2 mg/dL and phosphate 6.5 mg/dL. What is the product and the gap?
10.2 x 6.5 = 66.30 mg2/dL2, which is 11.30 above 55. The same draw converts to about 5.34 mmol2/L2 in SI.
Calcium 2.20 mmol/L and phosphate 1.29 mmol/L. What is the product?
2.20 x 1.29 = 2.84 mmol2/L2, below the 4.43 target. The US mirror is about 8.82 x 4.00 = 35.28 mg2/dL2.
Questions
What is the calcium-phosphate product?
It is serum calcium multiplied by serum phosphate. US labs give mg2/dL2, and SI labs give mmol2/L2.
What is the target?
KDOQI set a ceiling below 55 mg2/dL2, about 4.43 mmol2/L2. KDIGO 2017 prefers individual calcium and phosphate trends over the product.
Why does KDIGO discourage the product?
Studies did not show that the product predicts outcomes better than calcium and phosphate separately, and it can hide which mineral is driving the change.
Which calcium value should I use?
Use total calcium, ideally the albumin-corrected value if your lab reports one. Ionized calcium answers a different question and should be labeled clearly.
How do I convert mg2/dL2 to mmol2/L2?
Multiply by 0.08056. For example, 55 mg2/dL2 x 0.08056 = 4.43 mmol2/L2.
Does a normal product rule out vascular calcification?
No. The product is a rough lab summary. Imaging, phosphate load, PTH, and the clinical picture all matter.
Sources
- NKF KDOQI Guideline 6: serum calcium and calcium-phosphorus product
- KDIGO 2017 CKD-MBD guideline summary