Calcium in a blood sample should be estimated when required medically. Calcium should be precipitated out of 0.1 mL of the blood sample serum as calcium oxalate. After that, the decomposition of the calcium oxalate should occur by heat. Then, the sample should be estimated colorimetrically by o-cresolphthalein complexone. The required liquid complexone is made by dissolving 10 mg o-cresolphthalein complexone in 50 mL alkaline borate, and then 50 mL of 0.05 N HCl are added to make the solution's pH 8.5. This method for calcium determination is efficient and effective, requiring a minimal amount of blood serum sample and a reasonable amount of time.