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Use a scale that will give you the value in kilograms. Because children’s weights change rapidly, especially right after birth, it is important to have an up-to-date weight. If your scale gives the weight in pounds, then you will need to covert the values to kilograms. Otherwise the answer will not be accurate. 1 pound equals 0.45 kilograms. If you know the child’s age, you can use these values:  A newborn that is only 15 to 30 minutes old will have an average blood volume of 76.5 milliliters of blood per kilogram. A newborn that is 24 hours old has 83.3 milliliters per kilogram. A three month old has 87 milliliters per kilogram. A six month old has 86 milliliters per kilogram. A child one to six years old has 80 milliliters per kilogram. A ten year old has 75 milliliters per kilogram. A fifteen year old has 71 milliliters per kilogram. As teens reach adult sizes and proportions, their blood volume will be the same as that of adults. Multiply your child’s weight times the average value for his or her age group. The result will be your child’s estimated blood volume.  The values may differ slightly when you use a different method of calculation. The accuracy of the calculations may also vary slightly based on whether these averages were calculated from a population that is similar in body side and growth patterns to your child.
Weigh the child. Determine the average blood volume per weight. Calculate the blood volume.