# 15 Milligram Percents, Parts Per Million & Specific Gravity

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Depending on your working environment, you may be required to produce various clinical laboratory test results. On these institutional reports, you’ll see milligram percents (mg%) used to describe how much of a chemical is say present in a patient’s body such as glucose, cholesterol, creatinine, etc. Milligram percents are expressed in the form of mg/dL where dL stands for deciliter, or 0.1 L. Worth noting is that ferritin levels are typically measured in nanograms/mL and albumin levels are typically measured in g/dL.

Let’s say that a patient’s serum cholesterol level is 200 mg/dL. We can express this level in milligram percent as 200 mg%. Similarly, if this patient’s serum glucose level is 80 mg% then we can express this as 80 mg/dL; However, these small quantities are safer and easier to write as milligram percents.

In solutions even more dilute than mg% we may see concentrations written in parts per million (ppm) such as 7 ppm. ppm is typically expressed in mg/L or mg/kg and can be easily converted to other ratios using the unit factor method we learned in “13 Converting Measurements by Dimensional Analysis (Unit Factor Method)“. Furthermore, we can also write 7 ppm as 0.0007% or as a ratio of 7:1,000,000 (1:142,857.1).

Let’s say we’re working in a lab and our job is to test samples of different solutions and determine the concentration of a particular chemical. After performing several titrations, we’ve determined that the concentration in one of the solutions is 7 ppm. We would now like to do some further testing on another device so we fill up a 1 mL sample vial with this solution. Let’s figure out how many grams of the chemical is in our 1 mL sample vial. Since we know that the concentration is 7 ppm we can convert 7 mg/L to g/mL.

We’ll do this using the unit factor method:

$$\frac{\text{7 mg}}{\text{L}} \times \frac{\text{1 g}}{\text{1,000 mg}} \times \frac{\text{1 L}}{\text{1,000 mL}}$$

$$= \frac{\text{7 g}}{\text{1,000,000 mL}} = \frac{\text{0.000007 g}}{\text{mL}}$$

Our team has put together this fun and easy to use calculator for you below. It lets you quickly convert between ratio concentrations in real time. Give it a go and let us know what you think. When you change the values, it gives you the results instantly 🙂 !

For our purposes, specific gravity (sp gr) is the ratio of the weight of a volume of a substance to the weight of an equal volume of water at standard temperature and pressure. Thus, if we know the specific gravity of a volume of a substance we can find its weight in grams. Water for example, has a specific gravity of 1, meaning that 1 mL of water weighs 1 g at STP. Fun fact: standard temperature and pressure or STP is defined as 0 degrees Celsius (273.15 K) and 1 atm pressure (101.325 kpa (kilopascals), 760 mmHg or 760 torr).

To find the weight of a volume of a substance in grams when we know its specific gravity, simply multiply its volume in mL by its specific gravity. Let’s use the table below to find the weight of 1,000 mL of Lactated Ringers in 5% Dextrose:

$$\text{1,000 mL} \times \text{1.02}\frac{\text{g}}{\text{mL}} = \text{1,020 g}$$

Type of Solution Specific Gravity
Lactated Ringers in 5% Dextrose 1.02
SWI (Sterile Water for Injection) 1.00
70% Dextrose 1.24
Travasol 10% 1.03
Now it’s your turn. Find the following:
777 mg/dL to mg%
150 mg/dL to mg%
300 mg% to mg/dL
250 mg% to mg/dL
0.0010% to ppm
0.0005% to ppm
800 mg/kg to ppm
400 ppm to mg/L
500 ppm to g/mL
750 ppm to g/L
270 mg/L to g/1,000,000 mL
360 mg/kg to mg/g
Weight in g of 700 mL of 70% Dextrose
Weight in g of 44,106,818 mL of Travasol 10%