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A milligram is a measure of mass in the SI (metric) system, while a teaspoon is a measure of volume in the English system. Therefore, this conversion from milligrams to teaspoons requires two stages. First you will need to find the density of the material you are measuring. You will use the density to convert from mass to volume, or milligrams to milliliters. You will then convert from the SI volume measurement milliliters to the English system measurement teaspoons.
Steps
Part 1
Part 1 of 2:Converting Milligrams to Milliliters

1Find the density of the material you are measuring. Because you are trying to convert mass into volume, you need to know the density of the material. Density is a measurement of how much mass fits in a given volume for any substance. Materials that we consider "heavy" have a high density, while "light" materials have a low density.
 Density tables exist online or in many text books for a wide range of substances. You can find a table of density values by searching for “density table,” “liquid density table,” or “solid density table.”
 The density of pure water is set at 1000 kg per cubic meter. This is the same as 1000 mg/ml.
 For comparison, some other common liquids have the following densities:^{[1]
X
Research source
}
 Alcohol = 800 kg/m^{3}=800 mg/ml
 Gasoline = 737 kg/m^{3}=737 mg/ml
 Milk = 1050 kg/m^{3}=1050 mg/ml
 Mercury = 13590 kg/m^{3}=13590 mg/ml

2Check with your pharmacist for medication densities. If you are trying to convert amounts for a prescription, you should check with your pharmacist. A pharmacist is in the best position to provide you with the correct density of whatever compound you need to measure.
 You may find the density of your substance printed on the bottle. You might check that before contacting the pharmacist.
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3Prepare to convert from mass to volume. Recall that density is equal to an item’s mass divided by its volume.^{[2] X Research source } Rearranging this formula,
 .

4Perform the conversion. When you know the density of the material and the measurement of the sample you wish to convert, perform the division to convert from mass to volume of your substance.^{[3] X Research source }
 For example, suppose you wish to convert 20,000 milligrams each of water, alcohol, gasoline, milk and mercury into milliliters. 20,000 milligrams is equal to 20 grams, which is about the weight of 20 paper clips. The conversions for these five liquids are as follows:
 Water:
 Alcohol:
 Gasoline:
 Milk:
 Mercury:
Advertisement  For example, suppose you wish to convert 20,000 milligrams each of water, alcohol, gasoline, milk and mercury into milliliters. 20,000 milligrams is equal to 20 grams, which is about the weight of 20 paper clips. The conversions for these five liquids are as follows:
Part 2
Part 2 of 2:Converting Milliliters to Teaspoons

1Learn the conversion factor. Because milliliters and teaspoons are both measurements of volume, the density of the different materials no longer makes a difference. Converting from milliliters to teaspoons is fairly easy because the conversion factor is just 5 to 1. That is, 5 ml = 1 tsp.
 If you know a measurement in milliliters, you can convert to teaspoons by just dividing by 5.

2Perform the conversion. Using the five samples from above, prepare for the conversion to teaspoons.
 Water: 20,000 mg = 20 ml/5 = 4 teaspoons
 Alcohol: 20,000 mg = 25 ml/5 = 5 teaspoons
 Gasoline: 20,000 mg = 27.137 ml/5 = 5.4274 teaspoons
 Milk: 20,000 mg = 19.048 ml/5 = 3.8096 teaspoons
 Mercury: 20,000 mg = 1.4717 ml/5 = 0.29434 teaspoons

3Review the results. Pure water is set as the standard for conversion density, with a density of 1.0 grams per milliliter. The densities of all other substances are set in relation to the density of water. Notice that mercury, with an extremely high density, converts to only about a quarter teaspoon for the same mass of water that equals one teaspoon.Advertisement
Community Q&A

QuestionHow many teaspoons are in 10 mg of fiber?DonaganTop AnswererAs indicated above, you have to know the density of the material.

QuestionHow many teaspoons is 400 mg of turmeric?DonaganTop AnswererThere are roughly 2,267 mg of turmeric in a teaspoon, so 400 mg would be about 0.176 tsp (onesixth of a teaspoon). (A teaspoon is slightly different in Australia.)

QuestionHow do I convert milligrams to meters?DonaganTop AnswererMilligrams measure mass, and meters measure length, so one does not convert to the other.

QuestionI need to measure 10 mg per gallon of water, how do I do that?DonaganTop AnswererFirst you would need a scale that measures in milligrams. Second, you need a onegallon container. Fill the container with water. Use the scale to measure ten milligrams of the material. Pour the material into the water. If the container holds two gallons, you could fill it with water and measure out 20 milligrams of material.

QuestionIs 4 mg liquid equal to 1/4 teaspoon?DonaganTop AnswererIt depends on the density (weight) of the liquid. For example, one teaspoon of water weighs 5 grams, but a teaspoon of vegetable oil weighs about 4½ grams. (Yes, vegetable oil and motor oil typically weigh less than water. That's why oil floats on water.)

Question1000 mg = how many tsp?DonaganTop AnswererAs the above article explains, you have to specify what material you're dealing with (sugar, water, baking powder, vegetable oil, etc.) before you can convert milligrams to teaspoons.

QuestionI need to know, what 40 milligrams of flaxseed is equal to in teaspoon. I have no way of knowing what the density is to divide it.DonaganTop AnswererOne teaspoon of ground flaxseed weighs about 2500 milligrams. Therefore, 40 mg consists of roughly 1/60 of a teaspoon.

Question15 ml is how many spoonfuls?DonaganTop AnswererSince a "spoonful" has no specific volume, that question can't be answered. However, one milliliter is defined as roughly 0.2 US teaspoon, so 15 ml would be approximately 3 teaspoons.
Tips
 After you get good at converting, you can perform this conversion quite quickly. Divide the number of milligrams by the density (in mg/ml), then divide by 5 to get teaspoons.Thanks!