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SI and U.S. Standard Measurements: Conversion Confusion for Pet Owners

Becky Lundgren, DVM

SI is the modernized version of the metric system of measurement. It is the language universally used in science and medicine. SI is an abbreviation from the French Système International d’Unités, or International System of Units. Unlike the comparatively confusing U.S. measurement system that uses ounces and pounds, metric is easy to use because all units can be divided by 10.

In the U.S. today, we use several measurement systems, depending on what the person doing the measuring is used to. The Imperial System (foot, pound, second) and U.S. Customary Units (tablespoons, cups, pints, and quarts) both come from British units and are similar. Metric uses milliliters, liters, and kilograms. The ratios between the systems can be awkward to convert.

No matter what country you live in, your veterinarian uses metric to establish volume and mass. Metric is much easier, but the problem is client acceptance: Most Americans and Canadians understand their pet’s weight in pounds and ounces and have no idea what their pet weighs when given the number in kilograms. Most veterinarians will write medical information in the pet’s chart in metric and use U.S. standard units when talking to clients. One pound equals 0.4535924 kilograms, so a 50-pound dog weighs 22.68 kilograms. (Plurals are not used in metric abbreviations, so that dog would be 50 lbs and 22.68 kg.) Many clinics use metric everywhere, except in client discussions.

Veterinarians typically measure masses, lumps, and growths in centimeters. One inch equals 2.54 centimeters (cm), so a pet’s 2 cm mass is 0.787 inches. A half inch mass is 1.27 cm. A mass weighing one kilogram is slightly over two pounds.

This chart can help you understand conversions.

1 centimeter (cm)

=

0.4 inches (in)

1 inch (in)

=

2.54 centimeters (cm)

1 teaspoon (tsp)

=

4.928 milliliters or cubic centimeters (ml; cc)

1 gram (g)

=

0.035 ounces (oz)

1 ounce (oz)

=

28.349 grams (g)

1 kilogram (kg)

=

2.204 pounds (lb)

1 pound (lb)

=

0.453 kilogram (kg)


Body temperature can be given in either Fahrenheit or Celsius. For example, a dog’s normal temperature range in Fahrenheit is 100.5 to 102.5F; that’s 38.056 to 39.167 in Celsius.

A more comprehensive chart from the U.S. Occupational Safety and Health Administration (OSHA) can be helpful when you are trying to understand your pet’s medical situation.

Medications

Medications are a specific concern because it’s all too easy to give unintended doses if the conversion is not made correctly. For example, some medications in powder form are given with instructions to use ¼ teaspoon (tsp), which is 1 milliliter (ml). If the veterinarian prescribes the dose in milliliters and the client only has measuring spoons in teaspoons, the wrong amount can be given.

Because metric is the language of science and medicine, pharmaceutical manufacturers give medication dosages based on a number of milligrams (mg) per kilogram (kg) of the pet’s body weight. To establish a dose, your veterinarian will convert your pet’s weight in pounds to kilograms, and then multiply the prescribed dosage by the weight. The result will be something like 2 milligrams of medication per kilogram of body weight, for which the written abbreviation is 2mg/kg.

Check Before You Leave

If you have been given any information in a measurement system that you’re not familiar with, ask your veterinarian for clarification before you leave the clinic. In certain cases, such as medication dosages, the consequences of misunderstanding can be disastrous, so make sure you understand your veterinarian’s instructions.

 

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