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Converting units of measure

Christa Bedwin
Editors’ Association of Canada (www) English Hyperlink Notice
(The Editors’ Association of Canada began using the name Editors Canada on July 1, 2015.)

2014-08-18

To convert units from one system to another (imperial to metric, for example), conversion tables and conversion calculators are available on the Internet and in some dictionaries. The American Medical Association publishes a popular language style guide, and they offer a free conversion calculator (www) English Hyperlink Notice for everyone.

However, there is more to proper conversion than simply sticking numbers in the calculator. To properly handle conversions of units, editors need to be aware of four very important aspects of the arithmetic of measurement:

  • precision and accuracy
  • hard conversion and soft conversion.

How you handle these issues while editing a text can alter the accuracy of the message.

Precision means how many decimal places a measurement has. If an object was actually weighed on an extremely accurate scale (such as the kind found in a pharmaceutical laboratory), the resulting measurement might actually be very precise, for example, 53.125 g. However, if a conversion calculator adds several extra non-zero digits to a measurement (e.g., you enter “17 inches” and it gives you back “43.18 cm”), you need to assess if that is reasonable. (It is not! Round to 43 cm, a figure containing the same number of digits as the original.)

When converting from SI to traditional measurements or vice versa, use your judgment regarding how precise the converted quantity should be. Do not hesitate to query or flag the author where the correct choice is not obvious.

Rule for precision in unit conversion: Round the converted figure to a number of non-zero digits that is equal to or fewer than the number of digits in the original figure used in the calculation.

Accuracy means how close a measurement is to the real measurement. Using the analogy of a dart board, the accuracy is determined by how close the darts are to the bull’s eye. A very accurate bathroom scale that is not very precise might tell you that you weigh 170 pounds, day after day, though a more precise scale might tell you that you weigh 168.0 one day, 171.6 the next day, and 169.4 another day. The scale that says 170 is always still correct and accurate, it’s just not very precise.

Reasonable accuracy and precision in real life

Many decimal places in a measurement implies that the measurement is very precise, and by association, very accurate, and this might not be true. Make sure the accuracy portrayed by measurements makes sense.

For example, if you weigh broccoli on two hanging scales in the vegetable aisle and one says 0.25 kg and another says 0.4 kg, it would not be accurate to average the two and definitively say that you have 0.325 kg. It would be reasonably accurate, given the measurements you have, to average them, then round off, to say that you have about 0.3 kg.

Soft conversion In practice, soft conversion means using a calculator such as the one referenced above. You enter in a measurement and a conversion factor, and the result has a lot of decimal places. For example, a paint supplier who ships cans of paint containing one US gallon (128 fluid ounces) to Canada and labels them 3.785 L, has used soft conversion.

Soft conversions can preserve accurate relations between quantities better than hard conversions, when the accuracy and precision of the measurements and context warrant it.

Hard conversion usually means referring to standard, rounded-off numbers from tables for conversions. The appropriateness of this varies. Rounded numbers are easier to read and understand, of course, but may introduce inaccuracies. The supplier who ships paint cans with 128 fluid ounces and tells you they’re 4 L cans has used hard conversion.