Word's Native Measurement Unit

Richard wonders what Word uses as its underlying, native measurement unit. He notes that there are almost always rounding errors when switching among inches, metric, and points, and picking the right system would seem to minimize problems.

From everything that we can find, it appears that Word internally uses a measurement unit called a twip. This is equivalent to 1/20 of a point, so there are 1,440 twips in an inch or 567 twips in a centimeter. The interesting thing, however, is that you cannot specify measurements in twips in dialog boxes, nor can you change the default measurement unit (as used in dialog boxes) to twips. Instead, twips are used internally and can be accessed programmatically, using VBA.

The problem with converting between measurement units seems to crop up when using dialog boxes to enter measurements. Most dialog boxes have a limit on how many digits you can enter to the right of a decimal point, and most of them we tested seemed to place that limit at two digits to the right of the decimal. Thus, you could set a measurement (for example) at 1.25 inches or 1.25 centimeters, but those two measurements are very different in effect.

Let's look at the case of 1.25 inches. If this is converted to points, then it comes out to 90 points. If you convert it to centimeters, then you get (at 2.54 centimeters per inch) 3.175 centimeters. However, if you enter the value in centimeters, you can't use three decimal places; Word would round it to 3.18 centimeters which, if expressed as points, is 90.15 points. (That is 3.18 multiplied by 28.35 points per centimeter.)

The reason this occurs is because points are designed to be equally divisible into common inch-based fractions. Thus, 1/2 inch is exactly 36 points, 1/3 inch is exactly 24 points, 1/4 inch is exactly 18 points, 1/8 inch is exactly 9 points, etc. However, it is not that way with metric conversions; they come out quite different. Each millimeter is equivalent to approximately 2.834645669 points, and each centimeter is 10 times this amount. This allows for rounding errors in almost any conversion—particularly if your input mechanism (the common dialog box) only allows you to enter two digits to the right of the decimal point.

Most people can live with small amounts of imprecision in positioning elements in Word. For these folks, they can live with the difference between 90 points and 90.15 points. Other people may not find this difference acceptable. For them, the best solution is going to be to configure Word to work in points and make sure that all measurements you enter in dialog boxes are always in points. (I would suggest working in twips but, again, this seems to only be germane when working in VBA.)

WordTips is your source for cost-effective Microsoft Word training. (Microsoft Word is the most popular word processing software in the world.) This tip (11387) applies to Microsoft Word 97, 2000, 2002, and 2003. You can find a version of this tip for the ribbon interface of Word (Word 2007 and later) here: Word's Native Measurement Unit.