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What Is a Nominal Measurement?

What Is a Nominal Measurement?

Ceramic tile and lumber are both often sold in “nominal” sizes.  If you don’t understand that this means, you may be in for trouble when you begin your next remodeling project.  Read on to find out more about this design and construction industry term.

A “nominal” measurement means that the size listed on the item is not the actual size of the item.  Huh?  Yes, you read that right.  Let me explain:

You have, I’m sure, heard of 2x4s – those long boards used to build houses and other structures.  The numbers 2 and 4 refer to the size of the board:  it is 2 inches thick and 4 inches wide.  These boards are sold in a variety of lengths, from 8 feet long on up.  However, if you’ve never used 2x4s to build anything before, you may not realize that a 2×4 isn’t actually 2″ x 4″ at all.  It’s more like 1-1/2″ x 3-1/2″.

Ceramic and stone tiles are also often sold in nominal sizes.  The tag on a box of tile may say the tiles are 12×12.  That means they are 12″x12″ in size – unless, of course, that measurement is a nominal one.  In that case, those tiles are actually 11-7/8″ x 11-7/8″ in size.  Unlike lumber, which is nearly always sold in nominal sizes, tile measurements are not quite so universal.  If the tile doesn’t say which measurement is being used – actual or nominal – the only way to be sure of the tile’s size is to measure it.

In short, “nominal” sizes refer to the approximate size of the item, while “actual” measurements refer to the actual size of the item.

wall tile

In complex tile installations, knowing the exact measurements of the tile you plan to use is essential to a successful project.

Why is all this nominal vs. actual measurement stuff important?  Believe it or not, these seemingly tiny variations in size can actually make a huge difference in the way a remodeling project turns out.  Consider the following scenario.  Say, for example, that you want to use a 12×12 tile on your bathroom floor, but you plan to install a 6×6 accent tile at various locations for some extra pizzazz.  If the 12×12 tile is measured nominally, meaning the tile is 11-7/8″x11-7/8″ in size, but the accent tile has an actual measurement of 6″x6″, the two tiles will not fit together properly.  Instead, you will be left with uneven grout joints and gaps.  This is because (simple math here), 2 6×6 tiles laid side by side are 12-1/8″ wide (2 tiles plus the width of the grout joint), making it 1/4″ wider than the nominal 12×12 tiles (which are actually 1/8″ smaller in both width and length).

Or, if you need a tile of a particular size to fit under your kitchen window, for example, and you choose one with an actual measurement, you may find that you need to cut it down to make it fit properly.

In some cases, you may not order enough material if you don’t realize that the measurements you used for figuring the material amounts were nominal dimensions, not actual ones.  Even small differences in material sizes can add up quickly when you are working on a large project.

Details matter in all home improvement projects.  Even the tiniest measurements can have a major impact on the outcome of your project.  Be sure of all the measurements in your material selections to avoid any unpleasant surprises!


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One Response to What Is a Nominal Measurement?

  1. […] tile is exactly 1″x1″.  You would be wrong.  Often, this is a “nominal measurement.”  The little tiles may be only 3/4″ or 7/8″ square.  Or, they may be […]

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