How best to label a column that contains values of different units?

Question

I am presenting tabular data. One column represents the size of a shape, however the shape itself may be a line, area, or volume. So size values can be in units mm or mm² or mm³. However, I only display the values, not the units, in the table. For example:

| Type   | Size (mm) |
|--------|-----------|
| Line   | 123.4     |
| Area   | 456.7     |
| Volume | 234.5     |

There is a lot of confusion from users about the use of "Size (mm)". Some users will tend to only generate Area shapes, so they think it's a mistake that it says "mm" and not "mm²", etc. However, any user can make any shape and it is common for a table to be filled with different shape types.

I like putting "(mm)" in the heading because it conveys at least that the data is in some dimensions of millimeters, but this is also the source of the confusion.

Do I just remove "(mm)"? Add a tooltip? The table has a lot of columns already, so breaking it up into separate columns isn't a great option either -- and besides, all the values represent the same concept of "Size", just in different dimensions. I also don't want to add the units to the table cells, because that would prevent someone from copy/pasting the data into Excel.

Any good ideas for a solution to this?

Answer 1

When mixing data units, explicit is better

It's very easy to confuse mixed units in a table, so best practice is to make the units explicit.

• If you can avoid this situation (e.g. using sections or different columns) that is usually better. But sometimes it's unavoidable because of space constraints, or for other reasons.
• Avoid using icons because it actually increases, not decreases the complexity of an already-complex table with mixed units.

A good practice is to use grid alignment to distinguish the units.

Here are a few examples of explicit, grid-assisted layout:

Answer 2

A simple icon can clarify what units are implied in each case. This should solve the problem of the user inadvertently copying the units along with the actual value, and get a quick mental feedback of what's to be expected:

EDIT: See Tohster's answer below, which appeals to me as the better solution.

Answer 3

A quick solution is to get rid of the type column and just have different size columns:

But you mentioned that it will have a lot of columns so maybe the better solution is to seperate the table into one per type:

The reason for these two ideas come from one question I have with your current table concept: Why do you have a table with incomparable rows? What's the purpose of it? That's why the (mm) in the header is inaccurate and confusing for people. They expect the same type and thus comparable values in the column, which would make it easier for them to quickly understand what they see.

Consider splitting the table because it makes it a bit more useful when it comes to comparing and quickly understanding the data. If that's not the purpose of this table, why using a table at all?