Numerical data is read right-to-left; that is, we compare numbers by first looking at their ones digit, then their tens, then their hundreds, and so on. This is also how most people learn arithmetic — start on the right and move left, carrying digits as you go[1]. Therefore, tables should keep numerical data right-aligned.

Textual data is read (in English) from left-to-right. Comparing textual elements is typically done by sorting into alphabetical order: if two entries start with the same letter, the second letter is used, and so on. Trying to quickly scan text can be infuriating if it’s not left-aligned.

Headers, generally, should carry whatever alignment their data has. This keeps the vertical lines of the table clean, and provides consistency and context.

Center alignment causes the lines of the table to become “ragged,” which makes it much harder to scan entries, often necessitating extra dividers and graphical elements.

Consistent Significant Figures = Better alignment

One easy way to keep your tables properly aligned is to keep the same number of significant figures — usually the number of digits after the decimal — consistent within each column. Significant figures is a whole rabbit hole of a topic, so I’ll keep my advice here brief: the fewer sig figs you can get by with, the better.

Fewer, Clearer Labels

Providing labels to accompany your data is crucial. These pieces of accompanying context allow a data table to be read by a wider audience, in a broader set of circumstances.

Mississippi River Flood Stage Forecast — NOAA

Title

It might sound like conventional wisdom, but giving your data table a clear and succinct title is as important as any other design decision you make. With a good title, tables are portable: they can be used in a number of different contexts, as well as cited from external sources.

Units

The most common label used in tables is the unit of measurement for the data; often, it’s repeated along with every single data point. Instead of repeating the label, only include it with the first data point in each column.

Headers

Keep headers as short as possible; the design of the data table should be focused around the data itself, and long header labels can occupy a lot of visual space.

As little ink as possible

When deciding how to style the graphic elements of a table, the goal should always be to reduce the table’s footprint without losing structural fidelity. One of the ways you can accomplish this is by using as little “ink” as possible — that is, whenever possible, choose not to style an element.