Numbers

It is possible, but fiddly.

I'm assuming that 'center my spreadsheet' in your question refers to a spreadsheet document containing a single table on a single sheet, and that the width and height of the Table are small enough to fit within the print area of a single page.

In the View menu, choose Show Rulers.

This will place rulers across the top and down the left side of your document. Note that 0 on these rulers refers to the locations of the left and top margins of a page to be printed with your current settings.

Go to Print Setup > Arrange to review the margin settings.

Subtract the left and right margin values from the long dimension of the paper you will use to get the width of the print area.

Click Done to return to edit mode.

In Edit mode, click on an empty space of the Sheet to deselect the Table, then click the Shaped button and choose the square shape to place this shape on the Sheet.

Drag the shape up to just under the horizontal ruler and align its left edge with the ruler's 0 mark.

Drag the handle (small square) at the mid point of the right edge to the right to extend the shape in that direction until the right edge is aligned with the ruler position of the width of the print area.

You may want to lock the shape at this stage to prevent accidentally moving it.

Click a cell on the table to activate it, then click the circle at the intersection of the row and column reference tabs to select the Table itself.

Drag the circle to the right, stopping when a yellow vertical guideline appears at the horizontal centre of the shape and of the table. Release the mouse button.

Check your result by pressing command-P to go to Print Setup.

If you see two 'pages,' one to the right of the other, something in your current document (probably the shape) extends beyond the right edge of the print area, forcing a second page.

Return to edit mode, select (and unlock) the shape, then drag the right edge handle left, moving it as little as possible. Repeat as necessary until the second page does not appear in Print Setup view, then recenter the table with respect to the shape.

Delete the shape.

Repeat with a second rectangular shape and the vertical ruler if you also want to centre the table vertically on the page.

Regards,

Barry

Two notes:

1. This process takes FAR less time to do than to describe!
2. You can approximate the centering on subsequent pages only by manually splitting the table into two or more separate tables, then placing those tables using a similar process involving either moving the bar (shape) or placing a new copy of it on each 'page' and adjusting it's position to not cross the right margin. The table for that page can then be centred on its paired shape. (This extension gets more fiddly, an takes more time.)