I have an image of no particular aspect ratio. I could be 5x7 or 4x8 or anything else. I want to crop it, to change the overall ratio or to zoom in on a section of the image that I may print later. I know that I will want the end result to be 8x10, or multiples thereof, so that when I print it, it will fit an 8x10 frame.

I can use the select tool and I can use the crop tool, but how can I tell if I am cropping to the ratio I want?

In the bottom part of the crop dialog there is a "fixed" box. You tick this and select aspect ratio ,width, height or size. when you select aspect ratio you can select your ratio by entering 4/5 or 8/10 or 4:5 or 8:10 (both methods work) and now the width to height ratio is 8 : 10 , no matter how big or small your crop is. The help section on the crop tool is quite good!

Thanks you. I knew the solution was there somewhere. I found a clunky way of doing it. Use the guides and calculate the positions so they enclose the size you want. Then crop to the guides. I give you method a try tomorrow. I know it will be better.
Thanks.

There's also a script:precise-selection.scm.
http://registry.gimp.org/node/13245
Gerard.

wbool63. Thanks. Your advice was a charm.
Haven't tried gerard82's script yet. But I will when I get some more time.