Take note of the largest metric allen key that will fit inside the bolt, and then look at this chart.
http://www.metric-conversions.org/length/millimeters-to-inches-table.htm
Look at what falls between the allen key from above and the next size up. That will be the SAE size you need. If you have the room to work, you can get a set of allen keys that are set up like a Swiss Army knife for under $10. Its just easier to go that way, if possible. Also, if this is a really small bolt, shine a light on it and make absolutely sure its an allen key, and not something else, like a torx. It's very easy to mistake them when they're small.
http://www.metric-conversions.org/length/millimeters-to-inches-table.htm
Look at what falls between the allen key from above and the next size up. That will be the SAE size you need. If you have the room to work, you can get a set of allen keys that are set up like a Swiss Army knife for under $10. Its just easier to go that way, if possible. Also, if this is a really small bolt, shine a light on it and make absolutely sure its an allen key, and not something else, like a torx. It's very easy to mistake them when they're small.