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The base dimensions of the Sentinel Light is 9 3/4" x 9 3/4" Inches. If your post is 6" x 6" unfortunately this fixture would be too large. We do offer other post cap lights for a 6" x 6" post but they are not considered a marine light. If you are interested in these other post cap lights, please give us a call at 877-548-3387. Thank you!