INSpectadeck...there are 144 square inches in a square foot. so, for your area,

18" x 18" = 324 sq. in.
324/144 = 2.25 sq. ft.

You've got 2.25 sq. ft. Really, you want to have at least 4000 lumens per square foot...5000 and up is better. At 1600 lumens per bulb, you should have 6 bulbs or more. In that small of an area, I'd try it with 6 bulbs and make sure that the wall surfaces reflect well. Check out some of the Rubbermaid grows on here...they have grow areas similar in sized to yours. See what type/how many lights they use, and how they did.

turksteelman...yes, you'll do a GREAT job with a T5 like that. 40000 lumes is good for 8 sq. ft. with no problem...that's 48" x 24". And you can use the T5 in flowering too...just switch out some/all of the bulbs for 3000k bulbs. HTG supply sells those for 4 for $20.

High Tech Garden Supply

Good luck to both of you!