Armstrong

This is a great calculator for tiles and grid components. It lets you pick basic room shapes, then change the dimensions easily by "dragging" the walls. You select tile size and orientation, and can adjust grid position as desired (nifty for getting the border areas just right). When you're done, it gives you a comprehensive parts list of tiles and basic grid components.

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Ceilume

This great calculator lets you design your room, and estimates how many tiles or panels will be required for your installation. It also calculates necessary accessories such as grid covers and insulation!

CSG Network

This is a good, basic calculator for suspended ceiling grid components- and tiles---nothing fancy. You put in the room dimensions (total square footage, linear wall footage), it tells you what components you need.

DIY Online

Good room layout visualization with this one. You select one of three basic room shapes, then drag the walls to match your actual dimensions. Next, pick your tile size . . . and select the number of lights panels (this feature is a little clunky). At the end, you can print a good basic parts list.