tell application “Finder”
set screenSize to bounds of window of desktop
set screenWidth to item 3 of screenSize
set screenHeight to item 4 of screenSize
end tell

tell application “System Events”
tell dock preferences
if screen edge is bottom then
tell application “System Events” to tell process “Dock”
set dockSize to size in list 1
set dockHeight to item 2 of dockSize
set screenHeight to (screenHeight – dockHeight)
end tell
else if screen edge is left then
tell application “System Events” to tell process “Dock”
set dockSize to size in list 1
set dockHeight to item 1 of dockSize
set screenWidth to (screenWidth + dockHeight)
end tell
else
tell application “System Events” to tell process “Dock”
set dockSize to size in list 1
set dockHeight to item 1 of dockSize
set screenWidth to (screenWidth – dockHeight)
end tell
end if
end tell

set screenHeight to (screenHeight + 22)
set myFrontMost to name of first item of ¬
(processes whose frontmost is true)
end tell

try
tell application myFrontMost
set windowSize to bounds of window 1
set windowXl to item 1 of windowSize
set windowYt to item 2 of windowSize
set windowXr to item 3 of windowSize
set windowYb to item 4 of windowSize

set windowWidth to windowXr – windowXl
set windowHeight to windowYb – windowYt

if myFrontMost is “Terminal” then
set bounds of window 1 to {¬
round ((screenWidth – windowWidth) / 2) rounding as taught in school, ¬
round ((screenHeight – windowHeight) / 2) rounding as taught in school, ¬
round ((screenWidth + windowWidth) / 2) rounding as taught in school, ¬
round ((screenHeight + windowHeight) / 2) rounding as taught in school}
else
set bounds of window 1 to {¬
round ((screenWidth – windowWidth) / 2) rounding as taught in school, ¬
round ((screenHeight – windowHeight) / 2) rounding as taught in school, ¬
round ((screenWidth + windowWidth) / 2) rounding as taught in school, ¬
round ((screenHeight + windowHeight) / 2) rounding as taught in school}
end if

set the result to bounds of window 1

end tell
end try

tell application "Finder"
	set screenSize to bounds of window of desktop
	set screenWidth to item 3 of screenSize
	set screenHeight to item 4 of screenSize
end tell

tell application "System Events"
	set myFrontMost to name of first item of ¬
		(processes whose frontmost is true)
end tell

try
	tell application myFrontMost
		set windowSize to bounds of window 1
		set windowXl to item 1 of windowSize
		set windowYt to item 2 of windowSize
		set windowXr to item 3 of windowSize
		set windowYb to item 4 of windowSize

		set windowWidth to windowXr - windowXl
		set windowHeight to windowYb - windowYt

		if myFrontMost is "Terminal" then
			set bounds of window 1 to {¬
				round ((screenWidth - windowWidth) / 2) rounding as taught in school, ¬
				round ((screenHeight + windowHeight) / 2) rounding as taught in school, ¬
				round ((screenWidth + windowWidth) / 2) rounding as taught in school, ¬
				round ((screenHeight + windowHeight) / 2 + windowHeight) rounding as taught in school}
		else
			set bounds of window 1 to {¬
				round ((screenWidth - windowWidth) / 2) rounding as taught in school, ¬
				round ((screenHeight - windowHeight) / 2) rounding as taught in school, ¬
				round ((screenWidth + windowWidth) / 2) rounding as taught in school, ¬
				round ((screenHeight + windowHeight) / 2) rounding as taught in school}
		end if

		set the result to bounds of window 1

	end tell
end try

You can make this AppleScript a Service that is accessible from any application’s menu in the Menu Bar. Open Automator, create a new Service — setting the options at the top to “no input” and “any application” — drag “Run AppleScript” into the workflow, and paste one of the scripts posted above into the text field. Save the Service, and run it from the Services menu in the Menu Bar.

I prefer this method to having the Scripts menu always visible in my menu bar, since this is the only AppleScript I use regularly.

tell application "Finder"
	set screenSize to bounds of window of desktop
	set screenWidth to item 3 of screenSize
	set screenHeight to item 4 of screenSize
end tell

tell application "System Events" to tell process "Dock"
	set dockSize to size in list 1
	set dockHeight to item 2 of dockSize
end tell

tell application "System Events"
	set screenHeight to (screenHeight - dockHeight)
	set myFrontMost to name of first item of ¬
		(processes whose frontmost is true)
end tell

try
	tell application myFrontMost
		set windowSize to bounds of window 1
		set windowXl to item 1 of windowSize
		set windowYt to item 2 of windowSize
		set windowXr to item 3 of windowSize
		set windowYb to item 4 of windowSize

		set windowWidth to windowXr - windowXl
		set windowHeight to windowYb - windowYt

		set bounds of window 1 to {¬
			round ((screenWidth - windowWidth) / 2) rounding as taught in school, ¬
			round ((screenHeight - windowHeight) / 2) rounding as taught in school, ¬
			round ((screenWidth + windowWidth) / 2) rounding as taught in school, ¬
			round ((screenHeight + windowHeight) / 2) rounding as taught in school}
	end tell
end try

The two problems with what I’ve got are that Assistive Devices have to be enabled in System Preferences, and it’ll only work with the Dock on the bottom, but I never move mine anyway.

Cheers!

It should be possible to make it center vertically using basically the same approach.

Try this:

tell application "Finder"
	set screenSize to bounds of window of desktop
	set screenWidth to item 3 of screenSize
	set screenHeight to item 4 of screenSize
end tell

tell application "System Events"
	set myFrontMost to name of first item of ¬
		(processes whose frontmost is true)
end tell

try
	tell application myFrontMost
		set windowSize to bounds of window 1
		set windowXl to item 1 of windowSize
		set windowYt to item 2 of windowSize
		set windowXr to item 3 of windowSize
		set windowYb to item 4 of windowSize

		set windowWidth to windowXr - windowXl
		set windowHeight to windowYb - windowYt

		set bounds of window 1 to {¬
			round ((screenWidth - windowWidth) / 2) rounding as taught in school, ¬
			round ((screenHeight - windowHeight) / 2) rounding as taught in school, ¬
			round ((screenWidth + windowWidth) / 2) rounding as taught in school, ¬
			round ((screenHeight + windowHeight) / 2) rounding as taught in school}
	end tell
end try

I’m wondering if it would be at all possible to center windows vertically as well? I don’t know AppleScript, and can’t seem to figure this out. Ideally, I’d like my Mail, iTunes, Finder windows, etc. to be centered vertically between the dock and menu bar.

Normally it should work with whatever window is frontmost, but I can’t guarantee it will work properly in every application, because it depends on their support for AppleScript. I unfortunately don’t have a copy of Microsoft Word to test with, but if anyone has a solution, please let me know and I’ll update the post.