How do you select the interior of a circle given its center and radius, in Photoshop?

Photoshop question:

If I know the coordinates x and y of a point in an image and a radius r, is there a way to select all points in the image that lie inside the circle whose center is x,y and radius r by just telling photoshop x, y and r?

Firstly you can mark the x, y spot using the guides.

-Hit Ctrl-R to turn on the rulers along the sides of the image if you don't have it on.
-Point your cursor from within the rulers and drag out a guide for both horizontal and vert.
-You should have the two guides crossing over this x, y spot.
-Select your circular marquee tool, and look at the options bar.
-Under the 'style' option box, select 'Fixed Size' instead of 'Normal'
-A bit further to the right on the same bar, you can key in dimensions in pixels, inches, etc.
-Click on the image area, and a fixed size marquee will appear.
-Point within the circle of the marquee and click drag to move it. It should be able to snap onto the guides that you created earlier. If not goto view>snap and toggle it on.

Thank you very much. I didn't know about the guides.

The "location" of the marquee is not the center of the circle, but the upper left hand corner of the box that contains the circle. That's what snaps. I can live with that.

No, don't live with that. Hahaha.
When using the marquee tool, click and let go. No need to move it or snap yet. Then point inside the circle, and drag it. You should be able to snap on center.

OK. That works. Thanks again.


Just make sure the "New Selection" flag is set. I had "Intersect" set the first time I tried (and the second, and the third ...). Lot's of gotchas in photoshop.

Hi Lee!

It's actually a lot easier than that.

1. Go to View > New Guide. That will open a dialog. Type in the value for the vertical axis and click OK. It appears.

2. Go to View > New Guide again. The same dialog opens. Click to enable Horizontal Guide, and type in the value for that axis. While you're looking at this menu, just glance down and make sure Snap is enabled.

3. Get the Circular Marquee tool, change the Style to Fixed Size, and type in the Height and Width you want (2r, of course.)

4. Hold down the Option/Alt key, and click the spot where the guides cross. Your circle will be selected, just like that.

There's really no need to mess with the rulers, or pulling the guides out, or any of that stuff when you know where you want them.

Also, since you might not know this, you don't have to enter pixels in the New Guide dialog. You can enter pixels, inches, percent, or whatever you want. (Percent can be extremely helpful, as you might imagine.)

Hope this helps!

Mmm. Thanks! Thats about 600% faster.

Thank you, Robin. I was hoping there was some way to do it by entering numbers in boxes.


By the way, the "new selection" box in the tool bar has to be selected in order for the circle to end up centered on the intersection of the cross-hairs. If something other than "new selection" is on, the circle will end up in the lower right quadrant defined by the cross hairs, and tangent to them.

Hi Lee!

Yes; but you already knew that.

The way that Photoshop works, I'd recommend clicking the New Selection icon in the Options Bar (which is the default,) and just leaving it like that permanently.

That way, when you hold down Option/alt, all selections will form from the center and if you are using Normal style, (not Fixed Size or Fixed Aspect Ratio) the Shift key will constrain the selection to a perfect circle, or a perfect square.

Unless you have a selection already, of course, (something with Marching Ants.) Then, the Option/alt key will cause the Style to automatically change to Subtract from Selection, the Shift key will cause it to change to Add to Selection, and holding down both Shift and Option/alt will cause it to change to Intersect with Selection.

So, you see, there's really no reason to choose those things from the Options Bar.

By the way, when you're not using Fixed Size, if you want to move the selection around, you can do so by holding down the Spacebar. As long as it's depressed, dragging the mouse won't change the size of the selection, it'll change the placement. Just something that's handy to know.