How many megapixels to print regular size photos?

4x6 regular size photos, no special effects. I'd like to print them in Boots this week and I always resize my pictures from many thousands to 800 wide. I've done some pre-research but it just confuses me so any thoughts?
I use Nikon D90 which has 12.3 MP

The "Gold Standard" is 300 ppi. For 4x6, this would mean 1200x1800 pixels, which works out to 2.16MP.

You would be hard pressed to see the difference between 300 and 200 ppi prints at 4x6. 200 ppi gives 800x1200 or 0.96MP.

Some places will reject anything below 175 ppi.

Your 800 pixel width works out to 133 ppi. Stick to at least 1200 pixels wide.

Someone is sure to point out that "ppi" is screen resolution and "dpi" is print resolution, but since you are talking about resizing image files, I'm speaking in terms of ppi.

Any digital camera made in the past ten years can produce excellent 4x6 inch prints.

Since you have a D90, you should be shooting at the cameras highest resolution in any case.

Why?

You made a substantial investment in your excellent camera and there's no reason not to use it at its full potential.

A 7 mb image file will make a amazing 4x6 inch prints and there's NO need to make them smaller, none at all.

If you are running out of room on your computers hard drive, then you have two options.

* Buy an external 1 tb hard drive (less than £60) to store your old images.
* Use your DVD burner to keep copies of your old images and store them in a cool, dry place

I guess you are not a pro photographer, but you need to have a few different size memory cards you can carry with you when out shooting your favorite subject.

Don't re-size them, your just dumping data, take them on a memory stick to any print house, provided you have enough pixels you will get a good print with the computer working it all out for you for any normal print size. For a 12Mp camera that's any size up to A3.

The SMALLER the image the more ppi you need, the opposite of what you might think. You only really need to take charge of ppi when you want to print big, I mean BIG, like 4ft X 3ft, and that's not a maximum.

The only problem your likely to encounter is Aspect Ratio (Height to Width Ratio), it's not very often that the Aspect Ratio of the paper matches the Aspect Ratio of your file. More often than not you will have to crop some of your image or accept different width borders.

4x6" photo print sizing, is the inches as 4 by 6 or 6 by 4,
at 300 ppi/ dpi print quality, +
1800 by 1200 pixels,

… But original images can be placed into album sized window as is, for print,
drag and drop or insert tools for print do not need to edit actual image for print,

Boots Photography > prints
http://www.boots.com/en/Photo/Digital-Photo-Processing/

Instore Digital and Film Photo Processing…
http://www.boots.com/en/Instore-Digital-and-Film-Photo-Processing_1042844/?cm_re=c1020_image3-_-instore-_-product_list&cm_sp=cat_photo-_-c1020-_-c1020_image3&cm_re=c1020_image2-_-service-_-instore_photo_processing&cm_sp=cat_mandb-_-c1020-_-c1020_image2

http://www.bootsphoto.com/boots/photo-gifts/photo-printing
http://www.bootsphoto.com/boots/howitworks
http://boots.custhelp.com/app/answers/detail/a_id/838/brand/75/brand_host/www3.bootsphoto.com/

Boots Help
http://www.boots.com/en/Help/

they do not clarify dpi or resolution size, or memory file size, only that files be placed on memory sticks or devices for transfer,

12.3 MP, is 4100 x 3100 ~ish at 72 is 56 by 43,
http://www.hugge.netg.se/java/MegaPixels.html

if placed to 300, size goes to 13 by 10 ~
if placed to 300, but change to 800 by 1100 ish pixel, size goes to 3.5 by 2.5 inches, too small.

if you are using kiosk place images as is on usb or memory card,
if being processed requires file size or resolution, no lower than 300 for inches of 4 x 6

if for 5x 7 inches, then you want 2000 by 1500 range per image pixel size,
.