Scanning and Resolution

By Jim Hartsell; revised Feb. 16, 2001


This page assumes you have a scanner where you can set scanning resolution, and you are doing things beyond scanning straight to a printer. This page is to share what I've learned about scanning, image resolution, pixels per inch (ppi), dots per inch (dpi), image sizes when printed from a photo editor versus a web page, and how to scan for a monitor wallpaper picture. I use this page as a reference when I scan for different situations.

Pixels will be explained below. Color depth is the number of bits per color supported by your screen/video card. For example, 8 bits of color depth corresponds to 256 colors since 2 raised to the 8th power is 256.


Images consist of pixels, which are tiny squares that represent the color of a spot in the picture. Resolution is measured in pixels per inch (ppi). The higher the resolution, the sharper the picture, but the larger the file size. Images for display on a monitor screen only need to be about 75 ppi. Images printed on standard printers only need to be 150 ppi. For 2400 dpi photo-quality printers, resolution should be 400 ppi. Printers have a resolution measured in dots per inch (dpi) but have no relationship to ppi (see chart below).

NOTE: dpi is being used more and more when ppi is seemingly more appropriate. You will have to judge for yourself.


For the best output on: Use this resolution:
300 dpi printer100 ppi
600 dpi printer200 ppi
720 dpi printer240 ppi
1200 dpi printer300 ppi
1440 dpi printer360 ppi
2400 dpi printer400 ppi


According to most experts, the typical photographic print doesn't have more than about 200 dpi (ppi?) of resolution. To completely capture all the detail in a 200 dpi print, you would need to scan at 400 ppi.


Image editors, or photo editors, are like Microsoft Picture It, Adobe PhotoDeluxe, etc. You can scan to an image editor, make modifications, and then save the file.


This was a big puzzle for me at first. An image would be one size when printed straight from scanning software or a photo editor, another size when displayed on a computer screen, and another size when printed from a web browser (printing a web page image). Here's a brief overview, and details will follow.

From scanning software or a photo editor, the printed size is the size as shown by the software. If you don't resize, the printed size will be the size of the original.

When an image is displayed on a computer monitor, it depends on the monitor size and the scan resolution. There is a table below that shows pixels per inch for various screen sizes, but it is about 80 ppi for a 17" monitor. If you scan a 6" wide picture at 400 ppi to print on a high quality printer, the image will be 6 x 400 = 2400 pixels wide. Displayed on a 17" monitor, it will be 2400/80 = 30 inches wide! For actual size on a 17" monitor, the image will need to be scanned or sized to 80 ppi. This is also the image size when viewed as an email attachment.

When an image is printed from a web browser, it will be sent to the printer at 96 ppi, regardless of scan density. A 4" wide picture scanned at 150 ppi will be (4 x 150) / 96 = 6 1/4" wide printed.

Therefore, you need to scan twice, one at a resolution suitable for a photo-quality printer (even if you don't have one yet), and another at around 80 ppi. Or, scan high, save the file, and then resize to the lower resolution. It depends on whether you want to email, put the picture on a web page, or print a high quality picture.


1. Scan for general emailing and printing by recipient: 96 ppi.
2. Scan to print at best quality and to archive as a file: 400 ppi.
3. Scan at high quality to send to someone: 400 ppi; email or mail CD.
4. Scan for a computer monitor wallpaper background.
5. Scan for web page display only: 75 ppi.
6. Scan for a web page, but have easy "click on picture" for better quality image to print.
7. Scan for a web page, but provide for higher quality "Save Picture As".


If you scan at 150 ppi and increase the size, the same 150 ppi resolution is maintained and the photo editor fills in the extra pixels according to a best guess on the original neighboring pixels. Print resolution should still be good, but file size increases.

If you decrease the size, neighboring pixels are averaged to get the resulting pixel.

On my Adobe PhotoDeluxe, there is a "Constrain File Size" checkbox in the Photo Size window. If checked, all the original pixels are preserved when you resize, and the file size stays the same. When you enlarge, you get a lower ppi. In my experience, it seems better to do it this way so the photo editor doesn't have to fill in pixels by guessing. If I want the enlarged image to be 150 ppi, and say I am going to double the original image width, I scan at twice the resolution, or 300 ppi.

There is also a "Constrain Proportions", where if you change the height, the width is adjusted accordingly.

NOTE: You should only change image size once.

For optimal scanning, I use the formula

scan resolution = (desired width / actual width) * desired resolution

If the picture is 2.5" wide, and I want 6" at 400 ppi,
scan resolution = (6 / 2.5) * 400 = 960; resize to 6" wide at 400 ppi.

If the picture is 7" wide, and I want 4" at 400 ppi,
scan resolution = (4 / 7) * 400 = 229; resize to 4" wide at 400 ppi.


If you save a scanned image into, and print from, an image editor like Adobe PhotoDeluxe, it prints at the specified size. This is also true if you save a web page image into a file (right click and "Save Picture"), then open the file with an image editor.


Regardless of the print size specified for an image, the size of an image on-screen is determined by the pixel dimensions of the scanned image and the monitor size and setting. A large monitor set to 1024 by 768 pixels uses LARGER pixels than a small monitor with the same setting, so they are BOTH 1024 pixels wide. Below is a chart showing monitor size and APPROXIMATE pixels per inch for a 1024 x 768 screen.

15"14" 11.25"91"
17"16" 12.75"80"
19"18" 14.5"71"
21"19.8" 16"64"

A happy medium might be 75 or 80 ppi.

An original image 4" wide at 150 ppi (for a total of 600 pixels wide) will be 7" wide on a 12" wide monitor. This is because if 1024 pixels = 12", (600/1024) x 12" = 7". The image will be 9" wide on a 16" wide monitor (600/1024) x 16" = 9".

This page of notes assumes your monitor screen is 1024 x 768 pixels. Your setting is shown at the top of this page.


This assumes left and right print margins are set to 3/4".

When I print an originally 4" wide 150 ppi image (600 pixels wide) that's on a web page, it comes out 6 1/4" wide regardless of the size of the monitor screen. This is because in most cases, screen images are sent to the printer at 96 ppi. For an image 600 pixels wide, 600 pixels divided by 96 ppi = 6 1/4". Putting it another way, the 150 pixels that made up an inch are spread out to about an inch and a half.

NOTE: If an image is a high density ppi, but sized down in the HTML to fit the web page, it will print at its original density. For example, if an original 400 ppi 6-inch wide image is sized down, the original is 2400 pixels wide and will try to print 25 inches wide (2400/96)!


This depends on how the image will be displayed on the recipient's computer after it is downloaded. It could be displayed by a web browser or an image editor like Microsoft Picture It. Since it is assumed this is not serious scanning, it would be best to scan at 96 ppi for actual size printed from a browser, and a good resolution for a 300 dpi printer.


For the following, you can mail a CD with these image files.

To archive photographs is to scan them at the best resolution for a photo quality printer so that the printed image looks almost as good as the original, even if you don't have a photo-quality printer yet. This would require scanning at 400 ppi, and you will want to save the file most likely on a CD. For a 4x6-inch photograph, this would be 1600 x 2400 pixels = 3,840,00 pixels, or said another way, 3.84 million pixels, or 3.84 megapixels. File size will be approximately ___ KB. This is somewhat large to email, and too large to display on the computer screen, so you need a second scan at a lower resolution. For email and/or a web page, scan at 75 or 96 ppi. For 96 ppi, this would be 384 x 576 pixels = 221K pixels, with a file size of approximately ___ KB.

To be able to print the enlargement on a single sheet of paper, the enlarging can be up to 6" x 9" for a 4" x 6" photograph. To end up with a 400 ppi image at the enlarged size, scan at (6/4 x 400) = 600 ppi, then resize to 6" x 9" and set it to 400 ppi. If your computer lets you constrain file size, just set resolution to 400 and the size will happen automatically.

This takes more math than enlarging. After scanning, your software may let you select a portion of the scanned photograph. To be finished...


My Windows NT at work wants a bitmap file (.bmp), but Windows 98/2000 accepts either bitmap or jpeg (.jpg) files.

To change the background wallpaper picture on your monitor for Windows, right click the mouse on the desktop screen. Left click on Properties. In the Display Properties window, select the Background tab. Under Wallpaper, click Browse. Find a picture you want (of the right size - see below). Depending on your Display Properties window, center the picture and don't stretch the picture. Click Apply, and click OK.

Before scanning a photo for a wallpaper picture, you need the pixel width of your monitor screen as shown at the top of this page. 1024 is the usual setting.

Before scanning, the picture you want should be in the proportions of 3 x 4 to fill the whole screen. If your original is a normal 4" x 6" print, it's close enough, but you'll get a border at the top and bottom. 4" x 5 5/16" would be best.

What is needed for a wallpaper picture is a picture 1024 pixels wide. Use this simple formula: resolution (R) = monitor width in pixels (P) divided by the width (W) of the image you're scanning (R=P/W, or R=1024/W)).

For monitor width = 1024 and photo width = 5.33", scan resolution = 1024/5.33 = 192 ppi.

The image will be 1024 pixels wide, and will fill the screen no matter what size monitor you have.

Save the image in a file in the directory for wallpaper images. One way to find this directory is to use Windows Explorer and search for an existing wallpaper file name.

To be more accurate, pre-scan your photo and get the actual scanned width from your photo editor. Then use the formula R=P/W, and rescan using the recomputed resolution.



Here you want the smallest file size for quickest loading of the page. If you have a file that is larger than it needs to be, and in the HTML you force it to a smaller width and height, you're increasing download time for nothing.

For physical dimensions of the picture on a web page, I usually shoot for around 400 pixels as the maximum photo size (long side), but you may want something different. A 4" x 6" photograph would be 266 x 400 pixels. If the long side of a photo is "L", scan at 400/L. For a 4" x 6" photo, this would be 400/6 = 67 ppi.

Since an average screen resolution is about 75 ppi, you could scan at 75 ppi for a roughly actual size on the screen, which would be 300 x 450 pixels on the screen for a 4" x 6" photograph.

You can also scan at any resolution, resize to 400 pixels on the long side, and then save the file.


In this case, I have the picture by itself on a web page so that you just need to click "Print".

I have cases where I want a small image (and small file size) on a web page "album", but I make the image a clickable link to bring in a web page with only the picture on it, and at a larger size. Or, I have a text link that can be clicked to bring in the single picture on a web page.

When you print from a web page, the images will be 96 ppi no matter what scanning resolution was used. A 4" wide picture scanned at 75 ppi (300 pixels wide) will be 300/96 = 3 1/8 inches wide printed, so it will be smaller than the original. A 4" wide picture scanned at 150 ppi (600 pixels wide) will be 600/96 = 6 1/4 inches wide, which will be larger than the original.

Therefore: If you want the printed image to be the same size as the original picture, scan at 96 ppi (assuming it is less than 6" x 9"). A 4" wide picture scanned at 96 ppi (384 pixels wide) will be 384/96 = 4" wide, the same as the original.

Note that the picture will be larger than actual size on the computer screen. In the photo editor you can resize the picture at 75 ppi and save it as the web page image. You can also set a smaller image size in the HTML with the WIDTH and HEIGHT attributes, but this is not recommended.

Let's say I have a 5" x 7" that I want to be printable at this size, but smaller on the web page. I would scan at 96 ppi, giving an image 480 x 672 pixels, and save the file. Note that in HTML you can HREF directly to an image file. For the album page, I would resize the image to, say, 214 x 300 at 75 ppi, and save the file. In the HTML below, the < and > are replaced by { and } because the browser will do it rather than show the HTML.

Click on the picture to print actual size
{A HREF="actualfile.jpg"}
{img src="smallfile.jpg" border=0 WIDTH=214 HEIGHT=300}

Or, to bring in the picture by clicking on a link, use

{A HREF="actualfile.jpg"}Click here{/A} to print an actual size picture.

If you need to increase or decrease the print size of the originally scanned image, use this formula:

scan resolution = (desired width / original width) x 96,

then set to constrain file size and resize to desired width. You should end up with 96 ppi.


NOTE: This method will increase the web page loading time because a higher resolution image is downloaded, but it is resized to a lower resolution for display on the page. It is also not recommended to resize a picture in the HTML.

Scan at higher resolution, and size it for the web page using WIDTH and HEIGHT attributes. Note that the higher the resolution, the larger the file size, so this depends on how much disk space you have at your web server. For the most important pictures, I scan at 400 ppi. 150 ppi is a good compromise if you're scarce on disk space.

For example: scan a 4" x 6" photograph at 150 ppi or higher and save the file. Then, look at it's actual scanned size and actual pixel width. If you want the long side to be 400 pixels, get the other dimension with the formula
X = (short side pixels x 400) / long side pixels. In the HTML, use

IMG SRC="filename.jpg" HEIGHT=266 WIDTH=400

You can then right-click on the image, and save it at it's originally scanned resolution.


Summary, using Adobe PhotoDeluxe: Scan at 400 ppi and save at Maximum image quality on a CD no matter what I will be doing with the picture. With the same image, keep the same size, set resolution to 96 for emailing or printing from a single-image web page, and save at High image quality. Reload the 400 ppi image, set resolution to 75 ppi and long side to around 400 pixels for fast load & display on a web page, and save at High image quality. Files names are xxxxx_400.jpg (~700KB), xxxxx_96.jpg (~80KB), xxxxx_75.jpg (~47KB).

First, if the scanned image is to be a different size from the original, I compute the scan resolution with

R = (desired width of photo/actual width) * 400,

then resize to desired width and 400 ppi. If the original is 8" W x 10" H, and I want 5" W, R = (5 / 8) * 400 = 250 ppi. On Adobe PhotoDeluxe I set "Constrain File Size" and simply change the 250 ppi to 400 ppi.

If no size change is needed, I scan at 400 ppi.

I save this file usually with an xxxx_400.jpg file name. This is the master, and I store it on a "Photos" CD. If I want to email it, I resize it to 96 ppi.

If the picture is going on a web page, and people might want to print a copy, like an old family photograph, I've adopted a new scheme for all new (or redone) scans. The image on the web page is usually about 400 pixels on the long side at 75 ppi. Clicking on it brings up an actual size image at 96 ppi that can be printed by clicking "Print" on the browser menu. Right clicking on the actual size image will let you "Save Picture As" at the 400 ppi resolution, or lower, depending on importance of the picture.

Using Adobe PhotoDeluxe or comparable photo editor:

1. Scan for a 400 ppi image at less than 6" x 8.5".
2. Save as xxx_400.jpg as an archived photo on a CD.
3. Resize to about 400 pixels on long side with resolution of 75 ppi.
4. Save as xxx_75.jpg for a web page image.
5. Create an HTML file xxx_400.html that just pulls in the 400 ppi file, but forced in the HTML to a maximum 600 x 768 pixels. (Picture can be printed in landscape mode if the long side is horizontal.) If it was a 4" x 6", force to 384 x 576 pixels. Add a line that says "Click Print to print at 96 ppi, or right click to save picture as a 400 ppi file".
6. On the web page, display the xxx_75.jpg file. Make the image a clickable link to the xxx_400.html file.

Optional to email:
7. Re-open original xxx_400.jpg file.
8. Change resolution to 96 ppi.
9. Save as xxx_96.jpg for actual size email.

On the web page, the information can go with pictures as shown below, or where it is clearly visible, you can say:

If, when you put the mouse cursor on a picture and it turns into a pointing finger, this picture is a clickable link to a larger picture that can be printed. On the larger picture, you can right click to do a "Save Picture As" for a higher resolution file. This can be printed from a photo editor to a photo-quality printer on photo-quality paper.

Click on image for a printable full size image at 96 ppi.

Or, on that image, you can right click to save a high resolution 400 ppi file for future printing from a photo editor to a photo-quality printer.



When you are looking at your image file names with Windows Explorer, there will be a symbol by the file name. If you don't have an image editor, the symbol might be for a web browser like Internet Explorer, AOL, or Netscape.

When you double-click on the file name, the image will be displayed according to the symbol. If the symbol for a web browser is showing, the browser will run and display the image like a web page. If Internet Explorer is showing, and you then install an photo editor, the symbol should automatically change to the symbol for the photo editor. There is no change to the image itself when the display symbol changes. When I double-click, my images come up in Adobe PhotoDeluxe. When I print, they are actual size as scanned.


(This is being verified: I need to find out if there is a default photo size when you print, such as 3 x 4, or 4 x 5.33 (ratio 3:4), and can set resolution to 400 ppi.)

There was a hint above that answered a question for me. I had been puzzled on what, really, is a 1 megapixel or 3 megapixel digital camera? In the section above, I said a 4" x 6" photo at 400 ppi would be 1600 x 2400 = 3.84 megapixels. That's it! Digital camera images have an aspect ratio of 3:4 like a computer screen. This would be 768 x 1024 or 1800 x 2400 pixels. You specify the resolution for the picture. If you print the picture at 4" x 5.33", and take it at 400 ppi, the image will be 1600 x 2133 pixels. 1600 x 2133 = 3,412,800 pixels, or 3.4 megapixels. If you want to take pictures for the best quality 4" x 5.33" printed picture, buy a 3.4 megapixel camera!