Work
Pixels kept by a downscale
Halving both dimensions quarters the pixel count, which is why file sizes fall much faster than the numbers on the dimensions box. It is also why an upscale gains almost nothing noticeable.
The Formula
fraction kept = s squared, because area scales with the square of the length factor
Halving both dimensions quarters the pixel count, which is why file sizes fall much faster than the numbers on the dimensions box. It is also why an upscale gains almost nothing noticeable.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| s | Scale factor | ratio | The number both dimensions were multiplied by. |
| P_src | Source pixels | pixels | w times h of the original image. |
| P_out | Output pixels | pixels | Pixels in the resized image. |
| f | Fraction kept | ratio | P_out divided by P_src. |
What Each Symbol Means
sratio
Scale factor
The number both dimensions were multiplied by.
P_srcpixels
Source pixels
w times h of the original image.
P_outpixels
Output pixels
Pixels in the resized image.
fratio
Fraction kept
P_out divided by P_src.
See It Worked With Real Numbers
A formula on its own does not tell you which number goes where. The how to resize product images page applies this equation to a concrete case, shows every intermediate step, and names the mistakes that produce a wrong answer even when the algebra is right.
Where This Formula Appears
This equation is the working core of How To Resize Product Images, filed under Everyday Life. Every number the guide produces comes from substituting into this one line.