To resize an image without distortion, change width and height by the same scale factor, so the ratio between them stays fixed. This comes up in almost every practical task that places a supplied picture into a document, poster or web page.
All file names, sizes and documents in this lesson are invented for practice. Menu names differ between programs and versions, so the steps below describe the feature, not one brand’s buttons.
What is an aspect ratio?
The aspect ratio is the width divided by the height. An image 1600 pixels wide and 1200 tall has a ratio of 1600 : 1200, which simplifies to 4 : 3. Any smaller copy that is still 4 : 3, such as 800 × 600 or 640 × 480, has the same shape.
Most programs offer a setting often called lock aspect ratio or keep proportions. With it on, changing one dimension updates the other automatically. Dragging a corner handle usually keeps proportions, while a side handle changes one direction only.
How do you calculate the missing dimension?
- Write the original width and height.
- Decide which new dimension is fixed by the layout.
- Find the scale factor: new size ÷ original size.
- Multiply the other original dimension by the same factor.
- Check that the ratio matches the original.
Worked example
An invented image garden.png is 1600 × 1200 pixels. It must be 640 pixels wide on a page.
Step 1: scale factor = 640 ÷ 1600 = 0.4.
Step 2: new height = 1200 × 0.4 = 480.
Step 3: check the ratio: 640 ÷ 480 = 4 ÷ 3, the same as 1600 ÷ 1200. The new size is 640 × 480.
The mistake to watch for
A student sets the width to 640 and, wanting to save space, types 300 for the height.
Mistaken result: 640 × 300 has a ratio of about 2.13 : 1, but the original is 1.33 : 1. The garden looks flattened.
The correction is to let one dimension drive the other. If the space really is only 300 high, scale by 300 ÷ 1200 = 0.25 and use 400 × 300 instead. If the picture still does not fit the space, crop it, as the next lesson explains.
Check yourself
1. An invented image is 900 × 600 pixels. It must be 400 pixels high. What width keeps the proportions?
Show answer
Scale factor = 400 ÷ 600 = 2/3. Width = 900 × 2/3 = 600. The new size is 600 × 400.
2. A 1200 × 800 image is reduced to 30% of its size. Give the new dimensions.
Show answer
1200 × 0.3 = 360 and 800 × 0.3 = 240. The new size is 360 × 240.
3. A 1000 × 500 image is set to 350 pixels wide with the aspect ratio locked. What is the height?
Show answer
Scale factor = 350 ÷ 1000 = 0.35. Height = 500 × 0.35 = 175.
Where this leads next
Next, learn what to do when the proportions of the picture and the space simply do not match in Crop deliberately rather than stretch. Return to the module guide for the full route, and use the practical task and evidence checker to confirm your result.
Resizing is easy on its own but often goes wrong inside a longer task with a size limit. A teacher in online one-to-one ICT tuition can watch you work on your own files and spot where the ratio slips.