How to compress an image to a target size in KB or MB

Understand the target

KB and MB describe storage, not image dimensions

Pixel dimensions tell you how wide and tall an image is. KB and MB tell you how much storage its encoded file occupies. Two images with identical dimensions can have very different file sizes because a detailed photograph, a flat illustration and a transparent logo compress differently.

When a form says an image must be below 500 KB, the useful goal is usually any visually acceptable result at or under that ceiling. Trying to land on exactly 500.0 KB adds no value and can be unreliable because browser encoders can produce slightly different byte counts.

Use the right order

Resize first when the source is much larger than its use

A phone photo may be several thousand pixels wide even when a profile or website displays it at a fraction of that size. Reducing dimensions removes pixels that the final use never shows, often saving more data than forcing the encoder to discard visible detail.

If exact dimensions are specified, use the Resize Image tool first and keep the aspect-ratio lock enabled. Download that result, open Compress Image and then choose the required maximum. If dimensions are already correct, begin directly with compression.

• Keep the original file before starting.
• Match the final display or upload dimensions.
• Choose a suitable output format.
• Compress to the required maximum and inspect the result.

What ResizeImg does

Quality search first, dimensions only when necessary

For JPG and WebP, ResizeImg tests encoder quality levels in your browser and keeps the highest practical result below the target. If even a very low quality remains too large, the tool reduces dimensions and tries again. This avoids shrinking the image when an encoding change alone is enough.

PNG behaves differently because browser PNG export is lossless and does not expose the same adjustable quality control. When a PNG target is far below the natural encoded size, reducing dimensions may be the only reliable browser-based option. For photographs that do not need transparency, JPG or WebP is usually a more flexible choice.

Review before use

Inspect the places where compression becomes visible

Open the downloaded image at the size where people will see it. Look at hair, leaves, fabric, small text, high-contrast edges and smooth skies. Blockiness, ringing around edges, smeared detail and color banding are signs that the target may be too aggressive.

If the result looks poor, increase the allowed size, reduce the dimensions more sensibly or switch formats. A slightly larger clean image is usually more useful than a file that technically passes an upload limit but no longer communicates clearly.

• JPG: practical for photographs and broad compatibility.
• WebP: efficient for modern web delivery when accepted.
• PNG: useful for transparency and crisp graphics, but often large for photos.