How to reduce the size of videos, PDFs, images and audio

Shrinking a file requires neither paid software nor technical knowledge. You only need to know what actually takes up space in each kind of content.

Why zipping barely helps

MP4 videos, JPG photos, MP3 music and PDFs with images are already compressed. Putting them in a ZIP reduces the size by 1 or 2%, if at all.

ZIP pays off for uncompressed content: CSV spreadsheets, text files, BMP images, WAV audio and folders with many small files, which then travel as a single file.

Videos: size is bitrate times duration

The size of a video is determined by its bitrate, not by the resolution itself. At 5 Mbps, a common value for 1080p, each minute takes about 37 MB. At 2.5 Mbps, suitable for 720p, each minute takes about 19 MB.

The reverse calculation works too: to fit in 100 MB, divide 800 megabits by the duration in seconds. A 10 minute video has 600 seconds and needs roughly 1.3 Mbps in total, picture and sound combined.

HandBrake, which is free, does this conversion. Pick a 1080p or 720p preset, keep the H.264 codec for compatibility and set the average bitrate to the calculated value. Screen recordings compress far better than scenes with motion.

PDFs

PDFs generated from text are small. The large ones are almost always scans, where each page is a photograph. Scanning at 150 dpi in grayscale is enough for on-screen reading and makes the file several times smaller than 600 dpi in color.

For an existing PDF, use your reader's reduce size option: Preview on macOS offers a reduction filter when exporting, and word processors have a minimum size option when saving as PDF.

Images

A phone photo is 3 to 8 MB and has far more resolution than a screen can show. Resizing to 1920 pixels wide and saving as JPG at around 80 quality usually brings the file under 1 MB with no visible difference.

Use JPG for photographs and PNG for screenshots, diagrams and images with text, where JPG creates smudges. WebP produces smaller files in both cases.

Audio

One minute of CD-quality WAV takes about 10 MB. The same minute as MP3 at 192 kbps takes roughly 1.4 MB, and for voice recordings 96 kbps mono is enough. Free tools such as Audacity export to these formats.