File size limits have ruined more plans than people expect. The email attachment that's too big to send. The video that won't upload to the platform. The website images that make pages load slowly. Understanding how file sizes work — and how to manage them — is a genuinely practical skill for anyone who works digitally.
What Determines File Size?
A file's size is determined by the amount of data it contains, measured in bytes. 8 bits = 1 byte. A bit is a single binary digit (0 or 1). Our data storage converter converts between bits, bytes, KB, MB, GB, and TB. The unit prefix hierarchy: 1 KB ≈ 1,000 bytes | 1 MB ≈ 1,000 KB | 1 GB ≈ 1,000 MB.
Different file types have wildly different sizes for the same content because of how data is encoded. A 3-minute song as an uncompressed WAV file might be 30 MB. The same song as an MP3 might be 3-4 MB. The music is identical — the encoding is different. For download and transfer time calculations, our time calculator converts transfer seconds to minutes.
Typical File Sizes by Type
- Plain text document (1,000 words): 6-10 KB
- Word document (formatted, no images): 50-200 KB
- PowerPoint presentation (10 slides): 1-5 MB
- PDF (text-heavy): 100 KB-5 MB
- JPEG photo (smartphone, full resolution): 3-8 MB
- PNG screenshot: 500 KB-5 MB
- 1-minute video (1080p): 150-300 MB
- 1-minute video (4K): 400-800 MB
Common Upload Limits
- Gmail attachment: 25 MB per email
- Outlook.com attachment: 20 MB
- WhatsApp document: 100 MB
- Google Drive: 5 TB per file (practically unlimited for most uses)
- Dropbox: varies by plan (typically 50 MB per file on free tier)
- YouTube: 256 GB or 12 hours per video
- Instagram: 100 MB for video posts
- TikTok: 287.6 MB
How to Reduce File Size
Images: compress JPEGs (80-85% quality looks identical to the eye, cuts size by 60-70%); use JPEG for photos, PNG for screenshots and diagrams, WebP for websites. Online tools like Squoosh or TinyPNG compress without visible quality loss.
PDFs: Adobe Acrobat and free tools like Smallpdf can compress PDFs. Scanned PDFs (which are essentially image files) compress particularly well.
Videos: re-encode using H.264 or H.265 codec at appropriate bitrate for your delivery platform. 1080p video at 8 Mbps looks excellent for most screen viewing.
Documents: compress embedded images (most word processors have this option), remove unused styles and metadata.
File Sizes and Website Performance
Large image files are the single most common cause of slow-loading web pages. Google's research shows that 53% of mobile users abandon pages that take more than 3 seconds to load. For web use: photos should typically be 100-300 KB each; hero images under 500 KB; thumbnails under 50 KB. Compress every image before uploading to any website.
Calculating Transfer Time
Transfer time = file size (bits) ÷ transfer speed (bits/second). A 50 MB file uploaded at 10 Mbps: 50 MB = 400 Mb ÷ 10 Mbps = 40 seconds (theoretical). Real uploads take 25-50% longer due to protocol overhead and network conditions.
Further reading: Google's PageSpeed documentation provides guidance on image optimisation for web performance. Test your page speed with Google PageSpeed Insights.