A file size converter translates between digital storage units — bytes, kilobytes, megabytes, gigabytes, terabytes — which sounds simple until you realize there are actually two different "kilobyte" definitions, and that's why your 1TB drive shows 931GB in Windows.
This guide explains how digital file sizes really work, the critical difference between binary (1024-based) and decimal (1000-based) units, the correct conversion formulas, and why certain file size numbers don't match what you expected.
Convert File Sizes Instantly — Free
Switch between bytes, KB, MB, GB, TB, and PB with both binary (1024) and decimal (1000) standards. Clean, fast, and always accurate.
The Basics — Bits, Bytes, and Digital Storage
All digital storage is ultimately counted in bits — individual 0s and 1s. Eight bits = one byte, the smallest unit typically referenced in file sizes.
Core Definitions
- Bit (b) — single binary digit (0 or 1)
- Byte (B) — 8 bits. The fundamental unit of file sizes.
- Nibble — 4 bits (rarely used outside low-level programming)
Why Bytes, Not Bits, for Files
One byte can represent one ASCII character, one pixel color channel, or one audio sample — making it the natural unit for file sizes. Internet speeds, however, are measured in bits per second (Mbps), which is why "100 Mbps internet" is actually ~12.5 MB/s of file download speed.
MB = Megabyte (file size). Mb = Megabit (network speed). A 100 Mbps connection = 12.5 MB/s. When your ISP says "100 Mbps", they mean bits.
The Binary vs Decimal Confusion
Here's where it gets tricky. There are two valid definitions of "kilobyte":
| Unit | Binary (SI incorrect but common) | Decimal (SI correct) |
|---|---|---|
| 1 KB | 1,024 bytes (2^10) | 1,000 bytes (10^3) |
| 1 MB | 1,048,576 bytes (2^20) | 1,000,000 bytes (10^6) |
| 1 GB | 1,073,741,824 bytes (2^30) | 1,000,000,000 bytes (10^9) |
| 1 TB | 1,099,511,627,776 bytes | 1,000,000,000,000 bytes |
Who Uses What?
- Windows — uses binary (1024) but LABELS it as KB, MB, GB. Technically wrong per SI but widespread.
- macOS (since 10.6) — uses decimal (1000) for displayed file sizes. A 1 GB file shows as 1,000,000,000 bytes.
- Linux — varies by tool; most use decimal now with KiB/MiB for binary.
- Hard drive manufacturers — always decimal (that's why a "1 TB" drive is 931 GiB in Windows).
- RAM specs — always binary (8 GB RAM means 2^33 bytes exactly).
- Internet speeds (Mbps) — always decimal.
Manufacturers sell 1TB = 1,000,000,000,000 bytes (decimal). Windows displays it in binary — that same number divided by 1,073,741,824 = 931 GiB. No one is cheating; they're just using different systems.
The Proper Binary Units (KiB, MiB, GiB)
To resolve the confusion, the IEC introduced binary prefixes in 1998:
| Binary Unit | Equals | Meaning |
|---|---|---|
| KiB (Kibibyte) | 1,024 bytes | "kibi" = kilo binary |
| MiB (Mebibyte) | 1,048,576 bytes | "mebi" = mega binary |
| GiB (Gibibyte) | 1,073,741,824 bytes | "gibi" = giga binary |
| TiB (Tebibyte) | 1.0995 × 10^12 bytes | "tebi" = tera binary |
| PiB (Pebibyte) | 1.1259 × 10^15 bytes | "pebi" = peta binary |
KB vs KiB
Strictly speaking:
- KB = 1,000 bytes (decimal, SI-correct)
- KiB = 1,024 bytes (binary)
But in practice, most consumer software still uses KB to mean 1,024 bytes. Adoption of KiB/MiB has been slow outside Linux and scientific software.
Conversion Formulas (Cheat Sheet)
Decimal (SI, hard drive manufacturer) Conversions
- 1 KB = 1,000 bytes
- 1 MB = 1,000 KB = 1,000,000 bytes
- 1 GB = 1,000 MB = 1,000,000,000 bytes (10^9)
- 1 TB = 1,000 GB = 10^12 bytes
Binary (OS display, RAM spec) Conversions
- 1 KiB = 1,024 bytes
- 1 MiB = 1,024 KiB = 1,048,576 bytes
- 1 GiB = 1,024 MiB = 1,073,741,824 bytes
- 1 TiB = 1,024 GiB = 2^40 bytes
Quick Decimal Conversions
- To go from byte to next unit: divide by 1000 (or 1024 binary)
- To go up 1 unit: divide size by 1000 (or 1024)
- To go down 1 unit: multiply by 1000 (or 1024)
Common File Size Examples
| File Type | Typical Size |
|---|---|
| Plain text document (1 page) | 3-5 KB |
| MP3 song (3.5 min, 128 kbps) | 3-4 MB |
| MP3 song (3.5 min, 320 kbps) | 8-10 MB |
| JPEG photo (smartphone, default) | 2-5 MB |
| Raw smartphone photo | 20-50 MB |
| 1 hour of HD video (1080p) | 1-3 GB |
| 1 hour of 4K video | 15-40 GB |
| Typical PDF document (10 pages, text) | 200-500 KB |
| Typical PDF with images | 5-20 MB |
| Standard OS install (Windows/Mac) | 30-50 GB |
| Modern AAA video game | 50-150 GB |
Gmail: 25 MB per attachment (use Drive for larger). Outlook: 20 MB. Yahoo: 25 MB. If a file is bigger, compress it, split it, or upload to cloud storage and share the link.
Practical Tips for Managing File Sizes
- JPEG vs PNG — JPEG compresses photos 5-10x more than PNG. Use PNG only for screenshots, logos, or images with transparency.
- MP4 (H.264) vs MOV — MP4 H.264 compresses 3-5x smaller than MOV with similar visual quality. Convert for sharing.
- PDF optimization — "reduce file size" in Adobe/Preview can cut PDF sizes by 50-80% for email.
- ZIP compression — minimal savings for JPEG/MP4 (already compressed), huge savings for text/code (40-70% smaller).
- Cloud sync — use selective sync on Dropbox/Drive to avoid downloading everything locally.
- RAW photos — only keep if you edit. Convert to JPEG for archival to save 90% space.
How to Use the Tool (Step by Step)
- 1
Enter the File Size
Type the number (e.g., 500) into the converter input field.
- 2
Select Source Unit
Choose the starting unit — bytes, KB, MB, GB, TB, etc.
- 3
Select Target Unit
Pick the unit you want to convert to. The tool handles both binary (1024) and decimal (1000) standards.
- 4
See Result Instantly
The converter shows the result with the chosen precision. Most conversions happen without hitting a button.
- 5
Use in Context
Apply the converted value to your use case — email size limit check, cloud quota planning, or backup sizing.
Frequently Asked Questions
How many megabytes are in a gigabyte?+−
1,000 MB (decimal standard used by hard drive manufacturers) or 1,024 MiB (binary standard used by Windows and RAM specs). Both are commonly seen.
Why does my 500 GB hard drive show only 465 GB in Windows?+−
The manufacturer labeled it 500 GB using decimal (500,000,000,000 bytes). Windows divides by 1,073,741,824 (binary) to display = 465 GiB. No bytes are missing; it's just unit math.
What is the difference between Mbps and MB/s?+−
Mbps (megabits per second) is network speed. MB/s (megabytes per second) is file transfer. 8 bits = 1 byte, so 100 Mbps ÷ 8 = 12.5 MB/s. Download speeds are always lower than your Mbps in bytes.
Is 1024 the correct base for file sizes?+−
It depends on context. Operating systems, RAM, and older software use 1024 (binary). SI standard, hard drive vendors, network speeds, and newer OS like macOS use 1000 (decimal). IEC now recommends using KiB, MiB, GiB for binary.
What is larger, TB or TiB?+−
TiB (tebibyte) = 1.0995 TB. So TiB is ~10% larger than TB. A "1 TB" drive is about 0.91 TiB. This difference is where the mysterious "missing" storage goes.
How many bytes in 1 petabyte?+−
1 PB (decimal) = 10^15 bytes = 1,000,000,000,000,000 bytes. 1 PiB (binary) = 2^50 bytes = 1,125,899,906,842,624 bytes.
Why are my email attachments smaller than my files?+−
Email services compress attachments during transmission (especially images and office documents). The original file size stays the same on disk, but transmission can be 20-40% smaller.
Is KB always written with capital K?+−
By SI convention, KB should be "kB" (lowercase k for kilo). In practice, KB with capital K is universally used for file sizes. The lowercase rule is strictly enforced in scientific contexts only.
Convert File Sizes Instantly — Free
Switch between bytes, KB, MB, GB, TB, and PB with both binary (1024) and decimal (1000) standards. Clean, fast, and always accurate.
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