How to Calculate Compression Ratio

What You'll Need to Calculate Compression Ratio

Before you begin, gather the following:

• The original (uncompressed) file size – in bytes, kilobytes (KB), megabytes (MB), or gigabytes (GB).
• The compressed file size – in the same unit as possible.
• A calculator (or pen and paper for simple arithmetic).
• (Optional) The compression percentage if you want to compute storage or bandwidth savings.

Step-by-Step Instructions

Follow these steps to manually calculate compression ratio and related metrics. For a quick explanation, see What is Data Compression?.

1. Identify the original size (S_original) and compressed size (S_compressed). For example, a raw image might be 5 MB before compression and 1.5 MB after.
2. Convert both sizes to the same unit. If one is in KB and the other in MB, convert to the smaller unit (or any consistent unit). Example: 5 MB = 5,120 KB; 1.5 MB = 1,536 KB.
3. Divide the original size by the compressed size. The formula is: Compression Ratio = Original Size / Compressed Size. Example: 5 MB ÷ 1.5 MB = 3.33:1 (round to two decimals).
4. (Optional) Calculate the compression percentage. Use Compression % = (1 - S_compressed / S_original) × 100. Example: (1 – 1.5/5) × 100 = 70%.
5. (Optional) Compute storage savings. Subtract compressed from original: Storage Saved = S_original - S_compressed. Example: 5 MB – 1.5 MB = 3.5 MB saved.
6. (Optional) Estimate bandwidth savings per transfer. Multiply the compressed percentage (as a decimal) by the data amount: Data Saved per Transfer = Transfer Size × (Compression % / 100). This helps you gauge internet or cloud costs. For more context, see Compression Ratio Values Explained.

Worked Example 1: File Compression

Scenario: You have a database backup of 200 MB that compresses to 40 MB.

1. Original = 200 MB, Compressed = 40 MB (same unit).
2. Compression Ratio = 200 ÷ 40 = 5:1.
3. Compression Percentage = (1 – 40/200) × 100 = (1 – 0.2) × 100 = 80%.
4. Storage Savings = 200 – 40 = 160 MB saved.
5. If you transfer this backup monthly and pay $0.10 per GB, bandwidth savings = (160 MB / 1024) GB × $0.10 ≈ $0.0156 per transfer.

Worked Example 2: Image Compression

Scenario: A 12 MB photo from your camera is compressed to 2.4 MB for web use.

1. Original = 12 MB, Compressed = 2.4 MB.
2. Compression Ratio = 12 ÷ 2.4 = 5:1 again (though different sizes yield same ratio).
3. Compression Percentage = (1 – 2.4/12) × 100 = (1 – 0.2) × 100 = 80%.
4. Storage Savings = 12 – 2.4 = 9.6 MB saved.
5. If you have 1,000 such photos, total savings = 9.6 MB × 1,000 = 9,600 MB = 9.375 GB.

Common Pitfalls to Avoid

• Unit mismatch: Always convert to the same unit before dividing. Mixing MB and GB will give wildly inaccurate ratios.
• Dividing in the wrong order: Ratio = original ÷ compressed, not the reverse. A 5:1 ratio means original is 5 times larger; dividing the other way yields 0.2:1, which is misinterpreted.
• Confusing ratio with percentage: A 5:1 ratio corresponds to 80% space saved, not 500%.
• Rounding prematurely: Keep at least two decimals during intermediate steps to maintain accuracy. The official formula on our calculator handles precision.
• Forgetting that compression varies by type: Text files often compress 3:1 or 4:1, while JPEG images may only compress 2:1. Learn more in our guide on compression for different media.