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Arithmetic
Compression savings calculator
Two numbers in, and what they actually mean out: the ratio, the percentage, and what it adds up to across a month of traffic.
Percentage saved is the number people mean. Ratio is the number they quote.
Result
Ratio and percentage are not the same claim
A 10:1 compression ratio sounds twice as impressive as 90% saved, and they are the same thing. Ratio is original divided by compressed; percentage saved is how much of the original went away. A 2:1 ratio is 50% saved. A 20:1 ratio is 95%. The gap between those two ways of speaking is why compression marketing is confusing.
Notice how quickly the returns flatten. Going from 90% to 95% saved halves the file again, but going from 95% to 97.5% halves it once more for a number that barely moves. Past a point you are destroying the image for a rounding error, which is exactly where most over-compressed uploads end up.
The bandwidth figures are the argument you actually need when someone asks why image sizes are worth an afternoon. Multiply by your traffic and the saving usually stops being abstract.
Common questions
Is a higher ratio always better?
Only up to the point where the image still looks right. A 40:1 ratio on a photograph means visible blocking. Judge by eye, not by the number.
Does this account for caching?
No, so treat the bandwidth figure as an upper bound. With sensible cache headers, repeat visitors download nothing, and the real saving is smaller but the first-visit speed gain is exactly as calculated.
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