Savings achieved or reduction needed?
The term compression ratio usually covers two different questions. Perhaps you already have a reduced file and want to know how much was actually saved. Or perhaps you only know the original size and a limit, for example from an upload form, and want to know how much the file would need to shrink. The calculator has a separate mode for each case.
- Savings achieved: You enter the original size and the measured result size. You get the bytes saved, the savings as a percentage, the remaining size as a percentage and the ratio of original to result. A target value is optional in this mode.
- Reduction needed: You enter only the original size and the limit. The fields for a result are hidden, because you don't need a compressed file for this. You get the required reduction as a percentage and the number of bytes that would need to be removed. In this mode the target value is mandatory.
Entering units and numbers correctly
Each field has its own unit selector. This means you can enter the values in whatever units you have them, for example the original in MB and the result in MiB. Before comparing them, the calculator converts both values into bytes. This matters, because 1 MB is 1,000,000 bytes, whereas 1 MiB is 1,048,576 bytes. If you simply divide the numbers without taking the units into account, you get the wrong answer. 10 MB compared with 4 MiB does not give 60 % savings but about 58.06 %, and the ratio is roughly 2.38:1. The reason is that 4 MiB is 4,194,304 bytes. The calculator for comparing file size units shows you how the units relate to each other.
All sizes must be positive and must come to a whole number of bytes after conversion. The calculator does not accept a result size of zero. Use a period as the decimal separator; a comma works only as a thousands separator. That’s why the calculator reads “1.000” as 1 and not as a thousand. For thousands, write “1,000” or “1,000.00”. Spaces and decimal commas are not accepted, so “1 000” and “1.000,00” are rejected. Values in exponential notation such as 1e6 are not accepted.
Telling savings, remaining size and ratio apart
Several metrics can be derived from the same two sizes. Each one answers a different question. What matters, therefore, is what a percentage refers to and in which direction the calculation runs.
- Savings in percent = (original − result) ÷ original × 100. It shows what share of the original has been removed.
- Remaining size in percent = result ÷ original × 100. It shows how much of the original is left. Savings and remaining size always add up to 100 %.
- Ratio = original ÷ result, written as x:1. It shows how many times larger the original is than the result.
In the first example, 6 MB is removed, i.e. 6,000,000 bytes. Relative to the 10 MB of the original, that is 60 % savings. The remaining 4 MB corresponds to a remaining size of 40 %. The ratio 10 ÷ 4 = 2.5 is written as 2.5:1. The displayed values are rounded, but the calculation uses the exact byte counts.
What does “60 percent compression” mean?
On its own, this figure is ambiguous. Some people mean that 60 % was saved. Others mean that the file is still 60 % of its original size. Yet there is a clear difference between 10 MB → 4 MB and 10 MB → 6 MB. That's why the calculator shows savings and remaining size separately. If someone gives you only a compression ratio, it's best to ask what they mean or to check the calculation yourself using the two sizes. The ratio has the same pitfall: some programs divide the result by the original and show 0.4 instead of 2.5. Both numbers describe the same case, just from the opposite direction.
Ratio below 1:1 or exactly 1:1
If the result is larger than the original, the ratio is below 1:1. That is not a calculation error. It means the file has grown. For 4 MB → 5 MB, the ratio is 0.8:1. The file has become 1,000,000 bytes larger, which is a 25 % increase relative to the original. The calculator explicitly labels such a case as an increase. Not every file can be reduced, and saving a file again can even make it larger.
If the size stays the same, as with 10 MB → 10 MB, the result is 1:1, 0 % savings and 100 % remaining size. That only means the number of bytes hasn't changed. The two numbers alone can't tell you why.
Planning backwards for a size limit
If a form or a mailbox sets a size limit, it helps to calculate in the opposite direction: how much would need to be removed for the file to fit? You calculate the required reduction like this: (1 − limit ÷ original) × 100. If the original is already at or below the limit, the result is 0 %, because then nothing needs to be removed.
80 % sounds like a clear target, but it is pure mathematics. Whether a particular PDF actually shrinks that much depends on what it contains, for example photos, scans or mostly text, and on which method is used. Neither a suitable resolution nor a quality level can be derived from the percentage. Above all, it shows you how ambitious the target is.
If you already have a result, you can also enter the limit in the savings achieved mode. For example: 10 MB original, 4 MB result and 2 MB limit. You then see 60 % savings achieved alongside 80 % required reduction. From this you can tell that the file is still 2 MB over the limit.
If the limit just says “10 MB”, it isn't always clear whether 10,000,000 or 10,485,760 bytes is meant. If you don't know for sure, it's safer to plan with the smaller value. To try out the target on the actual document, you can reduce the PDF to fit a size limit.
What the compression ratio does not assess
Exactly two numbers go into the calculation. A high ratio therefore says nothing about whether the reduced PDF is still easy to read. Nor does it reveal whether text can still be selected and searched, whether form fields and signatures have been preserved, or whether the recipient can open the file without problems. It also doesn't predict how much the next document will shrink. Different content or a different method can lead to a completely different result.
Before you send a compressed file, therefore, check the result yourself:
- Open the compressed file and check at normal zoom whether text, numbers and images are clearly legible.
- Test whether you can select and search text, if that matters to the recipient.
- Check whether form fields as well as handwritten or scanned signatures are still complete and clearly visible.
- If the PDF contains digital signatures, check their validation status in a suitable PDF viewer. The fact that a signature is visible on the page does not by itself tell you whether signature validation succeeds.
- Compare the actual file size with the recipient's limit.
For the actual reduction, you can use the Compress PDF feature. You can then use Check upload limits to find out whether a finished file meets the requirements of a portal or mailbox.