How much data does VPN compression save?

It depends on the data, and this tool does not measure it. With its assumed ratios and 5% overhead, 500 MB saves between 18.1% (LZ4) and 63.7% (Zstd High). OpenVPN’s documentation notes that most traffic, such as video, is already compressed.

Should I turn on VPN compression?

OpenVPN’s documentation says compression should generally not be used except in unusual cases, calls its compress and comp-lzo options deprecated, and names the VORACLE attack as an example of the side effects of mixing compression with encryption. Check your own VPN’s documentation.

Why is the saving negative with no compression?

The tool adds a 5% tunnel overhead to every profile. With no compression 500 MB sends 525.00 MB, so the saving shows -25.00 MB (-5.0%): more is sent than the original. The 5% is an assumption, close to a Comparitech test that measured 4.05% to 5.23% extra mobile data on a 100 MB download with NordLynx, WireGuard and Lightway (4.76% on average).

Where do the 1.3x, 1.8x, 2.5x and 3.2x ratios come from?

They are assumptions, not measurements. The LZ4 project’s own benchmark on the Silesia corpus lists a factor of 2.101 for LZ4 and 2.108 for LZO 2.09, higher than the 1.3x and 1.8x used here; a different corpus gives different numbers.

Which units does it accept?

B, KB, MB, GB and TB, at 1,024 bytes per step, up to 1,024 TB and down to 1 byte. So 1 MB here is 1,048,576 bytes, not the 1,000,000 bytes of the SI megabyte, and 500 MB is about 4.9% more data than 500 MB on a meter that counts in decimal units. kB, KiB, MiB, GiB and TiB are read as the same 1,024-based sizes. Bit units such as Mb or Mbps are refused with a message, because 1 Mb is 0.125 MB. A comma decimal, a sign and an exponent are refused too.

VPN Data Savings Estimator

Estimate VPN data use with assumed compression ratios for LZ4, LZO and Zstd plus a 5% tunnel overhead. The ratios are scenarios, not measurements. How it works

Original Data Size

Units: B, KB, MB, GB, TB (binary: 1 KB = 1024 B; kB, KiB, MiB, GiB and TiB read the same). At least 1 byte; bytes must be whole.

Compression Profile

Saved, after overhead (negative means more is sent)

-25.00 MB

Change -5.0%

Savings estimate

Original Data

500.00 MB

Compressed (None (Overhead Only))

500.00 MB

Tunnel overhead (assumed 5% of original)+25.00 MB

Sent through the tunnel525.00 MB

Assumed compression ratio1x

Note: Compression ratios are assumed, not measured here. Real savings depend on the data type, the cipher suite and the MTU. The 5% overhead is an assumption applied to the original size, which overstates overhead for compressed traffic. Sizes use binary units (1 KB = 1024 B).

About the VPN Data Savings Estimator

Real VPN compression usually saves little: OpenVPN’s documentation says most data sent over the internet is already compressed, and it advises leaving compression off. This estimator shows what a tunnel would send if the data did compress, using fixed assumed ratios for LZ4 (1.3x), LZO (1.8x) and Zstd (2.5x and 3.2x). At an assumed 2.5x compression ratio, 500 MB becomes 200.00 MB; add a 5% tunnel overhead (25.00 MB) and the tunnel sends 225.00 MB, saving 275.00 MB (+55.0%). For video and other already-compressed data the None row, where the tunnel sends 5% more than it carries, is the closer guide.

500 MB through each profile

Profile	Assumed ratio	Compressed	Sent with overhead	Saved
None (Overhead Only)	1x	500.00 MB	525.00 MB	-25.00 MB (-5.0%)
LZ4 (Fast)	1.3x	384.62 MB	409.62 MB	90.38 MB (+18.1%)
LZO	1.8x	277.78 MB	302.78 MB	197.22 MB (+39.4%)
Zstd (Balanced)	2.5x	200.00 MB	225.00 MB	275.00 MB (+55.0%)
Zstd (High)	3.2x	156.25 MB	181.25 MB	318.75 MB (+63.7%)

The figures the tool shows for a 500 MB input, with 1 MB = 1,048,576 bytes and the 5% overhead (25.00 MB) added to every row.

How to use this tool

Type a data size such as 500 MB or 1.5 GB. A number alone means bytes.
Pick a compression profile.
Read the compressed size, the overhead, the size sent and the saving. Press Copy beside the original or compressed size to copy it.

Frequently asked questions

How much data does VPN compression save?: It depends on the data, and this tool does not measure it. With its assumed ratios and 5% overhead, 500 MB saves between 18.1% (LZ4) and 63.7% (Zstd High). OpenVPN’s documentation notes that most traffic, such as video, is already compressed.
Should I turn on VPN compression?: OpenVPN’s documentation says compression should generally not be used except in unusual cases, calls its compress and comp-lzo options deprecated, and names the VORACLE attack as an example of the side effects of mixing compression with encryption. Check your own VPN’s documentation.
Why is the saving negative with no compression?: The tool adds a 5% tunnel overhead to every profile. With no compression 500 MB sends 525.00 MB, so the saving shows -25.00 MB (-5.0%): more is sent than the original. The 5% is an assumption, close to a Comparitech test that measured 4.05% to 5.23% extra mobile data on a 100 MB download with NordLynx, WireGuard and Lightway (4.76% on average).
Where do the 1.3x, 1.8x, 2.5x and 3.2x ratios come from?: They are assumptions, not measurements. The LZ4 project’s own benchmark on the Silesia corpus lists a factor of 2.101 for LZ4 and 2.108 for LZO 2.09, higher than the 1.3x and 1.8x used here; a different corpus gives different numbers.
Which units does it accept?: B, KB, MB, GB and TB, at 1,024 bytes per step, up to 1,024 TB and down to 1 byte. So 1 MB here is 1,048,576 bytes, not the 1,000,000 bytes of the SI megabyte, and 500 MB is about 4.9% more data than 500 MB on a meter that counts in decimal units. kB, KiB, MiB, GiB and TiB are read as the same 1,024-based sizes. Bit units such as Mb or Mbps are refused with a message, because 1 Mb is 0.125 MB. A comma decimal, a sign and an exponent are refused too.

How much a compression ratio matters against tunnel overhead

Enter a data size such as 500 MB or 1.5 GB and pick a profile to see what a tunnel might send if its VPN supports compression. The profiles are scenarios: none, LZ4 at 1.3x, LZO at 1.8x, Zstd at 2.5x and Zstd at 3.2x. Use them to see how far a ratio has to rise before it beats a fixed 5% overhead, for a text-heavy sync job for example, not to predict a mobile data bill. Units are B, KB, MB, GB and TB at 1024 bytes each, and kB, KiB, MiB, GiB and TiB are read the same way. A size under 1 byte is refused, and bit units such as Mb are refused, since 1 Mb is 0.125 MB.

The arithmetic

sent = round(original ÷ ratio) + round(0.05 × original) and saved = original − sent. The 5% tunnel overhead is an assumption close to one published test: Comparitech downloaded a 100 MB file over mobile data and measured 4.05% more with NordLynx, 5.01% with WireGuard (Surfshark) and 5.23% with Lightway (ExpressVPN), 4.76% on average. Other protocols and settings differ. The overhead is applied to the original size, which overstates the overhead on compressed traffic, so the saving shown is the cautious one.

Example: 500 MB at 2.5x is 200.00 MB compressed plus 25.00 MB overhead, so 225.00 MB is sent and 275.00 MB (+55.0%) is saved. With no compression, 525.00 MB is sent and the result is -25.00 MB (-5.0%): negative means more is sent than the original.

The ratios do not come from a measurement. The LZ4 project's own benchmark on the Silesia corpus lists a factor of 2.101 for LZ4, against the 1.3x assumed here, and OpenVPN's documentation notes that most traffic, such as video, is already compressed.

Fun Fact: OpenVPN's documentation calls its compression options deprecated and names the VORACLE attack as an example of the side effects of mixing compression with encryption.

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