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Zstandard supports a wide range of compression levels, from ultra-fast modes to maximum compression, allowing you to fine-tune the trade-off between speed and compression ratio.

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Standard Compression Levels (1-19)

The library supports regular compression levels from 1 up to 22. Lower levels provide faster compression at the cost of compression ratio, while higher levels achieve better compression ratios at the cost of speed.

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Default Compression Level

The default compression level is 3 (ZSTD_CLEVEL_DEFAULT). 
#ifndef ZSTD_CLEVEL_DEFAULT
#  define ZSTD_CLEVEL_DEFAULT 3
#endif
You can also query the default level at runtime: 
int defaultLevel = ZSTD_defaultCLevel();  // Returns 3

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Setting Compression Levels

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Simple API

size_t const cSize = ZSTD_compress(cBuff, cBuffSize, fBuff, fSize, 1);
From examples/simple_compression.c:28

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Advanced API

ZSTD_CCtx* const cctx = ZSTD_createCCtx();
ZSTD_CCtx_setParameter(cctx, ZSTD_c_compressionLevel, cLevel);
From examples/streaming_compression.c:43

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Fast Modes (Negative Levels)

Zstandard also offers negative compression levels, which extend the range of speed vs. ratio preferences. These fast modes prioritize speed over compression ratio.

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Using Fast Modes

Fast modes are specified with --fast=# on the command line: 
zstd --fast=1 file.txt
zstd --fast=4 file.txt
The lower the level (more negative), the faster the speed at the cost of compression ratio.

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Performance Characteristics

Benchmark results on a Core i7-9700K CPU @ 4.9GHz using the Silesia compression corpus:
CompressorRatioCompressionDecompression
zstd 1.5.7 -12.896510 MB/s1550 MB/s
zstd 1.5.7 —fast=12.439545 MB/s1850 MB/s
zstd 1.5.7 —fast=42.146665 MB/s2050 MB/s
From README.md:44-49

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Ultra Modes (20-22)

Levels 20-22 are labeled as --ultra modes and require explicit enabling. These levels provide maximum compression but require significantly more memory.
Levels >= 20 should be used with caution, as they require more memory during both compression and decompression.

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Enabling Ultra Modes

On the command line: 
zstd --ultra -20 file.txt
zstd --ultra -22 file.txt
The --ultra flag is required to enable levels beyond 19, up to 22. From programs/README.md:196

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Compression Level Bounds

You can query the valid range of compression levels: 
int minLevel = ZSTD_minCLevel();  // Returns minimum negative level
int maxLevel = ZSTD_maxCLevel();  // Returns 22

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Speed vs Ratio Trade-offs

Zstandard’s compression levels offer configurable speed vs compression trade-offs in small increments. Key characteristics:
  • Decompression speed is preserved and remains roughly the same at all settings
  • Compression speed decreases as the level increases
  • Compression ratio improves as the level increases
For most use cases, levels 1-9 provide an excellent balance. Use levels 10-19 when compression ratio is more important than speed. Reserve ultra modes (20-22) for maximum compression requirements where memory usage is not a constraint.

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Environment Variables

The default compression level can be overridden using the ZSTD_CLEVEL environment variable: 
export ZSTD_CLEVEL=5
tar --zstd -cf archive.tar.zst files/
This is useful when the CLI is invoked in a way that doesn’t allow passing arguments directly. From programs/README.md:264-266

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Compression Strategies

Higher compression levels use more complex strategies: 
typedef enum {
    ZSTD_fast=1,
    ZSTD_dfast=2,
    ZSTD_greedy=3,
    ZSTD_lazy=4,
    ZSTD_lazy2=5,
    ZSTD_btlazy2=6,
    ZSTD_btopt=7,
    ZSTD_btultra=8,
    ZSTD_btultra2=9
} ZSTD_strategy;
From lib/zstd.h:336-347 The higher the value of the selected strategy, the more complex it is, resulting in stronger and slower compression.

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