Choose a version:
40% The original file has 148168 bytes (144.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 59511 bytes (58.1k, 40%).

After GZIP compression these minified files vary in size:
unpkg
  22327 bytes (21.8k)
CDN
gzip -6 (default)
  15895 bytes (15.5k)
local copy
jsdelivr
  15864 bytes (15.5k)
CDN
cdnjs
  15818 bytes (15.4k)
CDN
gzip -9
  15813 bytes (15.4k)
local copy
libdeflate -12
  15312 bytes (15.0k)
local copy
7zip -mx=9 -tgzip
  15296 bytes (14.9k)
local copy
zultra
  15269 bytes (14.9k)
local copy
pigz -11 -n
  15243 bytes (14.9k)
local copy
kzip -s0 -rn -b2
  15241 bytes (14.9k)
local copy
Zopfli
  15232 bytes (14.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/bootstrap/bootstrap-5.1.0.min.js (or via HTTPS)

You will automatically get the smallest Bootstrap 5.1.0 file, ETag caching is available and
if your browser doesn't support GZIP decompression then the uncompressed version will be sent.

Currently best Zopfli settings

Save 586 bytes by using my Bootstrap 5.1.0 Zopfli version instead of the best available CDN (3.85% smaller than cdnjs, 15232 vs. 15818 bytes):
You can use my super-compressed files for whatever purpose you like as long as you respect the library's original license agreement.
There are no restrictions from my side - but please avoid hot-linking if you run a high-traffic website.

These command-line settings yielded the best compression ratio so far (Linux version of zopfli-krzymod):
zopfli --i1000000 --mb8 --mls512 --bsr30 --lazy --ohh

(found August 5, 2021)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 512  --mls512
block splitting recursion 30  --bsr30
lazy matching in LZ77 yes  --lazy
optimized Huffman headers yes  --ohh
initial random W for iterations 1  --rw1
initial random Z for iterations 2  --rz2

Verify file integrity

After decompression, my uncompressed files are identical to the original ones:

MD5:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v5.1.0/dist/js/bootstrap.min.js --location | md5sum
7ffeb319739e6e218958e4cc897c25c4  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-5.1.0.min.zopfli.js.gz | md5sum
7ffeb319739e6e218958e4cc897c25c4  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v5.1.0/dist/js/bootstrap.min.js --location | sha1sum
f727245043560827ca85231e7524a7460d364f47  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-5.1.0.min.zopfli.js.gz | sha1sum
f727245043560827ca85231e7524a7460d364f47  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
unpkg 22327 bytes 7ffeb319739e6e218958e4cc897c25c4 (invalid)
jsdelivr 15864 bytes 7ffeb319739e6e218958e4cc897c25c4 August 5, 2021 @ 14:27
cdnjs 15818 bytes 7ffeb319739e6e218958e4cc897c25c4 (invalid)

Note: only the MD5 hashes are shown to keep things simple.

Other Versions

Available Bootstrap versions at minime.stephan-brumme.com:

5.1.3, 5.1.2, 5.1.1, 5.1.0,
5.0.2, 5.0.1, 5.0.0,
4.6.0,
4.5.3, 4.5.2, 4.5.1, 4.5.0,
4.4.1, 4.4.0,
4.3.1, 4.3.0,
4.2.1, 4.2.0,
4.1.3, 4.1.2, 4.1.1, 4.1.0, 4.0.0,
3.4.1, 3.4.0,
3.3.7, 3.3.6, 3.3.5, 3.3.4, 3.3.2, 3.3.1, 3.3.0,
3.2.0,
3.1.1, 3.1.0,
3.0.3, 3.0.2, 3.0.1, 3.0.0

The project site contains an overview how well these versions were compressed.
Other interesting projects are AngularJS, BackboneJS, D3, Dojo, Ember, jQuery, Knockout, lodash, React, Socket.IO, ThreeJS, UnderscoreJS and Vue.

Changelog

Best Zopfli parameters so far:
Size Improvement Parameters Found
15232 bytes -1 byte zopfli --i1000000 --mls512 --bsr30 --lazy --ohh August 5, 2021 @ 22:23
15233 bytes -1 byte zopfli --i100000 --mls4096 --bsr40 --lazy --ohh August 5, 2021 @ 17:05
15234 bytes -1 byte zopfli --i1000 --mls128 --bsr19 --lazy --ohh August 5, 2021 @ 14:38
15235 bytes -3 bytes zopfli --i1000 --mls4096 --bsr17 --lazy --ohh August 5, 2021 @ 14:35
15238 bytes zopfli --i100 --mls1024 --bsr18 --lazy --ohh August 5, 2021 @ 14:31

If there are multiple parameter sets yielding the same compressed size, only the first one found is shown.

Most recent activity on August 6, 2021 @ 12:21.

Heatmaps

This Zopfli heatmap visualizes how compression changes when modifying the --bsr and --mls parameter.
Cell's contents is the best filesize achieved (in bytes, hover with mouse over cells to see number of iterations).

Good parameters are green, bad are red. The best and worst are bold as well.
The brightness of the blue background color indicates how many iterations were processed:
10,000, 100,000 or 1,000,000.
bsr \ mls
2 4 8 16 32 64 128 256 512 1024 2048 4096 8192 16384 32768
bsr \ mls
2 4 8 16 32 64 128 256 512 1024 2048 4096 8192 16384 32768
15248 15248 15249 15249 15248 15249 15249 15249 15248 15247 15246 15248 15248 15246 15247
15241 15241 15240 15242 15238 15246 15246 15242 15240 15241 15243 15235 15243 15243 15244
15242 15241 15236 15240 15241 15241 15234 15236 15236 15236 15237 15235 15242 15244 15244
15243 15240 15240 15240 15240 15241 15235 15238 15236 15237 15237 15236 15244 15242 15243
15243 15242 15234 15240 15240 15234 15244 15236 15233 15237 15237 15235 15244 15243 15243
15240 15241 15240 15241 15241 15234 15235 15236 15237 15237 15237 15236 15243 15243 15242
15242 15241 15239 15241 15241 15241 15241 15242 15232 15237 15237 15235 15241 15243 15244
15240 15240 15240 15237 15241 15235 15245 15234 15235 15236 15237 15234 15244 15242 15244
15237 15241 15240 15241 15241 15241 15235 15236 15234 15238 15237 15236 15243 15240 15243
15242 15241 15241 15241 15240 15241 15241 15234 15234 15236 15237 15235 15243 15241 15243
15239 15241 15246 15239 15241 15234 15235 15236 15234 15237 15237 15233 15243 15242 15243
15243 15243 15241 15241 15240 15234 15235 15236 15234 15237 15237 15236 15243 15243 15243
15243 15240 15246 15241 15240 15234 15233 15242 15236 15236 15237 15236 15244 15242 15243
15240 15241 15233 15240 15241 15235 15234 15234 15234 15237 15234 15234 15244 15243 15238
15243 15241 15239 15241 15241 15240 15234 15234 15234 15236 15237 15236 15243 15243 15244
15239 15239 15234 15240 15242 15241 15234 15240 15242 15237 15237 15235 15244 15242 15243
15241 15243 15239 15241 15237 15235 15234 15234 15235 15237 15237 15236 15243 15242 15244
15243 15241 15242 15234 15240 15234 15242 15236 15234 15237 15237 15236 15243 15243 15243
15240 15240 15240 15241 15241 15241 15241 15234 15235 15237 15237 15236 15243 15242 15244
15240 15241 15234 15240 15242 15234 15235 15236 15234 15236 15237 15236 15243 15243 15243
15243 15240 15239 15241 15240 15241 15234 15234 15234 15236 15237 15236 15243 15242 15243
15241 15239 15241 15237 15240 15234 15235 15236 15232 15237 15237 15236 15243 15241 15243
15242 15243 15243 15241 15240 15241 15234 15234 15234 15236 15236 15233 15244 15243 15244

Due to the Monte Carlo design of my search algorithm, not all parameters have reached the same number of iterations yet:
Iterations Min. Bytes Reduction Coverage
100 15238 bytes 100%
1,000 15234 bytes -4 bytes 100%
10,000 15234 bytes 100%
100,000 15233 bytes -1 byte 19.13%
1,000,000 15232 bytes -1 byte 2.03%
10,000,000

KZIP has far less options available for tuning/optimization. I only played around with the number of blocks (parameter -n):
Blocks Min. Bytes Compared To Best Zopfli Compared To Best KZIP
15325 bytes +93 bytes (+0.61%) +84 bytes
15325 bytes +93 bytes (+0.61%) +84 bytes
15241 bytes +9 bytes (+0.06%)
15269 bytes +37 bytes (+0.24%) +28 bytes
15301 bytes +69 bytes (+0.45%) +60 bytes
15311 bytes +79 bytes (+0.52%) +70 bytes
15344 bytes +112 bytes (+0.74%) +103 bytes
15391 bytes +159 bytes (+1.04%) +150 bytes
15408 bytes +176 bytes (+1.16%) +167 bytes

Non-DEFLATE Algorithms

Archivers based on completely different compression algorithms often produce superior results.
Unfortunately, browsers only support gzip compression at the moment.
However, support for Brotli is constantly growing - for example, your browser actually supports it !
Algorithm Program Parameters Size Compared To Best Zopfli
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 12807 bytes -2425 bytes (-15.92%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 13192 bytes -2040 bytes (-13.39%)
Brotli (Wikipedia) brotli brotli -q 11 14153 bytes -1079 bytes (-7.08%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14868 bytes -364 bytes (-2.39%)
LZMA2 (Wikipedia) xz xz -9 15016 bytes -216 bytes (-1.42%)
Zstandard (Wikipedia) zstd zstd -19 15129 bytes -103 bytes (-0.68%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15391 bytes +159 bytes (+1.04%)

Detailled Analysis

I wrote a DEFLATE decoder in Javascript. Click the button below to start a client-side analysis of the smallest gzipped files (may take a second):


Notes: pigz is a fast open source multi-threaded implementation of gzip written by one of the original authors of gzip.
However, when using compression level 11, pigz actually switches to the slower Zopfli algorithm and isn't multi-threaded anymore.
KrzyMOD's extensions to Zopfli offer the highest level of configuration and is therefore used for my brute-force search.
Ken Silverman wrote the closed-source KZIP compression program and Jonathon Fowler ported it to Linux.
Defluff was created by Joachim Henke; DeflOpt is a tool by Ben Jos Walbeehm.

website made by Stephan Brumme in 2015 and still improving in 2021.
all timestamps are displayed in central european time. see my changelog.
no flash, not even images or external css files - and everything squeezed into a single html file.
which was handsomely compressed before releasing it into the wild internet - obviously.

please visit my homepage and my blog, too.
email: minime (at) stephan-brumme.com

All trademarks are property of their respective owners. You know, the boring legal stuff.