Choose a version:
40% The original file has 148821 bytes (145.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 59733 bytes (58.3k, 40%).

After GZIP compression these minified files vary in size:
unpkg
  22468 bytes (21.9k)
CDN
gzip -6 (default)
  15953 bytes (15.6k)
local copy
jsdelivr
  15924 bytes (15.6k)
CDN
cdnjs
  15885 bytes (15.5k)
CDN
gzip -9
  15875 bytes (15.5k)
local copy
libdeflate -12
  15353 bytes (15.0k)
local copy
7zip -mx=9 -tgzip
  15352 bytes (15.0k)
local copy
zultra
  15328 bytes (15.0k)
local copy
kzip -s0 -rn -b2
  15304 bytes (14.9k)
local copy
pigz -11 -n
  15300 bytes (14.9k)
local copy
Zopfli
  15292 bytes (14.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/bootstrap/bootstrap-5.1.1.min.js (or via HTTPS)

You will automatically get the smallest Bootstrap 5.1.1 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 593 bytes by using my Bootstrap 5.1.1 Zopfli version instead of the best available CDN (3.88% smaller than cdnjs, 15292 vs. 15885 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 --mls4096 --bsr17 --lazy --ohh

(found September 14, 2021)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 4096  --mls4096
block splitting recursion 17  --bsr17
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/v5.1.1/dist/js/bootstrap.min.js --location | md5sum
53f7f752152ec85a9b53e66e807930aa  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-5.1.1.min.zopfli.js.gz | md5sum
53f7f752152ec85a9b53e66e807930aa  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v5.1.1/dist/js/bootstrap.min.js --location | sha1sum
530ca54dfdb4d3e7f496fd782d87aaf0f1696946  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-5.1.1.min.zopfli.js.gz | sha1sum
530ca54dfdb4d3e7f496fd782d87aaf0f1696946  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
unpkg 22468 bytes 53f7f752152ec85a9b53e66e807930aa (invalid)
jsdelivr 15924 bytes 53f7f752152ec85a9b53e66e807930aa September 13, 2021 @ 14:08
cdnjs 15885 bytes 53f7f752152ec85a9b53e66e807930aa (invalid)

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

Other Versions

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

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
15292 bytes -1 byte zopfli --i1000000 --mls4096 --bsr17 --lazy --ohh September 14, 2021 @ 21:17
15293 bytes -3 bytes zopfli --i1000 --mls128 --bsr19 --lazy --ohh September 14, 2021 @ 10:54
15296 bytes -1 byte zopfli --i1000 --mls128 --bsr8 --lazy --ohh September 14, 2021 @ 10:54
15297 bytes zopfli --i100 --mls512 --bsr16 --lazy --ohh September 13, 2021 @ 16:38

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

Most recent activity on September 15, 2021 @ 04:20.

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 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
15309 15309 15310 15309 15310 15310 15311 15310 15303 15307 15306 15306 15307 15305 15310
15299 15300 15300 15304 15300 15302 15304 15303 15299 15295 15294 15296 15299 15302 15300
15302 15302 15303 15302 15301 15300 15303 15302 15299 15295 15294 15296 15302 15301 15301
15298 15297 15296 15301 15300 15301 15294 15302 15296 15296 15294 15294 15300 15302 15303
15299 15294 15299 15301 15299 15302 15295 15302 15303 15295 15294 15296 15298 15297 15298
15301 15301 15299 15301 15299 15300 15295 15302 15294 15295 15293 15296 15296 15301 15304
15302 15301 15301 15301 15301 15299 15300 15303 15295 15294 15293 15295 15303 15300 15299
15298 15301 15297 15298 15299 15300 15299 15301 15295 15295 15293 15296 15302 15299 15298
15303 15303 15301 15303 15303 15300 15295 15302 15300 15295 15294 15296 15297 15299 15303
15301 15300 15301 15301 15299 15300 15295 15294 15299 15295 15295 15295 15301 15299 15316
15299 15300 15299 15294 15300 15300 15295 15294 15297 15295 15294 15294 15301 15301 15300
15301 15300 15299 15301 15301 15302 15303 15302 15295 15295 15294 15296 15299 15296 15299
15299 15300 15300 15301 15301 15300 15293 15294 15292 15294 15294 15295 15297 15298 15299
15298 15307 15296 15298 15300 15300 15301 15294 15295 15294 15294 15292 15301 15301 15301
15298 15295 15298 15301 15301 15299 15294 15294 15297 15294 15294 15294 15297 15301 15301
15301 15303 15301 15301 15299 15300 15292 15302 15294 15295 15294 15296 15301 15299 15300
15298 15301 15297 15298 15300 15301 15294 15294 15294 15294 15294 15295 15297 15300 15300
15300 15301 15298 15302 15300 15301 15295 15300 15299 15294 15293 15294 15297 15300 15300
15300 15301 15299 15303 15299 15300 15301 15295 15295 15295 15293 15295 15297 15300 15300
15301 15300 15300 15303 15299 15301 15295 15294 15299 15294 15294 15296 15297 15301 15301
15300 15300 15299 15301 15301 15300 15295 15303 15296 15295 15294 15296 15300 15300 15303
15298 15303 15299 15294 15299 15302 15295 15294 15299 15295 15294 15294 15298 15297 15299
15299 15301 15300 15296 15298 15301 15303 15301 15295 15294 15294 15296 15296 15299 15299

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 15297 bytes 100%
1,000 15293 bytes -4 bytes 100%
10,000 15293 bytes 100%
100,000 15293 bytes 2.61%
1,000,000 15292 bytes -1 byte 2.61%
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
15390 bytes +98 bytes (+0.64%) +86 bytes
15392 bytes +100 bytes (+0.65%) +88 bytes
15304 bytes +12 bytes (+0.08%)
15333 bytes +41 bytes (+0.27%) +29 bytes
15366 bytes +74 bytes (+0.48%) +62 bytes
15374 bytes +82 bytes (+0.54%) +70 bytes
15409 bytes +117 bytes (+0.77%) +105 bytes
15454 bytes +162 bytes (+1.06%) +150 bytes
15472 bytes +180 bytes (+1.18%) +168 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 12857 bytes -2435 bytes (-15.92%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 13234 bytes -2058 bytes (-13.46%)
Brotli (Wikipedia) brotli brotli -q 11 14216 bytes -1076 bytes (-7.04%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14917 bytes -375 bytes (-2.45%)
LZMA2 (Wikipedia) xz xz -9 15068 bytes -224 bytes (-1.46%)
Zstandard (Wikipedia) zstd zstd -19 15188 bytes -104 bytes (-0.68%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15415 bytes +123 bytes (+0.80%)

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.