Choose a version:
41% The original file has 148253 bytes (144.8k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 60091 bytes (58.7k, 41%).

After GZIP compression these minified files vary in size:
unpkg
  22470 bytes (21.9k)
CDN
gzip -6 (default)
  15960 bytes (15.6k)
local copy
jsdelivr
  15927 bytes (15.6k)
CDN
gzip -9
  15880 bytes (15.5k)
local copy
cdnjs
  15867 bytes (15.5k)
CDN
libdeflate -12
  15347 bytes (15.0k)
local copy
7zip -mx=9 -tgzip
  15335 bytes (15.0k)
local copy
zultra
  15303 bytes (14.9k)
local copy
kzip -s0 -rn -b2
  15298 bytes (14.9k)
local copy
pigz -11 -n
  15293 bytes (14.9k)
local copy
Zopfli
  15284 bytes (14.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/bootstrap/bootstrap-5.0.1.min.js (or via HTTPS)

You will automatically get the smallest Bootstrap 5.0.1 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 583 bytes by using my Bootstrap 5.0.1 Zopfli version instead of the best available CDN (3.81% smaller than cdnjs, 15284 vs. 15867 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 --i100000 --mb8 --mls1024 --bsr10 --lazy --ohh

(found June 9, 2021)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 1024  --mls1024
block splitting recursion 10  --bsr10
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/v5.0.1/dist/js/bootstrap.min.js --location | md5sum
b5730588db13e71c65bdb1d234089260  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-5.0.1.min.zopfli.js.gz | md5sum
b5730588db13e71c65bdb1d234089260  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v5.0.1/dist/js/bootstrap.min.js --location | sha1sum
282209ef6065e8451a5623c1b208d256d7b14c27  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-5.0.1.min.zopfli.js.gz | sha1sum
282209ef6065e8451a5623c1b208d256d7b14c27  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
unpkg 22470 bytes b5730588db13e71c65bdb1d234089260 (invalid)
jsdelivr 15927 bytes b5730588db13e71c65bdb1d234089260 June 8, 2021 @ 10:03
cdnjs 15867 bytes b5730588db13e71c65bdb1d234089260 (invalid)

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

Other Versions

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

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
15284 bytes -1 byte zopfli --i100000 --mls1024 --bsr10 --lazy --ohh June 9, 2021 @ 12:25
15285 bytes -1 byte zopfli --i10000 --mls2048 --bsr9 --lazy --ohh June 8, 2021 @ 12:03
15286 bytes -1 byte zopfli --i1000 --mls8 --bsr6 --lazy --ohh June 8, 2021 @ 10:49
15287 bytes -1 byte zopfli --i1000 --mls2 --bsr40 --lazy --ohh June 8, 2021 @ 10:32
15288 bytes zopfli --i100 --mls2 --bsr8 --lazy --ohh June 8, 2021 @ 10:07

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

Most recent activity on June 9, 2021 @ 17:12.

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 100,000.
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
15290 15288 15287 15289 15291 15291 15290 15291 15290 15294 15292 15290 15290 15293 15292
15292 15294 15292 15292 15289 15286 15287 15287 15288 15288 15287 15287 15288 15288 15285
15286 15288 15286 15287 15287 15285 15287 15287 15287 15285 15288 15288 15288 15287 15287
15286 15287 15287 15287 15287 15286 15286 15287 15290 15285 15288 15287 15288 15287 15287
15286 15288 15287 15289 15287 15286 15287 15287 15287 15284 15288 15287 15289 15287 15284
15287 15287 15287 15287 15287 15287 15287 15287 15286 15285 15284 15287 15288 15287 15285
15287 15288 15287 15286 15286 15287 15286 15286 15286 15284 15288 15287 15288 15287 15286
15293 15287 15293 15287 15287 15286 15287 15287 15286 15288 15288 15287 15287 15285 15284
15293 15287 15292 15286 15287 15286 15286 15287 15287 15287 15287 15287 15288 15287 15299
15288 15287 15287 15286 15287 15286 15287 15287 15286 15284 15288 15287 15288 15284 15284
15292 15287 15292 15287 15287 15286 15287 15287 15286 15285 15287 15287 15286 15287 15287
15286 15286 15287 15287 15287 15286 15287 15287 15289 15285 15288 15287 15284 15287 15300
15286 15287 15287 15287 15286 15285 15286 15287 15286 15287 15288 15287 15288 15285 15285
15287 15287 15288 15286 15286 15287 15286 15287 15287 15285 15288 15287 15286 15285 15287
15286 15287 15287 15286 15286 15286 15287 15287 15287 15288 15288 15287 15289 15285 15284
15288 15287 15287 15287 15287 15286 15291 15287 15287 15287 15287 15287 15288 15287 15286
15287 15287 15287 15287 15287 15288 15288 15287 15286 15287 15288 15291 15287 15287 15286
15286 15286 15287 15286 15286 15286 15287 15287 15287 15285 15288 15292 15286 15287 15287
15288 15288 15287 15286 15286 15287 15286 15287 15287 15285 15287 15287 15286 15285 15284
15286 15287 15288 15286 15287 15286 15287 15285 15285 15285 15288 15285 15286 15285 15286
15287 15287 15287 15289 15287 15286 15285 15287 15287 15285 15287 15287 15286 15284 15285
15286 15287 15287 15288 15287 15286 15287 15287 15286 15284 15288 15287 15286 15285 15299
15286 15287 15288 15286 15286 15286 15287 15287 15287 15287 15288 15290 15286 15287 15286

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 15288 bytes 100%
1,000 15286 bytes -2 bytes 100%
10,000 15285 bytes -1 byte 100%
100,000 15284 bytes -1 byte 8.41%
1,000,000
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
15383 bytes +99 bytes (+0.65%) +85 bytes
15388 bytes +104 bytes (+0.68%) +90 bytes
15298 bytes +14 bytes (+0.09%)
15311 bytes +27 bytes (+0.18%) +13 bytes
15333 bytes +49 bytes (+0.32%) +35 bytes
15341 bytes +57 bytes (+0.37%) +43 bytes
15376 bytes +92 bytes (+0.60%) +78 bytes
15407 bytes +123 bytes (+0.80%) +109 bytes
15440 bytes +156 bytes (+1.02%) +142 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 12796 bytes -2488 bytes (-16.28%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 13169 bytes -2115 bytes (-13.84%)
Brotli (Wikipedia) brotli brotli -q 11 14209 bytes -1075 bytes (-7.03%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14909 bytes -375 bytes (-2.45%)
LZMA2 (Wikipedia) xz xz -9 15052 bytes -232 bytes (-1.52%)
Zstandard (Wikipedia) zstd zstd -19 15165 bytes -119 bytes (-0.78%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15411 bytes +127 bytes (+0.83%)

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.