https://www.tauday.com/

1<!DOCTYPE html>
2<html class="scrolled" lang="en">
3  <head>
4<title>Tau Day | No, really, pi is wrong: The Tau Manifesto by Michael Hartl  </title>
5<link href="/favicon.png" rel="icon">
6<link href="/apple-touch-icon.png" rel="apple-touch-icon">
7<meta content="IE=edge" http-equiv="X-UA-Compatible" >
8<link rel="stylesheet" href="/css/main.css?8d27b711c953267139a1e380fb0d7b78">

9<script src="https://code.jquery.com/jquery-3.7.1.slim.min.js" integrity="sha256-kmHvs0B+OpCW5GVHUNjv9rOmY0IvSIRcf7zGUDTDQM8=" crossorigin="anonymous"></script>

9
10<meta name="csrf-param" content="authenticity_token">
11<meta name="csrf-token" content="Sk2nitnxXFhOmKUbgtP02X+jtdq9lWjPK39C4vgZu+x7InP+oJPWZ6yTZiJpxq9FS/QjissinINL1K3K6XLuFw==">
12<!--[if lt IE 9]>

13<script src="http://html5shiv.googlecode.com/svn/trunk/html5.js"></script>

13
14<![endif]-->

15<script>
16MathJax = {
17  "HTML-CSS": {
18    availableFonts: ["TeX"],
19  },
20  tex: {
21    inlineMath: [["$", "$"], ["\\(", "\\)"]],
22    extensions: ["AMSmath.js", "AMSsymbols.js", "color.js"],
23    tags: "ams",
24    macros: {
25      sen: ["\\mbox{sen}\\,{#1}",1], // \sin in Spanish
26      splim: ["{\\underset{#1}{\\mathrm{lím}}}", 1], // \lim in Spanish
27      floor: ["{\\left\\lfloor{#1}\\right\\rfloor}", 1], // Floor function
28    },
29    autoload: {
30      upgreek: ["uptau"]
31    }
32  },
33  svg: {
34    fontCache: "global"
35  },
36  showProcessingMessages: false,
37  messageStyle: "none",
38  imageFont: null,
39  "AssistiveMML": { disabled: true }
40};
41</script>

41

42<script src="https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=es6"></script>

42

43<script id="MathJax-script" async src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"></script>

43

44<script async src="https://platform.x.com/widgets.js" charset="utf-8"></script>

44
45
46<meta content="Pi Is Wrong, Tau Manifesto, Tau Day" name="keywords">
47<meta content="The Tau Manifesto is dedicated to one of the most important numbers in mathematics, perhaps the most important: the circle constant relating the circumference of a circle to its linear dimension. For millennia, the circle has been considered the most perfect of shapes, and the circle constant captures the geometry of the circle in a single number." name="description">
48<meta content="Michael Hartl" name="author">
49<meta content="on" name="twitter:widgets:csp">
50<meta content="initial-scale=1.0, maximum-scale=1, width=device-width" name="viewport">
51
52<meta property="og:title" content="No, really, pi is wrong: The Tau Manifesto">
53<meta property="og:description" content="For millennia, the circle has been considered the most perfect of shapes, and the circle constant π captured the geometry of the circle in a single number. But π is wrong, and it’s time to set things right.">
54<meta property="og:site_name" content="Tau Day">
55<meta property="og:image" content="http://softcover-static.s3.amazonaws.com/og_image2.png">
56<meta property="og:image:secure_url" content="https://softcover-static.s3.amazonaws.com/og_image2.png">
57<meta property="og:url" content="https://tauday.com/tau-manifesto">
58<meta property="og:type" content="article">
59<meta property="article:author" content="https://www.facebook.com/mhartl">
60<meta property="fb:admins" content="3501584">
61

62<script src="//use.typekit.net/hox4ibs.js"></script>

62

63<script>try{Typekit.load();}catch(e){}</script>

63
64</head>
65
66  <body>
67    <div id="fb-root"></div>

68<script async defer crossorigin="anonymous" src="https://connect.facebook.net/en_US/sdk.js#xfbml=1&version=v22.0&appId=119843151394139"></script>

68
69
70    <nav class="nav">
71<a class="logo logo_nav" href="/">τ</a>
72<a class="hamburger" onclick="$(&#39;html&#39;).toggleClass(&#39;nav_open&#39;);" href="javascript://">☰
73</a><ul class="nav_list">
74<li>
75<a class="nav_link email" href="/">Home
76</a></li>
77<li>
78<a class="nav_link buyLnk" href="/tau-manifesto">The Tau Manifesto
79</a></li>
80<li>
81<a class="nav_link" href="/state-of-the-tau">State of the
82<i>τ</i>
83</a></li>
84<li><a class="nav_link" href="/a-parable">A Parable</a></li>
85<li>
86<a class="nav_link" href="/propaganda">Forward to Victory!
87</a></li>
88<li>
89<a class="nav_link" href="/a-tau-testimonial">A
90<i>τ</i>-stimonial
91</a></li>
92<li>
93<a class="nav_link" href="/tau-digits">10
94<sup>5</sup>
95Digits of
96<i>τ</i>
97</a></li>
98<li>
99<div class="nav_heading">Languages</div>
100<ul class="nav_sublist">
101<li>
102<a class="nav_link" href="/tau-manifesto">English
103</a></li>
104<li>
105<a class="nav_link" href="/el-manifiesto-tau">Español
106</a></li>
107<li>
108<a class="nav_link" href="/le-manifeste-de-tau">Français
109</a></li>
110<li>
111<a class="nav_link" href="/il-tau-manifesto">Italiano
112</a></li>
113<li>
114<a class="nav_link" href="/o-manifesto-tau">Português
115</a></li>
116<li>
117<a class="nav_link" href="/das-tau-manifest">

117Deutsch
118</a></li>
119<li>
120<a class="nav_link" href="/tau-manifesto-ru">НА РУССКОМ ЯЗЫКЕ
121</a></li>
122<li>
123<a class="nav_link" href="/tau-manifesto-cn">简体中文
124</a></li>
125</ul>
126</li>
127<li>
128<a class="nav_link email" target="_blank" onclick="$(&#39;html&#39;).removeClass(&#39;nav_open&#39;);" href="/signup">Join
129<i>τ</i>
130Email List
131</a></li>
132<li>
133<a class="nav_link buyLnk" onclick="$(&#39;html&#39;).removeClass(&#39;nav_open&#39;);" href="#buy">Ebook / Tees
134</a></li>
135</ul>
136</nav>
137
138    <div class="main_content">
139      <div class="announcement">
140<div class="extraNotice"></div>
141<div class="buyIcon ann-buyicon">
142<a target="_blank" rel="noopener" href="https://tauday.com/print"><div class="iconPrint">
143<img src="/images/buy_print.svg" alt="Buy print edition">
144</div>
145<br>
146<h3>Buy Print Edition</h3>
147<p class="buyIcon-text">
148Buy the print edition of <em>The Tau Manifesto</em> at Amazon!
149</p>
150</a></div>
151<div class="buyIcon ann-buyicon">
152<a target="_blank" rel="noopener" href="https://tauday.com/shirt"><div class="iconTee">
153<img src="/images/buy_tee.png" alt="Buy Tau T-shirt">
154</div>
155<br>
156<h3>Get your own Tau Tee</h3>
157<p class="buyIcon-text">
158Celebrate Tau Day in style with a Tau tee shirt, available only for a limited time from Teespring. Offered in both men’s sizes as well as fitted women’s.
159</p>
160</a></div>
161<div class="shareButtons">
162<div class="share">
163<ul>
164<li class="share_horiz">
165<a href="https://twitter.com/share" class="twitter-share-button" data-url="https://tauday.com/" data-count="horizontal" data-via="tauday" data-text="No, really, pi is wrong: The Tau Manifesto by Michael Hartl #TauDay">𝕏 Post</a>

165<script src="https://platform.twitter.com/widgets.js"></script>

165
166</li>
167<li class="share_horiz">
168  <div class="fb-like"
169     data-href="https://tauday.com"
170     data-width=""
171     data-layout="button_count"
172     data-action="like"
173     data-size="small"
174     data-share="false">
175</div>
176
177</li>
178<li class="share_vert">
179<a href="https://twitter.com/share" class="twitter-share-button" data-url="https://tauday.com/" data-count="vertical" data-via="tauday" data-text="No, really, pi is wrong: The Tau Manifesto by Michael Hartl #TauDay">𝕏 Post</a>

179<script src="https://platform.twitter.com/widgets.js"></script>

179
180</li>
181<li class="share_vert">
182  <div class="fb-like"
183     data-href="https://tauday.com"
184     data-width=""
185     data-layout="button_count"
186     data-action="like"
187     data-size="small"
188     data-share="false">
189</div>
190
191</li>
192</ul>
193</div>
194
195</div>
196</div>
197<div class="notice_bar">Read the
198<strong><a href="/state-of-the-tau-current">State of the Tau 2026</a></strong>
199<span class="notice_separator">&nbsp;|&nbsp;</span>
200<span class="email_list">
201Join the
202<strong><a target="_blank" href="/signup">Tau Email List</a></strong>
203</span>
204</div>
205
206<noscript id="no_math_warning">Please enable JavaScript to render the math properly.</noscript>
207
208      <div class="container page">
209  <header>
210<div class="subtitle">No, really, pi is wrong…</div>
211<h1>
212<a href="/tau-manifesto">
213<span>The Tau Manifesto</span>
214</a>
215</h1>
216<h3 class="author">by Michael Hartl</h3>
217</header>
218<div class="index_text">
219<p>Tau Day is a celebration of the circle constant $\tau = C/r =$ $ 6.283185\ldots$ <span class="intersentencespace"></span> Founded in 2010 with the publication of <a href="/tau-manifesto"><em>The Tau Manifesto</em></a> by <a href="https://www.michaelhartl.com/">Michael Hartl</a>, Tau Day takes place annually on June 28 (6/28 in the American calendar system).</p>
220<p><em>The Tau Manifesto</em>’s introduction of $\tau$ (tau) as the “true circle constant” has had a significant impact on geek culture, including changing the <a href="https://mitadmissions.org/blogs/entry/pi-day-tau-time/">time of day</a> MIT announces its admissions decisions, being featured in the official <a href="https://www.google.com/search?q=tau*1">Google calculator</a>, and being supported by a large number of computer programming languages (including Microsoft .NET, Java, Rust, and Python).</p>
221<p>Including the English-language original, <em>The Tau Manifesto</em> is currently available in eight different languages. <span class="intersentencespace"></span> Its associated holiday, <a href="https://tauday.com/">Tau Day</a>, has been  <a href="https://archive.fo/TrRqz">featured in <em>The Wall Street Journal</em></a> and is celebrated annually on <a href="https://www.google.com/search?q=tau+day">6/28</a> by thousands of math enthusiasts around the world.</p>
222<p>To get all the latest updates on tau, read the <a href="/state-of-the-tau">State of the Tau update</a>, and <a href="/signup" target="_blank">join the official Tau Email List</a>.</p>
223</div>
224</div>
225<ul class="buyIcons">
226<li class="buyIcon">
227<a target="_blank" class="buy_link" href="https://tauday.com/print"><div class="iconEbook">
228<img src="/images/buy_print.svg" alt="Buy print edition">
229</div>
230<div>
231<h3>Buy Print Edition</h3>
232</div>
233<p>Get the print edition of <em>The Tau Manifesto</em> at Amazon!</p>
234</a></li>
235<li class="buyIcon">
236<a target="_blank" class="buy_link" href="https://tauday.com/shirt"><div class="iconTee">
237<img src="/images/buy_tee.png" alt="Buy Tau T-shirt">
238</div>
239<div>
240<h3>Get your own Tau Tee</h3>
241</div>
242<p>Celebrate Tau Day in style with a Tau T-shirt from Teespring!</p>
243</a></li>
244</ul>
245
246<div class="container page" id="homeVideo">
247<h2>
248<a id="tau_talk_short"></a>
249Tau Videos
250</h2>
251<ul>
252<li>
253<div class="youtube-container">
254<iframe width="560" height="315" src="https://www.youtube.com/embed/2hhjsSN-AiU" title="YouTube video player" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>
255</div>
256<a class="front_caption" href="https://youtu.be/2hhjsSN-AiU">Original Tau Talk (short version)</a>
257</li>
258<li>
259<div class="youtube-container">
260<iframe width="560" height="315" src="https://www.youtube.com/embed/H69YH5TnNXI" title="YouTube video player" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>
261</div>
262<a class="front_caption" href="https://youtu.be/H69YH5TnNXI">Original Tau Talk (long version)</a>
263</li>
264<li>
265<div class="youtube-container">
266<iframe width="560" height="315" src="https://www.youtube.com/embed/kMtgV18Iew8" title="YouTube video player" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>
267</div>
268<a class="front_caption" href="https://youtu.be/kMtgV18Iew8">The Tau Manifesto (animated)</a>
269</li>
270<li>
271<div class='youtube-container'>
272<iframe width="560" height="315" src="https://www.youtube.com/embed/PIhNNnW1tUo?si=aATREIBTjg3PSdB-" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture;

272 web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>
273</div>
274<a class="front_caption" href="https://youtu.be/PIhNNnW1tUo">Pi Is (still) Wrong (Vi Hart)</a>
275</li>
276<li>
277<div class="youtube-container">
278<iframe width="560" height="315" src="https://www.youtube.com/embed/k7MuXCOlE6M" title="YouTube video player" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>
279</div>
280<a class="front_caption" href="https://youtu.be/k7MuXCOlE6M">The Tau Manifesto (Talks at Google)</a>
281</li>
282<li>
283<div class="youtube-container">
284<iframe width="560" height="315" src="https://www.youtube.com/embed/3174T-3-59Q" title="YouTube video player" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>
285</div>
286<a class="front_caption" href="https://youtu.be/3174T-3-59Q">What Tau Sounds Like (M. Blake)</a>
287</li>
288<!-- %li -->
289<!-- .youtube-container -->
290<!-- <iframe width="560" height="315" src="https://www.youtube.com/embed/83ofi_L6eAo" title="YouTube video player" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe> -->
291<!-- = link_to "Tau replaces Pi (Numberphile)", "https://youtu.be/83ofi_L6eAo", class: "front_caption" -->
292</ul>
293<div id="indexAuthor">
294<header>
295<img src="https://secure.gravatar.com/avatar/ffda7d145b83c4b118f982401f962ca6?s=400" alt="Michael Hartl">
296<h5>About the author</h5>
297</header>
298<p><a href="https://www.michaelhartl.com/" target="_blank" rel="noopener">Michael Hartl</a> is a physicist and entrepreneur.<span class="intersentencespace"></span> He is the author of over a dozen books, including <a href="https://www.learnenough.com/python" target="_blank" rel="noopener"><em>Learn Enough Python to Be Dangerous</em></a> and the <a href="https://www.railstutorial.org/" target="_blank" rel="noopener"><em>Ruby on Rails Tutorial</em></a>, and was cofounder and principal author at <a href="https://www.learnenough.com" target="_blank" rel="noopener">Learn Enough</a> (<a href="https://news.learnenough.com/big-news-about-learn-enough" target="_blank" rel="noopener">acquired</a> 2022).<span class="intersentencespace"></span> Previously, Michael taught theoretical and computational physics at the <a href="https://www.caltech.edu" target="_blank">California Institute of Technology</a> (Caltech), where he received a <a href="https://www.michaelhartl.com/ascit/awards2000.html" target="_blank" rel="noopener">Lifetime Achievement Award for Excellence in Teaching</a> and served as Caltech’s editor for <a href="https://www.michaelhartl.com/feynman-lectures/" target="_blank" rel="noopener"><em>The Feynman Lectures on Physics</em></a>.<span class="intersentencespace"></span> He is a graduate of <a href="https://college.harvard.edu/" target="_blank" rel="noopener">Harvard College</a>, has a Ph.D. in Physics from <a href="https://www.caltech.edu/" target="_blank" rel="noopener">Caltech</a>, and is an alumnus of the <a href="https://ycombinator.com/" target="_blank" rel="noopener">Y Combinator</a> entrepreneur program.</p>
299<p>Michael is ashamed to admit that he knows <span class="inline_math">\( \pi \)</span> to 50 decimal places—<a href="/tau-manifesto#fig-futurama_video" target="_blank" rel="noopener">approximately 48 more than Matt Groening</a>.<span class="intersentencespace"></span> To atone for this, he has memorized <a href="https://www.wolframalpha.com/input?i=N%5B2+Pi%2C+53%5D" target="_blank" rel="noopener">52 decimal places</a> of <span class="inline_math">\( \tau \)</span>.</p>
300</div>
301
302</div>
303
304      <footer class="footer">
305<a class="logo" href="/">τ</a>
306<ul class="footNav">
307<li>
308<a href="/tau-manifesto"><i>τ</i>
309Manifesto
310</a></li>
311<li>
312<a href="/state-of-the-tau">State of the
313<i>τ</i>
314</a></li>
315<li>
316<a href="/tau-digits">10
317<sup>5</sup>
318Digits of
319<i>τ</i>
320</a></li>
321<li><a href="/a-parable">A Parable</a></li>
322<li>
323<a href="/#tau_talk_short">The
324<i>τ</i>
325Talks
326</a></li>
327</ul>
328<div id="buy"><ul class="buyIcons">
329<li class="buyIcon">
330<a target="_blank" class="buy_link" href="https://tauday.com/print"><div class="iconEbook">
331<img src="/images/buy_print.svg" alt="Buy print edition">
332</div>
333<div>
334<h3>Buy Print Edition</h3>
335</div>
336<p>Get the print edition of <em>The Tau Manifesto</em> at Amazon!</p>
337</a></li>
338<li class="buyIcon">
339<a target="_blank" class="buy_link" href="https://tauday.com/shirt"><div class="iconTee">
340<img src="/images/buy_tee.png" alt="Buy Tau T-shirt">
341</div>
342<div>
343<h3>Get your own Tau Tee</h3>
344</div>
345<p>Celebrate Tau Day in style with a Tau T-shirt from Teespring!</p>
346</a></li>
347</ul>
348</div>
349<div class="page-email-signup"><ul class="buyIcons">
350<li class="buyIcon emailSignup">
351<a target="_blank" class="buy_link" href="/signup"><h3>Sign Up for the Tau Email List&nbsp;»</h3>
352</a></li>
353</ul>
354</div>
355<div class="share">
356<ul>
357<li class="share_horiz">
358<a href="https://twitter.com/share" class="twitter-share-button" data-url="https://tauday.com/" data-count="horizontal" data-via="tauday" data-text="No, really, pi is wrong: The Tau Manifesto by Michael Hartl #TauDay">𝕏 Post</a>
359</li>
360<li class="share_horiz">
361<div class="fb-like" data-href="https://tauday.com" data-layout="button_count" data-action="like" data-show-faces="false" data-share="true"></div>
362</li>
363<li class="share_vert">
364<a href="https://twitter.com/share" class="twitter-share-button" data-url="https://tauday.com/" data-count="vertical" data-via="tauday" data-text="No, really, pi is wrong: The Tau Manifesto by Michael Hartl #TauDay">𝕏 Post</a>
365</li>
366<li class="share_vert">
367<div class="fb-like" data-href="https://tauday.com" data-layout="box_count" data-action="like" data-show-faces="false" data-share="true"></div>
368</li>
369</ul>
370</div>
371</footer>
372
373    </div>
374    

374<script async src="https://platform.x.com/widgets.js"></script>

374
375
376  

376<script type="module" src="https://static.cloudflareinsights.com/beacon.min.js/v31edd6df95cf4e85bb4c19e7a9bdbcba1788362987495" integrity="sha512-iIg7k2xntmwu6/uSb5tpc/hySgZc4eoL31yB29W6tJFo2akwjPWcEqnCEdJvGexCL0KEQwVYv5BlowfhVz26hg==" data-cf-beacon='{"version":"2024.11.0","token":"2076e144259044deb00b04bf0efcc1b8","r":1,"spa":2}' crossorigin="anonymous"></script>

376
377</body>
378</html>