Featured number
Six constants, one app
Switch between π, τ, e, φ, Planck, and more. Every calendar, lookup, stat, and share card updates to match — long-press the logo to swap on the fly.
pi · 3.14159 26535… · live lookup
Today lives in pi.
PiDay opens to today and finds the earliest place your date appears in five billion digits of pi, using the same bundled index and live lookup rules as the app. Search birthdays, anniversaries, and anything else that matters.
Download freeiPhone, iPad and Mac
today in pi
waiting for today
searching 5,000,000,000 digits
3141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117067982148086513282306647093844609550582231725359408128481117450281403199527101938521105559644622948954930381964428810975665933446128
Day14today
Month03DDMMYYYY
Year1995demo fallback
Position 47,832,104 · format DDMMYYYY · 5 billion digits searched
How it works
Three steps to your place in infinity.
01
14031995
Pick your date
Open to today, or pick any birthday, anniversary, or date that matters. PiDay searches across five billion digits of π in every date format simultaneously.
02
See the heat map
A calendar fills with colour — hotter dates appear earlier in π. See at a glance how your whole month compares, date by date.
03
Day 14Month 03Year 1995
Share the discovery
Save your result as a card and share it. Six themes, multiple date formats, and a canvas of π digits you can scroll forever.
Version 1.3
Pi is just the beginning.
PiDay goes beyond π. Search your date across six famous mathematical constants, celebrate their unofficial holidays, and settle scores with Date Battles.
Party tricks
Celebration days
Confetti and a short explainer pop up on each constant's unofficial holiday — Pi Day, Tau Day, Euler's Day, and more.
Friendly rivalry
Date Battles
Pick any two dates and let pi decide the winner. Compare positions, winning margins, and rarity in a single polished matchup card.
What is π
The ratio at the heart of every circle.
π is the ratio of a circle’s circumference to its diameter — always, exactly, inevitably. Draw a circle of any size, measure its circumference, divide by its diameter, and you get the same number: 3.14159265…
But π refuses to stop. Unlike simple fractions (⅓ = 0.333…, which at least has a pattern), π is irrational — its decimal expansion neither terminates nor repeats. Mathematicians have computed it to over 100 trillion digits, and no pattern has ever emerged.
Transcendental. Infinite. Unknowable.
π is also transcendental— it cannot be the solution to any algebraic equation with rational coefficients. This is why squaring the circle is impossible. π isn’t just hard to pin down; it fundamentally resists every algebraic cage ever built for it.
And because it never repeats, it almost certainly contains every possible finite sequence of digits — your birthday, your phone number, every book ever written in numeric form, hiding somewhere in its infinite length. Probably. We can’t prove it yet. But we can search.
“Probably contains every possible sequence of digits. We can’t prove it yet. But we can search.”
Themes
Six ways to see your number.
From warm parchment to phosphor green — each theme is a distinct world. Change anytime.
Frost
· light ·
Slate
· dark ·
Coppice
· dark ·
Ember
· light ·
Aurora
· dark ·
Matrix
· dark ·
Find your date in π.
Free · iPhone, iPad & Mac · iOS 17+
Download on the App Store
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