Free web tool
Biorhythm
Your physical, emotional and intellectual cycles for today, a 30-day chart, best and critical days. Keep the whole family on one page.
Version 2.0 Updated October 11, 2026 What's new
What it does
Enter your birth date and see where your three classic biorhythm cycles stand today: physical (23 days), emotional (28 days) and intellectual (33 days). Each one shows as a level, whether it's rising or falling, and what it's said to affect, so you can take it in at a glance.
Make it yours, and the family's
Add your name and the page becomes Joe's biorhythm (with your name, of course). Tap ＋ Add someone to keep your partner, kids or friends on the same page; their names sit along the top, so switching is one tap. Compare any two of them in a tap, too.
Every person gets their own link that opens straight to them, great as a bookmark or a home-screen icon. And Link with everyone puts the whole list on another device in one go. Email it to yourself and it doubles as your backup: if your browser's data is ever cleared, open the link and everyone's back.
Private by design
Everything is worked out right in your browser. Names and birth dates are remembered on your own device, and in a link they sit after the #, a part of the address that browsers never send to any server. TheLabs.fyi never sees them.
What the research says
Biorhythms go back to Berlin doctor Wilhelm Fliess in the 1890s (23 and 28-day cycles), Vienna psychologist Hermann Swoboda around 1904, and Innsbruck engineering professor Alfred Teltscher in the 1920s (the 33-day cycle). They were a craze in the 1970s, and that's when researchers put them to the test.
This calculator follows the classic rules exactly: each cycle is sin(2π × days since birth ÷ cycle length), starting at the middle line on your birthday, and a critical day is when a cycle crosses that line. There's no newer or "more accurate" method, because the cycle lengths are fixed by the theory itself.
The verdict: a 1998 review by psychologist Terence Hines of 134 studies found 99 with no effect, and errors in the methods or math of the 35 that claimed one. Large careful tests, like one of more than 2,400 Army aviation accidents, found no link to critical days. So enjoy it as a curiosity and a piece of history, not a forecast. Sources: Wikipedia, Hines (1998), aviation study.
Features
- Today at a glance: each cycle as a percentage, rising or falling, in plain words (High, Good, Middle, Low).
- The whole family: save several people and switch with one tap; compare any two.
- Links and a backup: a link for each person, a link with everyone, and Email it to myself so you never lose the list.
- 30-day chart: tap any day to see its numbers; step a day or a week at a time, or jump to any date.
- Best days and critical days over the next month, with critical days marked green ↑ (turning up) or red ↓ (turning down); tap one to see it.
- Compare with someone: how in step your cycles are with another person's, overall and cycle by cycle.
- More cycles if you want them: intuitive (38 days), aesthetic (43) and spiritual (53).
- Private: nothing is sent anywhere. Dark by default, with light and automatic too. Works on phones, tablets and computers.
Screenshots
How to use
- Open the Biorhythm page and enter your birth date. Add your name if you like.
- Tap ＋ to add each family member; tap a name at the top to switch, and ✎ to edit names, dates and links.
- Under Link with everyone, tap Email it to myself (or Copy). That's your backup, and how you put the list on another device.
- On Android: open a link in Chrome, tap ⋮ then Add to Home screen for a one-tap icon.
Questions
Can I keep my wife's (or my kids') biorhythm too?
Yes. Tap ＋ Add someone and enter their name and birth date. Their names sit along the top; tap one to switch, and use Compare to see how in step you are.
What if my browser's data gets cleared, or I switch devices?
Tap Email it to myself under Link with everyone and keep that email. Opening the link on any device brings everyone back. No account needed.
Where's the backup / Email it to myself button?
Tap ✎ Edit at the right of the name buttons (or 💾 Back up your list further down the page). Under Link with everyone, tap Email it to myself.
Is my birth date stored anywhere?
Only on your own device, so the page remembers it. In a link it sits after the #, a part browsers keep to themselves and never send to any server, so TheLabs.fyi never sees it.
What's a critical day?
A day when a cycle crosses the middle line. Green ↑ means it's turning up into a high stretch, red ↓ means it's turning down into a low one. Biorhythm fans say either way you're a little less steady in that area that day.
Is any of this scientific?
No. A 1998 review of 134 studies found no real effect, and big careful tests found none either. Open What the research says at the bottom of the page for the history and sources.