Birthday Calculator
Find your day of birth, star sign, Chinese zodiac, days alive and upcoming milestone birthdays.
Source: Royal Museums Greenwich — Day of the week calculator
By Konstantin Iakovlev · Founder, Calks.uk
Last updated: · Verified against HMRC and GOV.UK 2026/27 rates
Disclaimer
This calculator is provided for informational purposes only and should not be considered as financial or tax advice. All calculations are performed locally in your browser — no personal data is collected or sent to our servers. Rates and thresholds are sourced from HMRC and GOV.UK and are updated for the current tax year. Always verify results with HMRC or consult a qualified professional before making financial decisions.
How It Works
Enter your date of birth and this calculator reveals the day of the week you were born, your star sign, your Chinese zodiac animal, your birthstone and how many days until your next birthday. It also shows fun facts like the number one song or major event on your birth date.
The day-of-week calculation uses Zeller's congruence, a well-known mathematical formula that works for any date in the Gregorian calendar. The Chinese zodiac follows a 12-year cycle — each year is associated with an animal sign based on the lunar new year date.
Most and least common UK birthdays. Most common UK birthday: 26 September (9 months after Christmas/New Year). Mid-September to early October peak births. Least common: 25-27 December and 1 January (planned C-sections avoided; induced labours rare on holidays). Day-of-week patterns: Tuesday/Thursday most common (planned C-sections); Saturday/Sunday least (no scheduled procedures). Approx UK births: 600,000-700,000/year (falling — 1.49 fertility rate 2023, well below 2.1 replacement).
UK birthday legal milestones. 13: light part-time work permitted. 16: leave school (England); legal sex consent; marry in Scotland with parent consent; buy lottery ticket. 17: provisional driving licence. 18: vote, alcohol, contracts, full legal adulthood throughout UK. 21: 'key of the door' tradition (largely symbolic since 18 = adult since 1969). 25: full National Living Wage. 55: minimum pension access age (rising to 57 from April 2028). 66: State Pension age. 75: free TV licence (Pension Credit recipients only since 2020).
Birthday paradox. 23 people in a room: 50.7% chance two share a birthday. 30 people: 70.6%. 50 people: 97%. 70 people: 99.9%. UK school class average 30 students = 70%+ chance of shared birthday. Mathematical reason: not 23/365 — instead 23 people = 253 PAIRS to compare. With each pair having 1/365 chance, probability of NO match is (364/365)^253 ≈ 0.493. Classic counterintuitive probability result. Twin probability ~1.6% UK pregnancies; triplets ~1 in 6,000.
UK birthday cards from the King. Cards sent for 100th birthday (free, apply via gov.uk/100th-birthday-card-from-the-king). Then every year after 100 — so 101, 102, 103, etc. UK currently has 16,000+ centenarians. Application: relative or friend submits via gov.uk 8 weeks before birthday. Need proof of age (birth certificate). 105th and 110th birthdays: additional special cards. Same for diamond (60) and 65th, 70th, 75th wedding anniversaries.
Birthday tax and benefits triggers. Specific birthdays unlock entitlements: 16 — start NI contributions, can claim some benefits; 18 — adult ISA limit £20,000; 25 — full National Living Wage; 55 — access defined-contribution pension (rises to 57 April 2028); 60 — winter fuel payment (currently restricted to Pension Credit), free NHS prescriptions in Wales/Scotland/NI (all ages), England 60+ free; 66 — State Pension; 75 — free TV licence (limited eligibility). Insurance premiums often change on your birthday (use birthday-before for cheaper quote).
Example: Born 23 July 1995
- Day of the week: Sunday
- Star sign: Leo (23 Jul – 22 Aug)
- Chinese zodiac: Pig (Year of the Pig, 1995)
- Days until next birthday: calculated from today's date
Source: Royal Museums Greenwich — Day of the week calculator
Frequently Asked Questions
- Most and least common UK birthdays.
- Most common UK birthday: 26 September (9 months after Christmas/New Year). Most births by day of year fall mid-September to early October. Least common: 25-27 December and 1 January (planned C-sections avoided; induced labours rare). Day of week most common: Tuesday & Thursday (planned C-sections); least common: Saturday/Sunday (no scheduled births). Day with fewest UK births annually: 25 December. Approx UK births: 600,000-700,000/year (falling — 1.49 fertility rate 2023, well below 2.1 replacement).
- UK birthday traditions and milestones.
- Card from the King at 100, 105, then yearly (apply via gov.uk; previously sent automatically). 16th: school leaving (England), legal sex consent. 17: provisional driving licence. 18: full adult — alcohol, voting, contracts, sit on jury, get married without parental consent throughout UK. 21: 'key of the door' tradition (largely symbolic since 18 = legal adult since 1969). 30, 40, 50: 'milestone' celebrations. 65 (formerly retirement, now 66+). 75: free TV licence (England, restricted to Pension Credit recipients since 2020).
- Birthday probability — the paradox.
- Birthday paradox: in a group of 23 people, there's a 50.7% chance two share a birthday. With 50 people: 97% chance. With 70: 99.9%. UK school class of 30 — 70.6% chance of shared birthday. Twin probability ~1.6% of pregnancies (UK 2023). Triplets ~1 in 6,000. Mathematical reason: not 23/365 but pairs of comparisons grow rapidly — 23 people = 253 pairs.
- Tax and benefits triggered by age/birthday.
- Specific birthdays unlock entitlements: 16 — start NI contributions, can claim JSA in some cases; 18 — adult ISA limit £20,000 (Junior ISA stops at 18); 25 — full National Living Wage; 55 — access defined-contribution pension (rises to 57 in April 2028); 60 — winter fuel payment (currently restricted to Pension Credit recipients), free NHS prescriptions in England (free at 60 in Wales, 60+ free in Scotland, all free in Wales/NI/Scotland); 66 — State Pension; 75 — free TV licence (eligibility restricted).