Three tools on this site were built as a set, because they answer three phases of the same eighteen-month question. This guide runs the whole chain with one consistent example so the numbers connect: where the fertile window comes from, why a due date is counted from a day before you were pregnant, and why the unit of measurement changes completely the moment the baby arrives.
This is not medical advice. These tools do date arithmetic on published conventions. They don't know your medical history, and calendar prediction is not contraception. Anything that matters belongs in a conversation with a midwife, GP or clinic.
Step one: finding the fertile window
Say your cycles run 30 days and your last period started on 1 March. Put that into the Period Calculator and the interesting output isn't the next period date — it's the fertile window.
Here's the part that trips almost everyone up. The calculator does not count 14 days forward from your period. It counts 14 days backwards from the next one. On a 30-day cycle, your next period is due 31 March, so ovulation is estimated at 17 March — day 17 of the cycle, not day 14.
The reason is that the two halves of a cycle behave differently. The luteal phase, from ovulation to the next period, is remarkably consistent at around 14 days in most people. The follicular phase, from your period to ovulation, is the elastic part — it's what stretches or shortens when your cycle runs 25 days one month and 33 the next. So counting backwards from the next period is the more reliable estimate, and it's why a 35-day cycle doesn't mean ovulating on day 14 and then waiting three weeks.
The window itself runs from seven days before that estimate to one day after — 10 March to 18 March in our example. That asymmetry is deliberate and biological: sperm can survive in the reproductive tract for up to five days or so, while an egg is viable for roughly 24 hours. The days that matter most for conception are therefore the two or three before ovulation, not the day itself, which is the opposite of what most people assume.
Where this genuinely helps: planning around travel or shift work, knowing which fortnight to book time off in, spotting that your cycles have quietly drifted from 28 to 33 days over a year, or just having a calendar to show a GP instead of trying to remember dates in an appointment.
Where it doesn't: the projection is an average, and ovulation moves for illness, stress, travel and weight change. If you're using this to avoid pregnancy, don't — use a method with a published failure rate.
Step two: the due date, and why it's counted from before you were pregnant
Now suppose that worked, and a test is positive in early April. Into the Due Date Calculator goes 1 March — the first day of that last period — and out comes a due date of 6 December. That's Naegele's rule: last period plus 280 days.
Which is odd, when you think about it, because you weren't pregnant on 1 March. Conception happened around 17 March. Counting from the period adds roughly two weeks of "pregnancy" that didn't happen.
The convention survives because it's anchored to something you can actually observe. Almost nobody knows their conception date; nearly everybody knows when their last period started. So obstetrics standardised on the observable date and absorbed the two-week offset into the numbering. It's a measurement convention, not a biological claim.
This is also the explanation for the thing that confuses every first-time parent: you take a test, it's positive, and you are immediately "four weeks pregnant". Four weeks since your last period. About two weeks since conception. The tool reports gestational age the way a midwife will — completed weeks since the last period — precisely so the number on your screen matches the number in your notes.
If you're tracking closely and know your conception date, the calculator will take that instead and work backwards 14 days to the equivalent starting point. For IVF it goes further and adjusts for the transfer day, since a day-5 blastocyst transfer is already five days into development at the point of transfer.
The caveat worth actually internalising: Naegele's rule assumes a 28-day cycle ovulating on day 14. Our example cycle is 30 days, ovulating on day 17 — three days later — so the true due date is probably nearer 9 December. And about 4% of babies arrive on their estimated date anyway. Full term is anything from 37 to 42 weeks. A first-trimester dating scan beats any arithmetic, and where they disagree, the scan wins. Treat the date as the centre of a five-week window, not an appointment.
Step three: the milestones in between
The due date tool marks trimesters at the end of week 12 and the end of week 27, which is the standard split, and lays out the milestones along the way. In practice the dates people care about are the ones that attach to appointments: the dating scan around weeks 11 to 14, the anomaly scan around 18 to 21, viability thresholds in the mid-twenties, and full term at 37.
The reason to have these as dates rather than week numbers is mundane but real — "week 20" is hard to plan around, whereas "the week of 20 July" tells you whether it clashes with a wedding.
Step four: the unit changes completely
The baby arrives, and within about a week you discover that nobody in healthcare measures babies in months. Every appointment, every feeding guideline, every developmental check and every vaccination schedule for the first two years is specified in weeks. Meanwhile every relative you speak to asks how many months old they are, and the two don't divide cleanly — a baby born 6 December is 8 weeks old on 31 January, which is not two months, and the mismatch compounds.
This is what the Age Calculator is genuinely useful for, and it's not the use most people would guess. Enter the birth date and you get the exact age in years, months and days, plus the total in weeks and days — so you can answer the health visitor in weeks and your mother-in-law in months without doing calendar arithmetic in your head at seven in the morning.
It walks the calendar rather than dividing by 365.25, which matters more than it sounds: a baby born in late February has a genuinely awkward relationship with month arithmetic, and a "months" figure derived from dividing days by 30.4 will drift by days.
The same tool keeps earning its place afterwards: working out school-year cut-off eligibility, which is defined by age on a fixed date; checking whether a passport or a car seat is still valid at a given point on a trip; and eventually being the thing you use to find out exactly how many days until a birthday, which children find far more interesting than adults do.
The chain, in one line
Period Calculator finds the fortnight that matters → Due Date Calculator converts a period date into a due date and a week count that matches your notes → Age Calculator takes over at birth and answers in the units everyone is asking in.
All three run entirely in your browser. The dates you enter — which are, for most people, among the most private things they'd ever type into a website — never leave your device, because there's no server here to send them to. If you want to confirm that, open your browser's network tab and use the tools.
The exact formulas behind all three, including their failure modes, are written out on the methodology page.