Ordered by stage, from planning to a day with a child in the house. All of it is calendar arithmetic and none of it is a medical judgement — each tool sets out its method and sources.
Worth being clear first: these are tools that count days, not tools that observe a body. They add 280 days to the start of a last period, work backwards from a cycle length to an ovulation date, or count how many days have passed since a birth.
So a result means «this date, if the cycle is average» — not that your body is doing that on that date. Around 5% of births land exactly on the estimated date, and ovulation moves between people and between cycles.
Anything that needs judgement — whether you are pregnant, whether a cycle is irregular, how a child is developing — sits outside this arithmetic. The dates here are useful as the reference point for booking an appointment or starting a conversation.
The common method, Naegele's rule, adds 280 days — forty weeks — to the first day of the last period. The part that sounds wrong is that those 280 days run from the last period rather than from conception.
The reason is practical. Most people do not know the date of conception, and most people do know when their last period started. So the method assumes an average — a 28-day cycle with ovulation on day 14 — and counts back from there.
Which means a cycle away from that average shifts the date. Someone with a 35-day cycle ovulates later, so the calculation runs early, and that is why a clinic adjusts the due date when an ultrasound measurement disagrees with the calendar. When they disagree, the clinic's date is the one to use.
«How many weeks pregnant» uses the same origin. It is counted from the last period rather than from conception, so being already four or five weeks along at the moment you find out is normal rather than surprising.
Ovulation is pinned to the next period rather than the last one: roughly 14 days before the next one is due. The luteal phase is relatively consistent at around fourteen days whether a cycle is long or short, which is why the calculation works from that end.
The fertile window being about five days rather than one also comes from this. Sperm survive for several days, so the window runs from a few days before ovulation up to the day itself.
But the whole calculation rests on the cycle being regular. If your cycle varies month to month, or you are under stress, unwell or breastfeeding, calendar arithmetic cannot locate ovulation. Other methods exist for that, and they are not what this tool does.
Before a birth you count days left; afterwards you count days since. Hundred-day marks, two hundred days, a first birthday — and vaccination schedules run on months.
One thing catches people out. Traditions that mark a hundredth day count the birth day as day one, which puts it 99 days later. Age in completed years counts the birth day as zero and increments on each birthday. The two conventions differ by a day, so knowing which one is in use prevents a lot of off-by-one confusion.
For a premature baby there is an additional axis: corrected age, counted from the due date rather than the birth date. Developmental milestones are frequently discussed in corrected age, so agreeing which age is meant makes the conversation much shorter.
Pregnancy calculators get used a handful of times. What gets used daily afterwards is far more mundane: how long to boil an egg, when the school is closed.
Boiling times are the classic. Soft and hard are a minute or two apart, an egg straight from the fridge behaves differently from one at room temperature, and if a child is eating it the sensible default is to err towards fully cooked. Not worth searching for every time, and forgotten every time.
Public holidays are the skeleton of any plan. The days nursery and school are shut are the days childcare has to come from somewhere, so leave and trips get built on that calendar. They differ by country and some move each year, which is exactly why counting from memory goes wrong.
And it is worth having one short game on the phone. For the ten minutes in a waiting room or before food arrives — and a card-matching game happens to be the one where adults regularly lose to a five-year-old.
It does not make medical judgements. Whether something is within a normal range, whether to get it checked, how a child is developing — none of those can be answered here. Each calculator carries a methodology and sources block setting out what it computes from and where it stops.
It does not send your dates anywhere. These calculations take in things like cycle dates and a child's birthday, so all of it runs in the browser and there is no account.
It does not keep a log either. If you want to record cycles month after month and watch the trend, that is a different kind of app; these tools stop at one calculation from the values you just typed.