Cycle, ovulation, and pregnancy — the calm, private way to do the math
This page brings together four free tools for tracking your cycle and your
pregnancy: a period calculator, an
ovulation and fertile-window calculator,
a due-date calculator, and a
pregnancy weight-gain calculator.
Before anything else, the part that matters most here: nothing you type
leaves your device. Every calculation runs in your browser. There is no
account, no login, and no pixel or analytics tracker on these pages — we turn
tracking off at the route level, not just promise it. Your dates, your cycle
length, and your weight are never sent to a server, never saved, and never read
back. When you close the tab, they’re gone. We explain exactly why below.
These tools give you estimates, not a diagnosis. They are not a form of
birth control. They don’t replace your clinician. What they do well is take a
few numbers off your mind and turn them into clear dates — so you can plan,
ask better questions at your next visit, and stop doing fragile mental math.
Tracking your cycle
If you’re trying to understand your own rhythm — when your next period is
likely, or when you’re most likely to conceive — start here.
The period calculator projects your next
three period start and end dates from one input: the first day of your last
period, plus your usual cycle length and how many days you typically bleed. The
math is simple and standard — each predicted period falls one cycle length after
the last (LMP + cycle, then + 2 cycles, then + 3). It also marks the estimated
ovulation day for that first cycle.
The ovulation and fertile-window calculator
zooms in on conception timing. It estimates your ovulation day as your cycle
length minus your luteal phase (the stretch between ovulation and your next
period), then marks a six-day fertile window ending on ovulation day — the
five days before, plus ovulation itself. That window reflects how conception
actually works: sperm can survive up to about five days, while the egg lasts
roughly a day after it’s released.
They share the same underlying calendar, so it’s fair to ask why there are two.
The answer is they answer two different questions:
- “When do I bleed next?” → the period calculator, which gives you a
three-cycle horizon of period dates you can put on a calendar.
- “When am I most fertile?” → the ovulation calculator, which focuses on the
fertile window and lets you adjust your luteal-phase length if you’ve charted
it.
Use the period calculator for planning around your period; use the ovulation
calculator when timing matters for conceiving. Many people use both.
The 14-day assumption (and where it breaks)
Both tools lean on a common rule of thumb: that ovulation happens about 14
days before your next period. The luteal phase — ovulation to period — is more
consistent person-to-person than the first half of the cycle, which is why
counting back 14 days is the textbook starting point. ACOG describes this
fertility-awareness approach the same way.
But “starting point” is the honest framing. Real ovulation drifts cycle to
cycle even when periods feel regular. The luteal phase isn’t always exactly 14
days (a common range is roughly 12 to 16), which is why the ovulation tool lets
you override it if you track basal body temperature or LH tests. And the further
out you predict, the more small variations stack up — so treat the second and
third predicted periods as a rough range, not a promise.
A clear cycle is worth a sentence to your clinician
A typical cycle runs about 24 to 38 days, per the HHS Office on Women’s
Health. Outside that range now and then is common. But if your cycles are
consistently shorter than 24 days or longer than 38, vary by more than about a
week, stop for 90 days or more, or come with unusually heavy or painful
bleeding, that’s worth raising with a clinician — these tools can’t evaluate
any of it. They predict dates; they don’t diagnose.
Fertility-awareness math is not birth control
This is the most important line on the page, so it gets its own heading.
A fertile-window estimate tells you when you’re more likely to conceive. It
does not reliably tell you when you’re safe from pregnancy. Reading the
calculator backwards — “avoid these six days and I won’t get pregnant” — is not
contraception, and treating it that way leads to unintended pregnancies.
Real fertility-awareness-based methods of contraception are far more involved
than a calendar. They combine daily basal-body-temperature charting,
cervical-mucus observation, and sometimes LH testing, and ACOG recommends
learning them with guidance from a clinician or a trained instructor. Our tool
does none of that — it’s a planning estimate built on averages. If you’re trying
to prevent pregnancy, talk to a clinician about a method that’s designed for
it. We won’t pretend a calendar is one.
Once you’re pregnant
If you already know you’re pregnant, the next two tools help you map the road
ahead.
Which due-date method should you trust?
The due-date calculator can estimate your due date
four different ways, and which one to use depends on what you know:
- Last menstrual period (LMP). The classic method (Naegele’s rule): your due
date is about 280 days (40 weeks) after the first day of your last period,
assuming a 28-day cycle. If your cycles run longer or shorter, the tool shifts
the estimate by the difference — a 32-day cycle pushes the date about four days
later. This is the most widely used method, but it leans on you remembering
your LMP and on a textbook cycle.
- IVF transfer. If you conceived through IVF, the transfer date gives a more
precise estimate because the embryo’s age is known. A day-5 (blastocyst)
transfer counts forward about 261 days; a day-3 transfer, about 263. This is
usually more accurate than LMP.
- Known conception date. If you know the date of conception, the due date is
about 266 days later (38 weeks from fertilization).
- A due date from an early ultrasound. If your clinician has already given
you a date from a scan, enter it directly.
Here’s the honest hierarchy: an early ultrasound is the most accurate dating
method, and your clinician’s date wins over any calculator. ACOG’s guidance is
that if an early scan disagrees with your LMP date by more than about a week,
the ultrasound date is the one to use. So treat the calculator as a first
estimate to discuss, not a fixed appointment.
And a number worth internalizing: only about 4% of babies arrive on their due
date. The “term” window is wide — roughly 37 to 42 weeks. The due-date tool
also lays out the milestones along the way (end of the first trimester around
13 weeks, the viability threshold near 24 weeks, full term at 37 weeks) so the
date feels like a center point, not a deadline.
Weight gain: a range, not a target
The pregnancy weight-gain calculator
compares your gain against the ranges from the Institute of Medicine (now the
National Academy of Medicine), 2009, which ACOG endorses. The key idea most
calculators bury: the recommended amount depends on your pre-pregnancy BMI,
not your current weight.
For a singleton pregnancy, the IOM ranges are:
- Underweight (BMI under 18.5): about 28–40 lb
- Normal (18.5–24.9): about 25–35 lb
- Overweight (25.0–29.9): about 15–25 lb
- Obese (30 and over): about 11–20 lb
Carrying twins shifts these higher, and the tool handles that too — with one
honest gap: the IOM did not publish a range for underweight twin
pregnancies because there wasn’t enough evidence. If that’s you, the tool says
so plainly and points you to your clinician rather than inventing a number.
Two more things the tool gets right that matter: gain isn’t steady. Most of the
first trimester is small (roughly 1 to 4.5 lb total), with the steadier weekly
pattern coming in the second and third trimesters. So a single week above or
below the band isn’t a verdict — the tool shows a band across the weeks, not a
pass/fail line. And these are population estimates. Gestational diabetes,
hyperemesis, carrying multiples beyond twins, or any other condition is a reason
for individual guidance, not a recalculated target.
A common path through the four tools looks like this:
- Mapping your cycle? Start with the
period calculator to see your next dates,
then open the
ovulation and fertile-window calculator
if you want to focus on conception timing.
- Just found out you’re pregnant? Use the
due-date calculator — your last period date or, if
you have it, an IVF or ultrasound date — to estimate your due date and how far
along you are.
- Settling into the pregnancy? Bring up the
pregnancy weight-gain calculator at a
point in your pregnancy to see where a healthy gain range sits for you.
Each tool stands on its own and recomputes from fresh input every time. We
deliberately do not carry your numbers from one tool to the next, and we
never pass them in the web address. That’s a privacy choice, explained next.
Why these pages have no tracking
Cycle and pregnancy data is some of the most sensitive information about you,
and the risk isn’t hypothetical. In 2021 the Federal Trade Commission ordered
the popular cycle app Flo Health to change its practices over allegations that
it shared sensitive menstrual and pregnancy information with advertising and
analytics services — the exact pattern these pages are built to avoid. Related
consumer litigation against the company has been widely reported since.
So on every page in this section, we do the opposite:
- No analytics and no pixels. These routes carry none — not a single
page-view ping. We enforce it at the route level so it can be checked, not
just claimed.
- Nothing in the web address. We never put your cycle length, dates, or
weight into the URL (no
?cycle=28). Sensitive values in a link leak into
browser history and server logs; we don’t create that trail.
- Nothing stored. No account, no cookie holding your numbers, no “save my
cycle.” Each visit starts clean. If a tool offers a calendar (.ics) export,
it’s built in your browser and the event is bare — a generic label, no notes,
nothing that describes you.
The honest version of this claim is “your numbers stay on your device and we
don’t store them,” and we keep it literally true.
These calculators give estimates and screening numbers, not medical advice or
a diagnosis. They can’t see your history, your labs, or your scan. They are not
a contraception method. For anything that feels off — irregular or missed
cycles, concerns about your pregnancy, or questions about healthy weight gain —
talk with your clinician, who can give you guidance built for your situation.
The math here is standard and cited; the judgment is theirs and yours.