Women's Health calculators

What these calculators measure

The Women's Health calculators put your cycle dates to work. The Ovulation calculator estimates your ovulation day, your six-day fertile window and your next period from the first day of your last period and your typical cycle length. The Period calculator predicts your next three period start dates. The Due Date calculator estimates when your baby is due, roughly when conception happened and how many weeks pregnant you are today, using Naegele's rule — the standard method used by midwives and doctors. The Pregnancy Weight Gain calculator compares your pre-pregnancy BMI with the weight-gain ranges recommended in the Institute of Medicine’s 2009 guidelines, for a singleton pregnancy.

Three assumptions sit behind the cycle-based tools: that cycle day 1 is the first day of full flow, that the luteal phase is roughly 14 days long, and that ovulation therefore happens about fourteen days before your next period. From that single deadline, the ovulation day, the fertile window and the projections all follow naturally. The weight-gain ranges instead follow the IOM 2009 evidence on how much gain supports healthy outcomes across different starting BMIs.

All of these run entirely in your browser. Your dates and calculations are never sent to a server, so you can track your cycle privately.

Who they are for

Anyone who tracks a menstrual cycle: couples trying to conceive, people planning around their period dates, and pregnant people — or those supporting them — who want an early due-date estimate before their first ultrasound. The predictions assume a reasonably regular cycle; treat them as guides, not guarantees. Because nothing leaves your device, these tools are a comfortable first step before you decide whether to share dates with an app or a provider.

How to use the results

Enter the first day of your last period and your most common cycle length. For conception, the two or three days before ovulation matter most. If your cycles are regularly outside the 24–35 day range, the calculators will flag that the estimates are less reliable — and for frequently irregular cycles, talking to a healthcare provider beats relying on predictions alone. Re-enter your dates each cycle if your cycle length changes, and remember that a single irregular month is common: stress, travel and illness all shift timing. Due dates should always be confirmed with early ultrasound.