Health

Ovulation Calculator

Estimate your most fertile days based on your last period and cycle length.

First day of your last period Day 1 of bleeding, not the day it ended.

Cycle length (days) Average length between periods. Default 28, normal range 21–45.

Estimated ovulation day

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Enter your data above to see your fertile window.

Estimates only. Not a substitute for medical advice or contraception.

What is ovulation?

Ovulation is the moment a mature egg is released from one of the ovaries into the fallopian tube, where it can be fertilized for roughly 12–24 hours. In a typical 28-day cycle, ovulation happens around day 14 — but real cycles vary, so this calculator adjusts based on your own cycle length.

The four phases of the menstrual cycle

Menstrual phase (days 1–5): the uterine lining sheds.
Follicular phase: follicles in the ovary mature; estrogen rises.
Ovulation: a luteinizing hormone surge releases the egg.
Luteal phase: the corpus luteum produces progesterone; if no pregnancy, the cycle restarts.

How to use this calculator

Two pieces of information are enough for a solid estimate:

Enter the first day of your last menstrual period (LMP) — the day bleeding started, not when it ended.
Enter your average cycle length in days. If you don't track it, 28 is the standard assumption.
Read your fertile window — the 6 days leading up to ovulation plus ovulation day itself are when conception is most likely.

How accurate is this?

Calendar-based predictions assume a regular cycle and that ovulation always occurs 14 days before the next period. In reality, this can shift by several days. Combine this estimate with one or more of the signals below for more precision:

Basal body temperature — rises slightly after ovulation.
Cervical mucus — becomes clear and stretchy near ovulation.
Ovulation predictor kits (OPKs) — detect the LH surge 24–36 hours before ovulation.
Wearables and apps that combine temperature and heart-rate data.

Your cycle at a glance

Day-by-day map of the cycle that started on your LMP. Highlighted rows are the fertile window.

Day	Date	Phase
Fill in the form to see your cycle map.

Frequently asked questions

Can I get pregnant outside the fertile window?

It's far less likely, but not impossible. Sperm can survive up to 5 days inside the female reproductive tract, and ovulation can shift unexpectedly — that's why the fertile window has a safety margin.

What if my cycle is irregular?

Calendar predictions become less reliable. Track several cycles, or pair this calculator with ovulation predictor kits, basal body temperature, or cervical mucus changes for a more accurate picture.

How long does ovulation last?

The egg is viable for about 12–24 hours after release. The fertile window is wider — around 6 days — because sperm can wait inside the body for the egg.

When should we have intercourse to maximize chances of conception?

Every 1–2 days during the fertile window, especially the 2–3 days right before ovulation, gives the best statistical chance.

Can I use this calculator for birth control?

No. Calendar methods alone are not reliable contraception. If you're trying to avoid pregnancy, use a method recommended by a healthcare professional.

Why ovulation = cycle length − 14?

The luteal phase (after ovulation, before the next period) is relatively constant at about 14 days. The follicular phase is the part that varies — so we count back 14 days from the next expected period.

Ovulation Calculator