Ovulation Calculator

Estimate your ovulation date and fertile window based on your menstrual cycle. Plan for conception or track your cycle.

Free to use. Runs in your browser.

Ovulation typically occurs 14 days before your next expected period, not 14 days after the last one. The fertile window spans 6 days: the 5 days before ovulation (sperm survive up to 5 days) plus ovulation day. For a 28-day cycle, ovulation is around day 14; for a 35-day cycle, around day 21.

Enter the first day of your last period and your cycle length to estimate ovulation and fertile days.

First Day of Last Period

Cycle Length (days)

Educational Estimates Only: These calculations are approximations based on population-level formulas and may not accurately reflect your individual health status. Body composition varies significantly due to factors like muscle mass, bone density, hydration, and ethnicity. This is not medical advice. Always consult a qualified healthcare provider for personalized assessment, diagnosis, or treatment decisions.

Methodology and sources

Formula or method

Estimates ovulation as cycle length minus 14 days after the first day of the last menstrual period, then marks the fertile window as the 5 days before ovulation plus ovulation day.

Basis and assumptions

The model assumes a regular cycle and a luteal phase of roughly 14 days.
Future cycles repeat the same average cycle length entered by the user.
The fertile window basis reflects sperm survival before ovulation, as explained in the tool.
Irregular cycles, PCOS, postpartum changes, and medication can shift or suppress ovulation.

What this tool does not decide

This tool does not confirm or diagnose pregnancy, ovulation, or fertility. Consult a GP, midwife, or fertility specialist.
Whether you ovulated in a specific cycle, whether you can conceive, or whether symptoms need medical assessment.

Sources

Standard ovulation estimate: ovulation about 14 days before the next period
Wilcox et al. (1995), New England Journal of Medicine

Last checked: 2026-06-05

Understanding Ovulation: The Key to Fertility

Ovulation is the release of a mature egg from the ovary. It happens once per cycle, typically 14 days before your next period, not 14 days after your last one. This distinction matters enormously if your cycle isn't exactly 28 days.

In a 28-day cycle, ovulation occurs around day 14. In a 35-day cycle, it's around day 21. In a 24-day cycle, it's around day 10. The time from ovulation to your next period (the luteal phase) is remarkably consistent at 12 to 14 days. It's the first half of the cycle (the follicular phase) that varies.

The egg survives only 12 to 24 hours after release. But sperm can live up to 5 days in the reproductive tract. This creates a fertile window of approximately 6 days, 5 days before ovulation plus ovulation day itself. Timing intercourse within this window is the single most important factor for natural conception.

The Menstrual Cycle Phases

Phase	Days (28-day cycle)	What Happens	Fertility
Menstruation	Days 1 to 5	Uterine lining sheds, your period	Very low
Follicular phase	Days 1 to 13	Follicles develop, one becomes dominant, oestrogen rises	Increasing after day 8
Ovulation	Day ~14	LH surge triggers egg release, egg viable 12 to 24 hours	Peak fertility
Luteal phase	Days 15 to 28	Corpus luteum produces progesterone, prepares uterus	Drops rapidly after day 15

What this means for you: Your most fertile days are the 2 to 3 days immediately before ovulation. By ovulation day itself, the window is closing. If you're trying to conceive, start timing intercourse from about 5 days before your expected ovulation date. Use our Conception Calculator if you're already pregnant and want to know when it happened.

Signs of Ovulation

Cervical Mucus Changes

As ovulation approaches, cervical mucus becomes clear, stretchy, and slippery, resembling raw egg white. This "fertile mucus" is designed to help sperm swim and survive. After ovulation, mucus becomes thicker and less abundant.

Basal Body Temperature

After ovulation, progesterone causes a 0.2 to 0.5°C rise in your resting temperature. Take your temperature at the same time every morning before getting up. The rise confirms ovulation occurred, useful for tracking but tells you after the fact, not before.

LH Surge (OPK Tests)

Ovulation predictor kits detect the LH surge that triggers egg release 24 to 36 hours before ovulation. A positive OPK means ovulation is imminent, the most time-sensitive signal available without medical monitoring.

Mittelschmerz (Ovulation Pain)

About 20% of women feel a twinge or mild cramp on one side of the lower abdomen during ovulation. It can last minutes to hours. While not reliable enough to use alone, combined with other signs it helps confirm your fertile window.

Conception Probability by Timing

Days Relative to Ovulation	Probability of Conception
5 days before	~4%
4 days before	~8%
3 days before	~15%
2 days before	~25%
1 day before	~30% (peak)
Ovulation day	~12%
1 day after	~0% (egg has typically expired)

Source: Wilcox et al., New England Journal of Medicine, 1995. These are per-cycle probabilities for healthy couples. Peak conception probability is actually the day before ovulation, not ovulation day itself, because it takes sperm several hours to reach the egg after ejaculation.

Worked Ovulation Example

If the first day of your last period was 1 March and your usual cycle is 32 days, the calculator estimates ovulation at cycle day 18 because 32 minus 14 equals 18.

Step	Example	Result
Last period	1 March	Cycle day 1
Ovulation estimate	Cycle length minus 14	Day 18
Fertile window	5 days before ovulation plus ovulation day	Days 13 to 18

A longer cycle usually means later ovulation. A shorter cycle usually means earlier ovulation. The luteal phase assumption is why the calculator counts back from the next period rather than simply using day 14 for every cycle.

How to Improve Calendar Estimates

Track Several Cycles

Use the average of recent cycles rather than one unusual month. Illness, travel, and stress can shift ovulation.

Add OPK Results

Ovulation predictor kits detect the LH surge and can narrow timing beyond a calendar estimate.

Watch Cervical Mucus

Fertile-quality mucus often appears before ovulation. It is a useful companion signal, not a diagnosis.

Use Temperature Carefully

Basal body temperature rises after ovulation, so it confirms a pattern retrospectively rather than predicting the best day in advance.

When to Ask for Fertility Advice

Speak with a GP if cycles are very irregular, absent, unusually painful, or associated with heavy bleeding.
If you are trying to conceive and are over 35, fertility advice is commonly sought sooner than for younger couples.
PCOS, thyroid disease, endometriosis, previous pelvic infection, and recurrent miscarriage all deserve clinician-led guidance.
A calculator cannot confirm ovulation, pregnancy, or fertility. Use medical testing when the answer matters.

Ovulation Estimate Review Checklist

Before relying on a predicted fertile window, check whether the cycle data supports a calendar estimate.

Use an average cycle length from recent cycles.
Pair the estimate with OPKs or other fertility signs when timing matters.
Seek medical advice if cycles are absent, highly irregular, or newly changed.

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How to use this tool

Enter the first day of your last period

Select your average cycle length

Calculate ovulation, fertile window, and future cycles

Common uses

Planning conception timing
Tracking your menstrual cycle
Identifying your most fertile days
Predicting your next period
Understanding your reproductive cycle

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Frequently Asked Questions

How is ovulation day estimated?

Ovulation typically occurs 14 days before your next expected period, not 14 days after your last one. This distinction matters if your cycle isn't exactly 28 days. A 35-day cycle means ovulation around day 21.

What is the fertile window?

The fertile window is approximately 6 days: the 5 days before ovulation plus ovulation day itself. Sperm can survive up to 5 days in the reproductive tract, so intercourse before ovulation can still lead to conception.

How accurate is an ovulation calculator?

It provides a good estimate for women with regular cycles. Actual ovulation can vary by 1 to 2 days even in regular cycles. For greater precision, combine calendar tracking with ovulation predictor kits or basal temperature monitoring.

What cycle length should I use?

Use the average of your last 3 to 6 cycles. If your cycles range from 26 to 30 days, use 28. If they're very irregular (more than 7 days variation), OPKs or cervical mucus tracking may be more useful.

Can I ovulate without having a period?

Yes, but it's uncommon. Some women ovulate before their first postpartum period while breastfeeding, or before their period returns after stopping hormonal birth control. Anovulatory cycles (periods without ovulation) are also possible, especially with PCOS.

Can stress delay ovulation?

Yes. Physical or emotional stress can delay or suppress ovulation by disrupting the hormones that trigger egg release. Delayed ovulation usually means a later period, since the luteal phase stays fairly constant at 12 to 14 days.

How do ovulation predictor kits work?

OPKs detect the surge in luteinising hormone (LH) that occurs 24 to 36 hours before ovulation. A positive result means ovulation is imminent, it's the most time-sensitive non-medical signal available. Test with afternoon urine for best results.

Can I ovulate more than once per cycle?

You can release multiple eggs within the same 24-hour ovulation window (which is how fraternal twins happen), but you don't have two separate ovulation events weeks apart in one cycle. Once ovulation occurs, progesterone prevents further egg release.

Does birth control affect when I ovulate after stopping?

Most women resume ovulating within 1 to 3 months of stopping hormonal birth control. The pill, patch, and ring typically allow faster return than Depo-Provera injections, which can delay ovulation for 6 to 12 months.

What is anovulation?

Anovulation means a cycle where no egg is released. You may still bleed but can't conceive. Common causes include PCOS, stress, extreme exercise, low body weight, and thyroid disorders. If you suspect anovulation, see your GP.

How does age affect ovulation?

Ovulation becomes less regular approaching perimenopause (typically mid-40s). Egg quality also declines with age, fertility drops noticeably after 35 and more steeply after 40. Cycle length tends to shorten slightly in your late 30s.

What is the luteal phase?

The luteal phase is the second half of your cycle, from ovulation to your next period. It's remarkably consistent at 12 to 14 days. If yours is shorter than 10 days, it may indicate a luteal phase defect that can affect implantation.

Results are for general informational purposes only and should be checked before use. They are not professional advice. See our Disclaimer and Terms of Service.