Ovulation Calculator: Fertile Window and Next Period Dates

1. Enter the first day of your last period. Day one is the first day of full bleeding, not spotting.
2. Enter your average cycle length. Count from the first day of one period to the first day of the next. The default is 28 days; the typical range is 24 to 38.
3. Leave the luteal phase at 14 days unless a clinician gave you a different number. It is the fairly constant gap from ovulation to your next period.
4. For irregular cycles, add your shortest and longest cycle lengths to see a wider, less certain fertile range.

This is an educational estimate from calendar data. It is not medical advice, a diagnosis, or a method of contraception.

Ovulation is the release of an egg from the ovary. It usually happens once in each menstrual cycle, roughly in the middle, and the egg can be fertilised for only about 12 to 24 hours afterwards.

The fertile window is wider than that single day because sperm can survive in the reproductive tract for up to about five days. Adding the egg survival time gives a six-day window.

The originating study by Wilcox and colleagues, published in the New England Journal of Medicine in 1995, found that conception occurred only when intercourse happened during a six-day period that ended on the day of ovulation. Pregnancy was most likely on the two days before and the day of ovulation.

The Office on Women's Health describes a typical cycle as about 28 days, with a normal range of 24 to 38 days. The luteal phase, from ovulation to the next period, is relatively steady and is commonly modelled as 14 days.

The calculator uses the standard calendar method. It assumes a steady luteal phase, so it counts backwards from your expected next period to place ovulation, then adds the fertile days around it.

1. Find ovulation. Add your cycle length to the start of your last period, then subtract the luteal phase. With 28 and 14, ovulation lands 14 days after the period started.
2. Mark the fertile window. Take the five days before ovulation and ovulation day itself, giving six days in total.
3. Project the next period. Add your cycle length to the start of your last period.
4. Repeat for later cycles. Add one more cycle length each time to estimate the next two or three cycles.

All dates use real calendar arithmetic, so month and year changes are handled correctly.

Take a first day of last period of January 1, 2026, a 28-day cycle, and a 14-day luteal phase.

1. Ovulation = January 1 + 28 - 14 = January 1 + 14 days = January 15, 2026.
2. Fertile window = January 15 minus 5 days to January 15 = January 10 to January 15, 2026.
3. Next period = January 1 + 28 days = January 29, 2026.

These are the three dates the tool shows for this example, which is loaded as the first preset. The calculator then repeats the pattern to project the next two or three cycles.

Where the Six Days Come From

The arithmetic is exact, but the real day of ovulation shifts with the body, so the estimate is only as good as the inputs.

Cycle Length Variation

Few people ovulate on exactly the same cycle day every month. Even a regular cycle can move the fertile window by a few days from one month to the next.

Irregular Cycles

With irregular, long, or short cycles the day of ovulation is much harder to pin down. The shortest and longest cycle inputs widen the window to reflect that uncertainty.

Luteal Phase Length

The 14-day luteal phase is a modelling default. The Office on Women's Health notes the real gap can range roughly 7 to 19 days, which changes where ovulation falls.

Stress, Illness and Life Changes

Stress, illness, travel, weight change, breastfeeding, and approaching menopause can all delay or skip ovulation, which calendar math cannot predict.

Hormonal Contraception

Recently stopping hormonal birth control can make cycles unpredictable for a while, so calendar estimates are less reliable during that time.

Calendar math predicts ovulation in advance from past cycles. Two body-based signals can confirm or sharpen the timing.

Because BBT only rises after the fact, it is useful for learning your pattern over several cycles rather than catching the fertile days in advance. LH kits are better for forward timing. To learn more, see our guide to the fertile window and ovulation and to ovulation signs and how to track them.

Planning for Pregnancy

If you are trying to conceive, the tool points to the days when timing intercourse is most likely to help. Pair it with a pregnancy calculator once you have a result.

Understanding Your Cycle

Seeing ovulation, the fertile window, and the next period on a calendar helps you learn your own pattern across a few months.

Timing Other Tests

Knowing the likely fertile window helps you decide when to start LH testing or when to track basal body temperature with our body temperature converter.

Estimating a Due Date Early

Once a cycle leads to pregnancy, the start date feeds into how a due date is calculated.

1. Using It as Birth Control

Calendar timing is not a reliable contraceptive. Fertility-awareness methods have a high typical-use failure rate. Do not use this tool to prevent pregnancy.

2. Trusting It with Irregular Cycles

If your cycles vary a lot, a single predicted day is misleading. Use the shortest and longest cycle inputs and treat the range as uncertain.

3. Counting from the Wrong Day

Cycle day one is the first day of full bleeding. Counting from spotting or from the end of a period shifts every result.

4. Expecting BBT to Predict Ovulation

A temperature rise confirms ovulation has already passed. It cannot warn you in advance.

5. Assuming Ovulation Is Always Day 14

Day 14 only applies to a textbook 28-day cycle. On other cycle lengths the ovulation day moves.

6. Ignoring Body Changes

Illness, stress, and travel can shift or skip ovulation in a given month, which no calendar can predict.

The date math is exact, but calendar prediction has real limits for any one person.

Treat the output as a planning aid. For fertility or cycle concerns, see a clinician, who can add tests and clinical context.

It is an estimate based on calendar data. It works best for regular cycles and a steady luteal phase, and it is much less reliable with irregular, short, or long cycles because the real day of ovulation shifts from month to month.

The fertile window is six days: the five days before ovulation plus ovulation day. Pregnancy is most likely in the two days before ovulation and on ovulation day, because sperm can survive up to about five days and the egg about 12 to 24 hours.

No. Calendar timing is not a reliable method of contraception. Fertility-awareness methods carry a high typical-use failure rate. If you want to prevent pregnancy, speak to a clinician about effective options.

The luteal phase runs from ovulation to the next period and is relatively constant. It is commonly modelled as 14 days, though the Office on Women's Health notes it can realistically range from about 7 to 19 days.

A typical cycle is about 28 days, and the current authoritative normal range is 24 to 38 days. Cycles shorter or longer than that are flagged by the tool and make calendar estimates less reliable.

No. Basal body temperature rises after ovulation, so it confirms that ovulation has already happened. It does not predict ovulation in advance. An LH ovulation predictor kit is a more direct forward-looking test.

LH kits detect the luteinising hormone surge, which begins about 36 hours before and peaks about 12 hours before ovulation. That makes them a more direct forward signal than calendar math, which only estimates from past cycles.

Enter your shortest and longest recent cycle lengths. The tool then shows a wider fertile range instead of a single day, reflecting that the real timing is uncertain when cycles vary.

Day one is the first day of full menstrual bleeding, not spotting and not the last day of your previous period. Counting from the wrong day shifts every result.

No. The math runs in your browser. Inputs are not sent to our servers, and anything you Save stays only in your own browser storage.

• Wilcox AJ, Weinberg CR, Baird DD. Timing of intercourse in relation to ovulation. NEJM 1995;333:1517-1521 (originating study for the six-day fertile window; accessed 2026-10-05).
• Your Menstrual Cycle (Office on Women's Health; cycle length, luteal phase, BBT and LH timing, accessed 2026-10-05).
• Fertility Awareness-Based Methods of Family Planning (American College of Obstetricians and Gynecologists; accessed 2026-10-05).
• Fertility Awareness-Based Methods for Pregnancy Prevention (Cochrane review, PMC8855505) (confirms BBT is retrospective and FABM efficacy is uncertain; accessed 2026-10-05).