Ovulation Calculator
Use this ovulation calculator to estimate your ovulation date, fertile window, and next period from your last period and cycle length.
Updated October 22, 2026
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Your Ovulation Date Is Calculated
Ovulation is when an ovary releases an egg. This calculator counts back from your next expected period to estimate that day, then marks the six-day fertile window that ends on it. It is a calendar estimate of when ovulation is likely, not a record of when it happened.
What Your Ovulation Estimate Measures
The calculator estimates two things: the day you are likely to ovulate, and the fertile window around it. In a study of 221 women who tracked ovulation with daily urine hormone tests, pregnancy happened only after sex in the six days ending on the day of ovulation¹. The chance of conception rose from about 10% five days before ovulation to about 33% on the day of ovulation¹. The calculator cannot tell you whether you ovulated this cycle.
The Formula
Ovulation cycle day = cycle length minus luteal phase length
Fertile window = 5 days before ovulation, through the day of ovulation
Next period = first day of last period plus cycle length
The luteal phase is the time after ovulation, up to your next period. The calendar method assumes it lasts about 14 days and counts back from the next period². Counting back works better than counting forward because the first half of the cycle varies much more. In a study of over 600,000 cycles, the time from period start to ovulation had a 95% range of 10 to 30 days, while the luteal phase ranged from 7 to 17 days³.
How This Calculator Computes Your Result
Enter the first day of your last period and your average cycle length (21 to 45 days, default 28). You can also enter your luteal phase length (10 to 16 days, default 14) and how many days your cycle length usually varies. Cycle day 1 is the first day of bleeding, and all results are whole days. If your last period was more than one cycle ago, the calendar moves forward to your current cycle. You get your ovulation date, fertile window, next period, and a table for the next 3 cycles.
What Your Result Means
Result | What it shows |
Estimated ovulation date | The day your cycle length and luteal phase point to, with its cycle day |
Fertile window | The six days ending on the estimated ovulation day¹ |
Highest chance days | Ovulation day and the two days before it, when the chance is highest² |
Possible range | Only if you enter variability: the earliest and latest dates if your cycle runs short or long |
Next period | First day of last period plus cycle length |
Next 3 cycles | The same estimates repeated forward, less certain the further ahead |
A result of cycle day 14 does not mean you will ovulate that day. Treat the fertile window as a planning guide. The American Society for Reproductive Medicine (ASRM) notes that sex every 1 to 2 days during the fertile window gives the best odds per cycle, and cautions that strict timing can add stress².
How Cycle Length Changes Your Ovulation Day
Ovulation falls on day 14 only in a 28-day cycle with a 14-day luteal phase. This table uses the default 14-day luteal phase.
Cycle length | Ovulation cycle day | Fertile window (cycle days) |
21 days | 7 | 2 to 7 |
24 days | 10 | 5 to 10 |
26 days | 12 | 7 to 12 |
28 days | 14 | 9 to 14 |
30 days | 16 | 11 to 16 |
32 days | 18 | 13 to 18 |
35 days | 21 | 16 to 21 |
40 days | 26 | 21 to 26 |
Real cycles are not always 28 days. In the 600,000-cycle study, the average cycle was 29.3 days, with a follicular phase (period start to ovulation) of 16.9 days and a luteal phase of 12.4 days³. That luteal phase is shorter than the 14 days most calculators assume. If you know yours is shorter, you can enter it, and your ovulation date will move later.
Why Your Real Fertile Window May Differ
Timing varies a lot, even for people with regular cycles. In a study of 221 women and 696 cycles, every day from cycle day 6 to day 21 had at least a 10% chance of falling in a woman's fertile window⁴. Only about 30% of women had their whole window inside days 10 to 17, the range often quoted in guidelines⁴. The authors called the timing highly unpredictable, even with regular cycles⁴. ASRM also cites a study of 949 volunteers where calendar-based apps predicted the exact day of ovulation with at most 21% accuracy².
How to Confirm Ovulation
A calendar cannot confirm ovulation. These methods look for signs of it.
LH test strips detect the surge in luteinizing hormone before ovulation. ASRM says ovulation may occur any time in the 2 days after the surge, and false positives happen in about 7% of cycles².
Basal body temperature charts show a rise after ovulation, so they confirm it after the fact, not before³.
Cervical mucus changes predicted fertile days as well as or better than temperature or LH monitoring in the research ASRM reviewed².
A doctor can check for ovulation with a blood progesterone test in the second half of the cycle⁵.
ACOG lists these same methods and notes that a temperature chart can confirm ovulation but cannot predict it⁵.
Ovulation Day and Fertile Window by Cycle Length
Each bar is one cycle, starting on day 1 of your period. The fertile window ends on the ovulation day, and longer cycles move both later.
- Fertile window
- Estimated ovulation day
- Other cycle days
Show data table
| Cycle length | Fertile window | Estimated ovulation day | Other cycle days |
|---|---|---|---|
| 21-day cycle | Fertile window: cycle days 2 to 7, ending on ovulation day | Estimated ovulation: cycle day 7, the last day of the fertile window | Cycle day 1, Cycle days 8 to 21 (14-day luteal phase) |
| 24-day cycle | Fertile window: cycle days 5 to 10, ending on ovulation day | Estimated ovulation: cycle day 10, the last day of the fertile window | Cycle days 1 to 4, Cycle days 11 to 24 (14-day luteal phase) |
| 26-day cycle | Fertile window: cycle days 7 to 12, ending on ovulation day | Estimated ovulation: cycle day 12, the last day of the fertile window | Cycle days 1 to 6, Cycle days 13 to 26 (14-day luteal phase) |
| 28-day cycle | Fertile window: cycle days 9 to 14, ending on ovulation day | Estimated ovulation: cycle day 14, the last day of the fertile window | Cycle days 1 to 8, Cycle days 15 to 28 (14-day luteal phase) |
| 30-day cycle | Fertile window: cycle days 11 to 16, ending on ovulation day | Estimated ovulation: cycle day 16, the last day of the fertile window | Cycle days 1 to 10, Cycle days 17 to 30 (14-day luteal phase) |
| 32-day cycle | Fertile window: cycle days 13 to 18, ending on ovulation day | Estimated ovulation: cycle day 18, the last day of the fertile window | Cycle days 1 to 12, Cycle days 19 to 32 (14-day luteal phase) |
| 35-day cycle | Fertile window: cycle days 16 to 21, ending on ovulation day | Estimated ovulation: cycle day 21, the last day of the fertile window | Cycle days 1 to 15, Cycle days 22 to 35 (14-day luteal phase) |
| 40-day cycle | Fertile window: cycle days 21 to 26, ending on ovulation day | Estimated ovulation: cycle day 26, the last day of the fertile window | Cycle days 1 to 20, Cycle days 27 to 40 (14-day luteal phase) |
A Note on Accuracy
This is a calendar estimate based on averages. It is not birth control, and your real fertile window can fall on days outside the dates shown. It is also not a fertility test or a diagnosis. Irregular cycles, a recent change in hormonal birth control, recent childbirth or breastfeeding, polycystic ovary syndrome (PCOS), and the years near menopause can all make the dates less reliable. If you have been trying to conceive for 12 months (6 months if you are 35 or older), consider talking to a doctor².
Frequently Asked Questions
How many days after your period do you ovulate?
With a 28-day cycle and a 14-day luteal phase, ovulation lands on cycle day 14, which is about two weeks after your period starts. In a study of over 600,000 cycles, the average time from period start to ovulation was 15.4 days in 28-day cycles, and 16.9 days across all cycles³.
How long is the fertile window?
The fertile window is six days: the five days before ovulation and the day of ovulation¹. Pregnancy was not seen from sex outside those six days in the study that defined it¹.
How accurate is an ovulation calculator?
It gives a useful planning window, but not an exact day. Calendar-based apps predicted the day of ovulation correctly in at most 21% of cases in one study cited by ASRM², and fertile windows vary widely even with regular cycles⁴.
Can I use an ovulation calculator as birth control?
No. Timing varies from cycle to cycle, and the fertile window can fall on days a calendar does not predict⁴.
Sources
Timing of Sexual Intercourse in Relation to Ovulation, Wilcox et al., 1995
Optimizing Natural Fertility, ASRM Practice Committee, 2021
Real-World Menstrual Cycle Characteristics of More Than 600,000 Menstrual Cycles, Bull et al., 2019
The Timing of the Fertile Window in the Menstrual Cycle, Wilcox et al., 2000
Evaluating Infertility, ACOG, 2020