Ovulation Calculator
Find your fertile window and ovulation day.
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Knowing when you ovulate helps whether you're trying to conceive or simply tracking your cycle. Enter the first day of your last period and your usual cycle length, and this calculator estimates your ovulation day, fertile window, and next period.
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Ovulation — The Quick Answer
Ovulation usually happens about 14 days before your next period — so cycle length minus the luteal phase. The fertile window spans roughly the 5 days before ovulation plus ovulation day itself.
Shortcut
Ovulation day = cycle length − luteal phase (≈14) | Fertile window = ovulation − 5 to +1 days
Ovulation day by cycle length (14-day luteal phase)
| Cycle length | Approx. ovulation day |
|---|---|
| 24 days | Day 10 |
| 28 days | Day 14 |
| 30 days | Day 16 |
| 32 days | Day 18 |
| 35 days | Day 21 |
Sperm can survive up to about 5 days, which is why the fertile window starts before ovulation. The egg itself only survives around 12–24 hours after release.
How It Works
The Method
Counting from your cycle
Formula
Ovulation = LMP + (cycle length − luteal phase) Fertile window = ovulation − 5 days to ovulation + 1 day Next period = LMP + cycle length
Variables
Days per cycle
From the first day of one period to the first day of the next — commonly 28, but 21–35 is all normal.
Post-ovulation phase
The days from ovulation to your next period, usually fairly constant at around 14 days.
Days of highest fertility
The roughly six days ending on ovulation day, when conception is most likely.
Note: Calendar estimates assume regular cycles. For more precision, ovulation predictor kits, basal body temperature tracking, and cervical mucus monitoring detect ovulation directly.
Worked Example: A 28-Day Cycle
Finding ovulation and the fertile window
Note the first day of the last period
Call it cycle day 1
Subtract the luteal phase from cycle length
28 − 14 = ovulation around day 14
Open the fertile window five days earlier
Sperm survive up to ~5 days → window starts about day 9
Close it the day after ovulation
Egg survives ~12–24 h → window ends about day 15
Reference Guide
| Cycle length | Approx. ovulation day | Fertile window |
|---|---|---|
| 24 days | Day 10 | Days 5–11 |
| 28 days | Day 14 | Days 9–15 |
| 30 days | Day 16 | Days 11–17 |
| 32 days | Day 18 | Days 13–19 |
| 35 days | Day 21 | Days 16–22 |
How to Read Your Fertile Window
What the dates mean — and an important limit
Conception is most likely in the five days before ovulation plus ovulation day itself. The days just before ovulation tend to be the most fertile, because sperm can already be waiting when the egg is released.
Best for: Timing when trying to conceive
Ovulation happens roughly 14 days before your next period, not necessarily on day 14. The second half of the cycle is fairly constant while the first half varies — which is why a longer cycle means later ovulation.
Best for: Estimating ovulation when your cycle isn't 28 days
Calendar estimates assume regular cycles, but ovulation can shift with stress, illness, and natural variation, and sperm survive for days. Do not rely on this to avoid pregnancy. For dependable contraception, use a proven method and speak to a healthcare provider. This tool is not medical advice.
Best for: Understanding your cycle, not preventing pregnancy
How the Fertile Window Works
Each menstrual cycle has only a short window in which conception is possible — about six days, ending on ovulation day. Ovulation is when a mature egg is released, and it usually happens around 14 days before the next period starts, because the luteal phase (ovulation to period) stays fairly constant while the follicular phase (period to ovulation) varies. This calculator estimates ovulation and the fertile window from your last period and cycle length. It's a useful guide for understanding your cycle or trying to conceive, but ovulation timing varies month to month, so it is an estimate — and it should never be used as a form of contraception.
Key Features
💡 Pro Tips
- →Enter your true average cycle length rather than assuming 28 days — it shifts ovulation and the whole fertile window.
- →For more accuracy than a calendar can give, combine this with ovulation predictor kits, basal body temperature tracking, or cervical mucus changes.
- →If you're trying to conceive, the days just before ovulation are generally the most fertile, so don't wait for ovulation day itself.
- →Track a few cycles to learn your own pattern; one irregular month can move ovulation by several days.
Common Mistakes
Assuming everyone ovulates on day 14
Day 14 only fits a textbook 28-day cycle. Ovulation is better estimated as about 14 days before the next period, so a 32-day cycle ovulates nearer day 18.
Using the calendar method as birth control
Ovulation timing varies and sperm survive for days, so calendar prediction is not reliable contraception. Use a proven method if you want to avoid pregnancy.
Timing intercourse only on ovulation day
The egg lives less than a day, but sperm can survive several. The days leading up to ovulation are often the most fertile, so focusing on a single day can miss the best window.
Expecting calendar estimates to be exact
Stress, illness, travel, and natural variation shift ovulation. For irregular cycles especially, direct methods like ovulation tests are far more reliable than dates alone.
Research & Citations
Sources and standards referenced for the factual claims on this page
- [1]
Wilcox, A. J., Weinberg, C. R., & Baird, D. D. (1995). Timing of Sexual Intercourse in Relation to Ovulation. New England Journal of Medicine, 333(23).
Defined the ~6-day fertile window relative to ovulation.
View source - [2]
American College of Obstetricians and Gynecologists (ACOG) (2019). Fertility Awareness-Based Methods of Family Planning. ACOG.org.
On the menstrual cycle, fertile window, and limits of calendar methods.
View source - [3]
National Health Service (NHS) (2023). Periods and fertility in the menstrual cycle. NHS.uk.
Plain-language guidance on ovulation timing and conception.
View source
This calculator is a reference tool, not a substitute for professional advice. Verify important health, financial, legal, or safety decisions with a suitably qualified professional.
Last updated: October 8, 2026
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Frequently Asked Questions
When do I ovulate?
Typically about 14 days before your next period starts. For a 28-day cycle that's around day 14; for a 32-day cycle, around day 18. This calculator estimates it from your cycle length and luteal phase.
What is the fertile window?
The roughly six days when conception is most likely: the five days before ovulation plus ovulation day. It reflects that sperm can survive several days while the egg lives for less than a day.
Can I use this as birth control?
No. Calendar-based estimates are not a reliable form of contraception because ovulation timing varies and sperm survive for days. If you want to avoid pregnancy, use a proven contraceptive method and speak to a healthcare provider.
Why might my ovulation estimate be wrong?
Cycle length varies month to month, and stress, illness, travel, and some conditions can shift ovulation. Irregular cycles make calendar estimates less reliable — ovulation test kits or temperature tracking are more accurate.