Ovulation Calculator
Estimate when you’re likely to ovulate from your last period and usual cycle length. See your 6-day fertile window, most fertile days and next period, plus the same dates for your next six cycles.
What ovulation and the fertile window are
Ovulation is when an ovary releases an egg, usually once a cycle. The NHS says it happens around 10 to 16 days before the next period in most women — that stretch is the luteal phase — and that sperm can survive for up to 7 days after sex.
The American Society for Reproductive Medicine (ASRM) defines the fertile window as the 6 days ending on the day of ovulation. In the study behind that definition, 221 women trying to conceive became pregnant only from sex during those 6 days, and the chance rose from about 10% five days before ovulation to about 33% on ovulation day. ASRM notes that fertility peaks within the 2 days before ovulation, so the calculator marks those days and ovulation day as your most fertile.
How to use this calculator
Enter the first day of your last period (the first day of real bleeding) and your average cycle length: the days from the first day of one period to the first day of the next, between 20 and 45. Leave the luteal phase at 14 days unless you know yours from ovulation tests or temperature charting (10–16 days). Results update as you type.
How it’s calculated
Day 1 of your cycle is the first day of your period. The calculator counts whole calendar days, so time zones and daylight-saving changes can’t shift a date:
- Next period = first day of last period + cycle length
- Ovulation = next period − luteal phase
- Fertile window = the 5 days before ovulation plus ovulation day
- Most fertile days = the 2 days before ovulation plus ovulation day
- Due date if you conceive = ovulation + 266 days (38 weeks)
For a 28-day cycle that puts ovulation on cycle day 15, 14 days after your period started — the “around day 14” you’ll often read about. With a 14-day luteal phase the due date matches our due date calculator, which adds 280 days to your last period and adjusts for cycle length.
Worked example
Your last period started on September 1, 2026, your cycles average 30 days and the luteal phase is left at 14. The next period is expected on October 1, 2026, so ovulation is estimated for September 17, 2026 (cycle day 17). The fertile window is September 12–17, 2026, the most fertile days are September 15–17, 2026, and the due date if you conceived would be June 10, 2027. The table’s first cycle starts Oct 1, 2026 (fertile window Oct 12–17, 2026; ovulation Oct 17, 2026) and its sixth starts Feb 28, 2027 (fertile window Mar 11–16, 2027). With a 12-day luteal phase, ovulation moves to September 19, 2026 and the fertile window to September 14–19, 2026.
Ovulation day and fertile window by cycle length
From the calculator’s own formula with a 14-day luteal phase. “Day 15” means 14 days after your period started.
| Cycle length | Estimated ovulation | Fertile window | Most fertile days |
|---|---|---|---|
| 21 days | Day 8 | Days 3–8 | Days 6–8 |
| 22 days | Day 9 | Days 4–9 | Days 7–9 |
| 23 days | Day 10 | Days 5–10 | Days 8–10 |
| 24 days | Day 11 | Days 6–11 | Days 9–11 |
| 25 days | Day 12 | Days 7–12 | Days 10–12 |
| 26 days | Day 13 | Days 8–13 | Days 11–13 |
| 27 days | Day 14 | Days 9–14 | Days 12–14 |
| 28 days | Day 15 | Days 10–15 | Days 13–15 |
| 29 days | Day 16 | Days 11–16 | Days 14–16 |
| 30 days | Day 17 | Days 12–17 | Days 15–17 |
| 31 days | Day 18 | Days 13–18 | Days 16–18 |
| 32 days | Day 19 | Days 14–19 | Days 17–19 |
| 33 days | Day 20 | Days 15–20 | Days 18–20 |
| 34 days | Day 21 | Days 16–21 | Days 19–21 |
| 35 days | Day 22 | Days 17–22 | Days 20–22 |
| 40 days | Day 27 | Days 22–27 | Days 25–27 |
| 45 days | Day 32 | Days 27–32 | Days 30–32 |
Limitations: irregular cycles, contraception and advice
Irregular cycles. Calendar estimates assume every cycle is the same length. In more than 600,000 app-tracked cycles, the luteal phase averaged 12.4 days, but the phase before ovulation (16.9 days on average) varied far more, so ovulation day moved around. If your cycles differ by more than a few days, treat these dates as a rough guide; ovulation (LH) tests, temperature or cervical-mucus tracking, or a healthcare provider give a better picture. The NHS describes regular cycles of 21 to 35 days as normal, so if yours are often outside that range, mention it to a doctor.
Not contraception. Please don’t rely on any calendar count to prevent pregnancy. The CDC puts typical-use failure rates for fertility awareness-based methods, which track body signs as well as dates, at 2% to 23% a year. A healthcare provider can help you choose a method.
Not a diagnosis. These are general estimates, not medical advice. If you have questions about your cycle, have been trying to conceive for a while, or think you might be pregnant, talk to a doctor, nurse or midwife.
Frequently asked questions
How does the calculator estimate my ovulation date?
It adds your average cycle length to the first day of your last period to find your next period, then counts back by your luteal phase (14 days unless you change it). In a 30-day cycle that puts ovulation on cycle day 17.
How long is the fertile window?
About 6 days: the 5 days before ovulation plus ovulation day. The American Society for Reproductive Medicine uses this definition, based on research showing that nearly all pregnancies come from sex during those 6 days.
Which days am I most fertile?
The 2 days before ovulation and ovulation day, when the chance of conceiving peaks. Having sex every 1 to 2 days through the fertile window covers them without needing an exact date.
Does the calculator work if my periods are irregular?
Only as a rough guide. It assumes every cycle is the same length, so if yours vary by more than a few days your fertile days may fall outside the dates shown. Ovulation test kits or a healthcare provider can help pinpoint ovulation.
Can I use an ovulation calculator as birth control?
No. A calendar estimate can’t confirm whether or when you ovulate, and cycles shift from month to month, so it isn’t a reliable way to avoid pregnancy. A healthcare provider can help you choose contraception that suits you.
What is the luteal phase, and what should I enter?
It’s the time from ovulation to your next period. The NHS says ovulation usually happens around 10 to 16 days before the next period. If you don’t know your own luteal phase from ovulation tests or temperature charting, leave it at 14 days.
Sources & references
- American Society for Reproductive Medicine — Optimizing natural fertility (2022) — the 6-day fertile window and the most fertile days.
- Wilcox, Weinberg & Baird — Timing of sexual intercourse in relation to ovulation (NEJM, 1995) — the study behind the 6-day fertile window.
- NHS — Periods and fertility in the menstrual cycle — ovulation 10–16 days before the next period; sperm survival; normal cycle lengths.
- Bull et al. — Real-world menstrual cycle characteristics (npj Digital Medicine, 2019) — average luteal phase 12.4 days and follicular phase 16.9 days.
- U.S. Office on Women’s Health — Ovulation calculator — calendar estimates are no guarantee of pregnancy or birth control.
- CDC — Contraception and Birth Control Methods — 2–23% typical-use failure for fertility awareness-based methods.
Last updated: September 2026 ·How we calculate
BriskToolbox provides estimates for general information only and is not medical advice. Talk to a healthcare professional about your individual health.