Ovulation Calculator
Enter the first day of your last period and your usual cycle length to estimate when you ovulate, your most fertile days and your next period, with a 6-month calendar you can add to your phone.
Estimate only. Cycles vary, and ovulation can come earlier or later than predicted. This calculator is for planning and information – it is not medical advice and must not be used as contraception. Nothing you enter leaves your device.
Your next 6 months
Upcoming cycles
| Cycle | Period starts | Fertile window | Ovulation | Next period |
|---|
How to use the Ovulation Calculator
- Enter the first day of your last period – the day bleeding started, not the day it ended.
- Set your average cycle length, counted from the first day of one period to the first day of the next. 28 days is typical, but anything from 21 to 35 is common.
- Adjust the luteal phase and period length if you know them. If not, leave the defaults of 14 and 5 days.
- Read your estimates: ovulation day, fertile window, next period and the earliest reliable pregnancy test date.
- Check the 6-month calendar and press Download calendar to add every fertile window and expected period to your calendar app.
How ovulation is estimated
A menstrual cycle has two halves. The follicular phase runs from the first day of your period until ovulation and is the part that varies most between people and between cycles. The luteal phase runs from ovulation to the next period and is usually close to 14 days (normally 10 to 16). That is why the calculator works backwards from your next expected period:
- Next period = first day of last period + cycle length
- Ovulation = next period − luteal phase length
- Fertile window = 5 days before ovulation to 1 day after
- Pregnancy test = from the day your next period is due
Example: your last period started on March 1 and your cycles are 30 days long. The next period is due on March 31, so ovulation is estimated on March 31 − 14 = March 17, and the fertile window runs from March 12 to March 18.
Ovulation day by cycle length
With a typical 14-day luteal phase, the expected ovulation day (counting the first day of your period as day 1) moves with your cycle length:
| Cycle length | Ovulation around | Fertile days |
|---|---|---|
| 24 days | Day 11 | Days 6–12 |
| 26 days | Day 13 | Days 8–14 |
| 28 days | Day 15 | Days 10–16 |
| 30 days | Day 17 | Days 12–18 |
| 32 days | Day 19 | Days 14–20 |
| 35 days | Day 22 | Days 17–23 |
You may see “day 14” quoted for a 28-day cycle. That counts the days after your period started (day 0 = first day of bleeding), while the table above counts the first day as day 1 – both describe the same date.
Signs that ovulation is near
- Cervical mucus becomes clear, slippery and stretchy, like raw egg white, in the days before ovulation.
- LH surge: ovulation predictor kits detect the rise in luteinising hormone 24 to 36 hours before ovulation.
- Basal body temperature rises by about 0.2–0.5 °C (0.4–1 °F) after ovulation and stays higher until your period, which confirms that ovulation has happened.
- Some people notice mild one-sided pelvic pain (mittelschmerz), breast tenderness or a higher sex drive.
Tracking one or more of these signs alongside the calendar estimate gives a much clearer picture of your own cycle.
Adding the dates to your calendar
The downloaded .ics file contains all-day events for each expected period, fertile window and ovulation day over the next six months. Open it on your phone or import it into Google Calendar (Settings › Import), Apple Calendar (File › Import) or Outlook. The file is created in your browser and contains only the dates shown on this page.
Frequently asked questions
How is the ovulation date calculated?
Ovulation usually happens about 14 days before the next period starts, because the luteal phase (from ovulation to the period) is fairly constant. The calculator adds your cycle length to the first day of your last period to find the next period, then subtracts your luteal phase length. For a 28-day cycle that is day 14.
What is the fertile window?
It is the roughly six days in each cycle when sex can lead to pregnancy: the five days before ovulation and the day of ovulation itself, plus a day after to allow for timing errors. Sperm can survive up to five days, while the egg lives only about 12 to 24 hours, so the two or three days before ovulation are the most fertile.
How accurate is an ovulation calculator?
It gives a good estimate for people with regular cycles, but ovulation can shift by several days from one cycle to the next because of stress, illness, travel or hormonal changes. For more certainty, combine it with ovulation predictor kits (LH tests), cervical mucus changes or basal body temperature tracking.
Can I use this calculator to avoid pregnancy?
No. Calendar estimates are not reliable enough to be used as contraception, because ovulation can happen earlier or later than predicted. If you want to avoid pregnancy, talk to a doctor or nurse about effective birth control methods.
When should I take a pregnancy test?
Most home tests are reliable from the first day of your missed period, which is about 14 days after ovulation. Some sensitive tests can detect pregnancy a few days earlier, but a negative result before your period is due may be wrong, so test again if your period doesn't arrive.
What if my cycles are irregular?
Use the average of your last few cycles, and treat the results as a rough guide only. If your cycles vary by more than about a week, are shorter than 21 days or longer than 35, or you have been trying to conceive for a year (six months if you are over 35), it's worth seeing a doctor.