Ovulation Calculator for a Regular Cycle
Enter the first day of your last period and your usual cycle length to see a presumed ovulation day, a six-day fertile window and your next period on a calendar.
Not birth control. A calendar estimate for pregnancy planning, for regular cycles only.
A calendar estimate isn't a good fit right now
This calculator only counts days on a regular cycle, so it doesn't show dates for the answers you chose:
What can help instead: a doctor, nurse practitioner or midwife can help now. If your periods are irregular or absent, or you have unexplained bleeding, you don't need to wait before asking. At-home ovulation tests (which check urine for the LH hormone) can help some people time sex or insemination, though they can be less reliable with PCOS or some fertility medicines.
Your estimate
- Estimated next period
- Presumed ovulation
- Six-day fertile window
The fertile window is the presumed ovulation day and the five days before it. It's a planning reference, not a “safe” or “unsafe” list of days.
- Day 1 of your last period
- Fertile window (6 days)
- Presumed ovulation
- Estimated next period
- Today (bold, underlined)
How this was worked out
Other calculators sometimes count 14 days back from the expected next period instead, which lands one day later. Both are rough calendar rules; neither is more precise for your body.
Keep in mind: even regular cycles vary from month to month, so ovulation can come earlier or later than this. At-home ovulation tests can narrow it down. If you're trying to get pregnant, a doctor, nurse practitioner or midwife can help; the usual advice is to get checked after 12 months of trying if you're under 35, after 6 months if you're 35 or older, and sooner if you're over 40 or your periods are irregular or absent.
The copy or file includes your period dates. Where it goes is up to you; nothing is sent to ToolNeko.
How to use the ovulation calculator
- Pick the first day of your most recent period (day 1).
- Enter your average cycle length in days, from 24 to 38.
- Answer the two questions about your cycle. They decide whether a calendar estimate fits you; nothing is preselected.
- Select Calculate to see your estimated next period, presumed ovulation day and six-day fertile window, marked on a calendar.
How the estimate is worked out
This calculator uses the calendar rule described by the American Society for Reproductive Medicine (ASRM): in a regular cycle, ovulation is presumed on cycle day (cycle length − 14), counting the first day of your period as day 1. The fertile window is that day and the five days before it, because sperm can survive up to 5 days and the egg 12 to 24 hours.
Example: a period that started on September 1, 2026, with a 28-day cycle, gives cycle day 28 − 14 = 14, which is September 14. The six-day window is September 9–14, and the next period is expected on September 29. With a 30-day cycle the presumed ovulation day is day 16.
Some calculators count 14 days back from the expected next period instead, which gives September 15 for the same example. That one-day difference is a difference in counting rules, not in accuracy.
When a calendar estimate doesn't fit
Counting days only works when cycles are fairly regular. The calculator shows no dates if your cycles are irregular or you aren't sure, or if you're postpartum, breastfeeding or chestfeeding, use or recently stopped or changed hormonal birth control, are in perimenopause, have irregular bleeding, or have an ovulation condition or take a medication that affects your cycle. In those situations a doctor, nurse practitioner or midwife can give you better information; at-home ovulation tests can help some people, though they can be less reliable with PCOS.
Not birth control
The fertile window is for trying to get pregnant. It isn't a list of “safe” days: ovulation can move from month to month, and pregnancy can happen outside these dates. CDC puts the typical-use failure rate of fertility-awareness methods at 2–23%. If you want to avoid pregnancy, ask a doctor, nurse practitioner or pharmacist about an effective method.
Privacy
Everything is calculated in your browser. Your dates and answers aren't added to the page address, stored in the browser or sent to ToolNeko, and there's no email or cloud-calendar link. Site analytics record only actions such as calculating or copying, never dates or answers. Copying or downloading the summary happens only when you choose it.
Method and advice checked against the ASRM committee opinion, the Office on Women's Health and the CDC pages listed below on September 25, 2026. This page gives general information, not medical advice, and hasn't been reviewed by a clinician.
Frequently asked questions
Which day counts as day 1, and how do I find my cycle length?
Day 1 is the first day of full flow, not spotting. To find your cycle length, count from the first day of one period to the first day of the next. If it changes a little, use the average of your last few cycles. This calculator covers average cycles of 24 to 38 days.
Why do ovulation calculators give different dates?
They use different calendar rules. This one uses the ASRM rule: ovulation is presumed on cycle day (cycle length − 14), counting the first day of your period as day 1, so a 28-day cycle gives day 14. Some calculators count 14 days back from the expected next period instead, which lands one day later. Both are rough rules, and neither measures your body.
Can I use the fertile window as birth control?
No. The six days are a reference for trying to get pregnant, not a list of safe days. Ovulation can come earlier or later than a calendar predicts, even in regular cycles, and pregnancy is possible outside these dates. For birth control, talk with a doctor, nurse practitioner or pharmacist about a reliable method.
What if my cycles are irregular, or I just had a baby?
Then a date counted from one period isn't a good guide, so this calculator won't show one. The same goes if you're breastfeeding or chestfeeding, use or recently stopped or changed hormonal birth control, are in perimenopause, have irregular bleeding, or have a condition or medication that affects ovulation. A doctor, nurse practitioner or midwife can help, and you don't need to wait if your periods are irregular or absent.
How accurate is this estimate?
It's a starting point, not a measurement. The six-day window covers the days when sex or insemination can lead to pregnancy, and the calendar only guesses where it falls. At-home ovulation tests, which detect the LH hormone surge before ovulation, can narrow it down; some people also track cervical mucus or basal body temperature. This page can't tell you whether you ovulated.
When should I talk to a doctor?
See a doctor, nurse practitioner or midwife if your cycles are often shorter than 24 days or longer than 38, change a lot from month to month, or if you have bleeding between periods or no period for 90 days when you aren't pregnant or breastfeeding. If you're trying to get pregnant, the usual advice is to get checked after 12 months of trying if you're under 35, after 6 months if you're 35 or older, and sooner if you're over 40 or your periods are irregular or absent.
Is what I enter saved or sent anywhere?
No. Your dates and answers stay in this browser tab and are gone when you reset or close it. They aren't put in the page address, saved in the browser or sent in analytics; site analytics only count actions like calculating or copying. Copying or downloading a summary is your choice, and the copy is yours to keep or delete.