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Browse all calculatorsEnter the first day of your last period and your cycle length to see an estimated ovulation day, fertile window, pregnancy test date and due date. An estimate only, not contraception.
An estimate, not contraception. Ovulation calculators give estimates, and cycles vary from month to month. The fertile window is not a method of contraception and not a way to find safe days: ovulation can fall on a different day, so pregnancy is possible outside the window. Only a doctor, midwife or an ultrasound dating scan can confirm dates or assess fertility. These dates are worked out for cycles of typical length.
This is a general estimate for educational use. It is not medical advice, a diagnosis or a treatment plan. Talk to a doctor or other qualified professional before you make any decision about contraception, pregnancy or health care.
Cycle length is counted from the first day of one period to the first day of the next, from 21 to 45 days. The luteal phase is the number of days from ovulation to the next period; 14 is the usual starting point and the NHS gives 10 to 16.
| S | M | T | W | T | F | S |
|---|
Each cycle repeats the same cycle length and luteal phase. The highlighted row is the first cycle. Scroll sideways for every column.
| Cycle | Period starts | Fertile window | Ovulation day | Next period | Due date if conceived |
|---|
Private by design. Everything runs in your browser and is never uploaded.
An ovulation calculator estimates the day an egg is released and the fertile window around it. It adds your cycle length to the first day of your last period, then counts back the luteal phase, usually 14 days. The fertile window is ovulation day plus the 5 days before it. Every date is an estimate.
Type the first day of your last period and your usual cycle length, and the answer appears at once, with a month calendar, a table of the next six cycles and the working. For period dates a year ahead use the period calculator; to work back from a due date use the pregnancy conception calculator. This page is for learning, and the fertile window is not a method of contraception.
| Cycle and luteal phase | Ovulation day | Fertile window | Next period | Due date if conceived |
|---|---|---|---|---|
| 24 days, luteal 14 | Nov 19, 2026 | Nov 14 to Nov 19, 2026 | Dec 3, 2026 | Aug 12, 2027 |
| 28 days, luteal 14 | Nov 23, 2026 | Nov 18 to Nov 23, 2026 | Dec 7, 2026 | Aug 16, 2027 |
| 28 days, luteal 12 | Nov 25, 2026 | Nov 20 to Nov 25, 2026 | Dec 7, 2026 | Aug 18, 2027 |
| 28 days, luteal 16 | Nov 21, 2026 | Nov 16 to Nov 21, 2026 | Dec 7, 2026 | Aug 14, 2027 |
| 32 days, luteal 14 | Nov 27, 2026 | Nov 22 to Nov 27, 2026 | Dec 11, 2026 | Aug 20, 2027 |
| 35 days, luteal 14 | Nov 30, 2026 | Nov 25 to Nov 30, 2026 | Dec 14, 2026 | Aug 23, 2027 |
The first answer is already on screen from a sample date. Change any value and every date updates at once, with no Calculate button.
Pick the first day of your most recent period with the date picker, or type it as 2026-11-09 or Nov 9, 2026. The sample date is 7 days before today.
Cycle length is the days from the first day of one period to the first day of the next, from 21 to 45. The luteal phase starts at 14 days and can be set from 10 to 16.
You get the most likely ovulation day, the 6-day fertile window, the peak days, a pregnancy test date, the next period and a due date if conception happened that cycle.
Look at the calendar and the table for the next six cycles, open the working, then copy the result or share a link that opens with the same date and cycle.
All dates are whole calendar days. The rules below are the ones used in the calculator, each with the page it comes from, listed under Sources.
| Date | Rule used | What the sources say |
|---|---|---|
| Next period | last period + cycle length |
The NHS says the average is a period around every 28 days and gives 21 to 35 days as the usual range. |
| Ovulation day | next period - luteal phase (14 days unless changed, 10 to 16 allowed) |
The NHS says that in most women ovulation happens around 10 to 16 days before the next period. NICHD says ovulation occurs at about day 14 of an average 28-day cycle. |
| Fertile window | ovulation day - 5 days to ovulation day, 6 days |
A 2022 review says the window usually begins about 5 days before ovulation and ends within 12 to 24 hours after it, and is generally considered to be about 6 days. NICHD gives a 6-day window that ends on ovulation day. |
| Peak days | ovulation day - 2 days to ovulation day |
NICHD names the 2 days before ovulation through ovulation day as the best time to try to get pregnant. |
| Pregnancy test from | The predicted first day of the next period | The NHS says tests are most reliable from the first day of a missed period, and some very sensitive tests can be used earlier. |
| Due date if conceived | ovulation day + 266 days |
A 2016 dating study says the due date has traditionally been worked out by adding 280 days to the last period, and that ovulation-based dating subtracts 14 days from the ovulation date. 266 is 280 minus 14. |
Cycle day 1 is the first day of the period. Because ovulation is counted back from the next period, a longer cycle puts ovulation later. With a 14-day luteal phase a 28-day cycle gives ovulation on cycle day 15, and the 6-day fertile window covers cycle days 10 to 15.
NICHD says ovulation occurs at about day 14 of an average 28-day cycle, and the NHS says women are likely to be fertile around day 14. Counting back 14 days from the next period gives day 15 when the first day of the period is day 1. A difference of one day is well inside the uncertainty of any calendar method.
The last two columns show how a luteal phase of 12 or 16 days moves the ovulation day for the same cycle length. The table is arithmetic from the rules above, not a measurement of any person.
| Cycle length | Ovulation, luteal 14 | Fertile window | Ovulation, luteal 12 | Ovulation, luteal 16 |
|---|---|---|---|---|
| 21 days | 8 | 3 to 8 | 10 | 6 |
| 24 days | 11 | 6 to 11 | 13 | 9 |
| 26 days | 13 | 8 to 13 | 15 | 11 |
| 28 days | 15 | 10 to 15 | 17 | 13 |
| 30 days | 17 | 12 to 17 | 19 | 15 |
| 32 days | 19 | 14 to 19 | 21 | 17 |
| 35 days | 22 | 17 to 22 | 24 | 20 |
| 38 days | 25 | 20 to 25 | 27 | 23 |
| 40 days | 27 | 22 to 27 | 29 | 25 |
| 45 days | 32 | 27 to 32 | 34 | 30 |
Many pages ask for a date and a cycle length, then show one date after a button press. These are the differences that matter when you want to understand the dates.
| Feature | Basic ovulation calculator | This calculator |
|---|---|---|
| Results | Often needs a Calculate button and a page reload | Updates as you type, with an answer on first load |
| Luteal phase | Fixed at 14 days, not shown | Starts at 14 and can be set from 10 to 16, with the NHS range beside it |
| Dates shown | An ovulation window and a few dates | Ovulation day, the 6-day fertile window, peak days, test date, next period and due date |
| Six cycles | A short list of start dates | A table with the window, ovulation day, next period and due date for each cycle, plus a month calendar |
| Working | Answer only | Every step written out, so you can check the arithmetic by hand |
| Safety wording | Often a small note at the foot of the page | An estimate and not-contraception note beside the result, and a note when the cycle is outside the usual range |
| Sources | Rarely listed | Pages from the NHS, NICHD and peer-reviewed articles are listed, with what each says |
| Sharing | Rare | Copy the result, or share a link that opens with the same date and cycle |
Find which days of the next few cycles fall inside the estimated window, and how the calendar looks from one month to the next.
The predicted next period date is the first day a test is most reliable, according to the NHS. Change the cycle length and watch that date move.
Change it from 12 to 16 days and see the ovulation day shift by the same number of days while the next period stays put.
If conception happened this cycle, the due date is shown. The due date calculator works it out from a period, a conception date, an IVF transfer or a scan.
Once a pregnancy is confirmed, the pregnancy calculator shows the week, trimester and key dates from a due date or a last period.
Copy a plain-text summary or share a link that opens with the same date and cycle length, for a partner or a clinician to look at.
A calendar estimate uses two numbers you type. It cannot see your hormones, so it can be off by days. These are the points the sources on this page make.
Not contraception. The fertile window is not a method of contraception and not a way to find safe days. Ovulation can fall on a different day, so pregnancy is possible outside the window. Only a doctor or midwife can assess fertility.
A 2022 review of fertility awareness methods says calendar formulas are not precise enough to provide reasonable estimates of the day of ovulation.
A 2016 dating study notes that only about 10 percent of women with a regular 28-day cycle ovulate on day 14, and that ovulation leading to pregnancy has been shown to occur anywhere from day 9 to day 30.
The same 2022 review says the time before ovulation is inherently more variable than the time after it, so the dates here are counted back from the next period.
NICHD says an egg can be fertilized for only a short time, usually 8 to 12 hours, and that sperm can stay functional for 5 days. The NHS says sperm can survive up to 7 days in the fallopian tubes.
The NHS says tests are most reliable from the first day of a missed period. A negative result taken too early is less reliable; the NHS suggests waiting a few days and trying again.
A doctor, midwife or an ultrasound dating scan can confirm dates. The NHS says the 12-week scan can more accurately estimate how many weeks pregnant you are.
It does not tell you which days are safe. For contraception options, speak to a doctor or nurse. The NHS page on fertility in the menstrual cycle explains the cycle in plain words.
It uses the last period date, one cycle length and one luteal phase, and repeats them for six cycles. Real cycles change. NICHD describes the timing of conception for an average 28-day cycle.
Cycle lengths from 21 to 45 days are accepted, with a note above the 35-day end of the NHS range. It does not replace a test: see the NHS guide to doing a pregnancy test.
Every rule and range on this page comes from the pages below, which were opened while building it. The fertile window is counted as the 5 days before ovulation plus ovulation day, six days in all. NICHD words its window as the 6 days before ovulation through the day of ovulation, and the 2022 review gives about 5 days before ovulation and about 6 days in total, so this page uses 6 days. The 266-day figure is 280 minus 14, as the 2016 dating study describes for ovulation-based dating. It is arithmetic from that statement, not a number the study prints. Where sources differ, for example on how long sperm survive, both figures are given.
Short answers about ovulation dates, the fertile window and the limits of this calculator.
An ovulation calculator estimates the day an egg is released and the days around it when a pregnancy can start. It works from the first day of your last period, your cycle length and the length of the luteal phase. The result is an estimate, not a measurement.
Add your cycle length to the first day of your last period to find the next period, then count back the luteal phase, 14 days by default. With a period that started on Nov 9, 2026 and a 28-day cycle, the next period is Dec 7, 2026 and ovulation is Nov 23, 2026, which is cycle day 15 when the first day of the period is day 1.
This calculator uses 6 days: the 5 days before ovulation plus ovulation day. A 2022 review of fertility awareness methods says the window usually begins about 5 days before ovulation and ends within 12 to 24 hours after it. NICHD says an egg can be fertilized for only a short time, usually 8 to 12 hours, and that sperm can stay functional for 5 days.
No. The fertile window is an estimate, not a method of contraception, and it is not a way to find safe days. Ovulation can fall on a different day from the one shown, and the 2022 review says calendar formulas are not precise enough to estimate the day of ovulation. A doctor or nurse can explain contraception options.
Ovulation moves with the length of the cycle, because this calculator keeps the luteal phase at 14 days unless you change it. A 24-day cycle puts ovulation on cycle day 11 and a 35-day cycle on cycle day 22, counting the first day of the period as day 1.
The luteal phase is the time from ovulation to the start of the next period. This calculator starts at 14 days and lets you pick 10 to 16, because the NHS says that in most women ovulation happens around 10 to 16 days before the next period. A different number moves the ovulation day earlier or later by the same number of days.
The NHS says pregnancy tests are most reliable from the first day of a missed period, and that some very sensitive tests can be used earlier. This calculator shows the predicted next period date for that reason. If you do not know when your period is due, the NHS says to test at least 21 days after unprotected sex.
The due date shown assumes conception on the estimated ovulation day and adds 266 days, which is the usual 280-day count from the last period minus the 14 days to ovulation. For a 28-day cycle it matches the last period plus 280 days. Only a dating scan, doctor or midwife can confirm a due date, and the Due Date Calculator covers more methods.
It is a rough guide. A 2016 study of pregnancy dating notes that only about 10 percent of women with a regular 28-day cycle ovulate on day 14, and that ovulation leading to pregnancy has been shown to occur anywhere from day 9 to day 30 of the cycle. Cycles also vary from month to month, so treat every date as an estimate.
This calculator uses the one cycle length you type, so cycles that change a lot make every date less certain. It accepts 21 to 45 days, and for cycles longer than the 21 to 35 days the NHS gives as the usual range it adds a note. A doctor can look at cycles that are very short, very long or hard to predict.
Some centre a five-day window on the ovulation day, some add a longer window for trying to conceive, and some place the pregnancy test on a different day. This page counts 6 days ending on ovulation day, lets you change the luteal phase and puts the test date at the predicted next period. For a 28-day cycle the ovulation day itself is the same.
No. The calculation runs in your browser and nothing is sent to a server or stored on your device. The date and cycle length appear in the page address only after you change a value or tap Share link.