Fertile Window Calculator: Find Your Fertile Days
Enter the first day of your last period and your usual cycle length to see your estimated fertile window. This is for awareness only and is not contraception. It runs in your browser, so nothing is sent or stored.
Enter your details
Calculated in your browser. Nothing you enter is sent or stored. Estimates only, and not a method of contraception.
How the fertile window calculator works
You can only become pregnant on a few days of each cycle. The fertile window starts about five days before ovulation, because sperm can survive in the body for up to five days, and ends about a day after ovulation, when the egg is no longer available. The calculator finds your estimated ovulation day first, and then builds the window around it.
The formula
- Next period = first day of last period + cycle length
- Ovulation = next period minus 14 days
- Fertile window = ovulation minus 5 days, up to ovulation plus 1 day (seven days in total)
Example: your last period started on 1 April and your cycle is 28 days. The next period is due on 29 April, ovulation is estimated for 15 April, and the fertile window runs from 10 to 16 April. The calculator adjusts automatically for a longer or shorter cycle.
Which days matter most if you are trying to conceive
The two days before ovulation and the day of ovulation are generally the most fertile. If you are trying for a pregnancy, having sex every day or every other day through the window gives the best chance, rather than aiming at a single day that may be wrong. The NHS suggests having sex every two to three days through the month, which avoids the need to pinpoint ovulation at all.
If your cycles are irregular
The fertile window moves when your cycle changes. If your cycles are 26 days one month and 34 the next, the window could start more than a week apart. Use the shortest and longest cycle boxes to see the earliest and latest the window could be. If cycles are very irregular, or you have gone more than three months without a period, a calculator will not be much use and a doctor can help work out why. See irregular cycles and PCOS, PCOD and PMOS.
When to see a doctor
See a doctor if you have not conceived after a year of trying (six months if you are 35 or older), if your periods are absent or very irregular, if you have severe pelvic pain, or if you have had more than one miscarriage. If you are trying to avoid pregnancy and want advice, a doctor, clinic or health worker can explain the options and their effectiveness.
The fertile window is not contraception
Please read this part. A calculator uses an average and cannot know when you will actually ovulate. Illness, stress, travel and normal variation can move ovulation by days, so a date marked "not fertile" can still be fertile. ACOG describes fertility awareness methods as less effective than most other methods, and says they need consistent daily tracking of body signs to work well. Cycella shows the fertile window for awareness only and never as a plan for avoiding pregnancy. See the fertile window and why it is not contraception.
Your privacy
This calculator runs only in your browser. What you enter is not sent, stored or shared, and there are no analytics or ads on this site. If you track in the Cycella app, your logs also stay on your phone by default. See our privacy policy.
Frequently asked questions
What is the fertile window?
The fertile window is the days in each cycle when pregnancy is possible: about the five days before ovulation, the day of ovulation, and one day after as a margin for error (seven days in total). It is that long because sperm can survive for up to five days, while the egg survives only about a day.
How do you calculate the fertile window?
Work out ovulation by counting back 14 days from your next expected period, then take the 5 days before ovulation up to 1 day after. The calculator above does this from the first day of your last period and your cycle length.
Which days in the fertile window are most fertile?
The two days before ovulation and the day of ovulation itself are usually the most fertile. Because the exact day is only an estimate, the whole window is worth treating as fertile.
Is the fertile window safe for avoiding pregnancy?
No. A fertile window calculator is not contraception. Ovulation shifts, cycles vary, and the result is an estimate. Talk to a doctor or health worker about methods that are proven to prevent pregnancy.
Does the calculator work if my cycle is irregular?
Only roughly. Enter your shortest and longest cycle to see a wider range, and keep in mind that the real window could fall outside it.
Is my data stored when I use this calculator?
No. Everything is calculated in your browser. Nothing is sent or stored, and this site has no analytics.
Want this in your pocket, without typing dates in each time? Cycella is a free Android period tracker with no account, no ads and no analytics. It learns your own cycle length from what you log, works offline, and is free on Google Play. See what it does for Nepal, or read about how it keeps your data private.