Ovulation calculator: when am I most fertile?
Your likely fertile days for this cycle and the next two, worked out on your phone.
In short
Add the first day of your last period and your usual cycle length. The calculator shows your likely ovulation day and fertile days for this cycle and the next two. It is an estimate, not contraception, and nothing you type leaves your phone.
- Ovulation usually happens 12 to 16 days before your next period.
- Sperm can live for up to 7 days, so the days before ovulation count too.
- Having sex every 2 to 3 days is the best way to boost your chances.
How does the ovulation calculator work?
It adds your usual cycle length to the first day of your last period to find when your next period is due, then counts back 14 days, the middle of the 12 to 16 days the HSE gives, for your likely ovulation day. It shows the 5 days before ovulation and the day after as your fertile days.
It is an estimate. Ovulation usually happens 12 to 16 days before your next period, so your real fertile days can move, especially if your cycle length changes from month to month. It is not a form of contraception.
When am I most fertile?
- Ovulation usually happens 12 to 16 days before your next period.
- You are more likely to get pregnant if you have sex around the time you ovulate.
- Sperm can live for up to 7 days inside the body, so sex in the days before ovulation can still lead to a pregnancy.
- Having sex every 2 to 3 days is the best way to boost your chances of getting pregnant.
For folic acid, what helps your chances and how long it usually takes, see our guide to trying for a baby in Ireland.
Can I track my fertile days in an app?
Yes. Lullagram, which we make in Ireland, has a free trying for a baby section, with no trial and no card. It keeps a cycle calendar with your likely fertile days, lets you log ovulation and pregnancy tests and tick off your folic acid each day. Sharing your cycle with your partner is your choice, and a positive test starts your free pregnancy tracker in the same app. Its fertile days are estimates too, and it is not a form of contraception.
When should I see my GP?
- If you are 35 or under, see your GP if you have been trying for more than 1 year without getting pregnant.
- If you are 36 or over, see your GP if you have been trying for more than 6 months without getting pregnant.
- If you or your partner have any health conditions, talk to your GP after 6 months of trying.
Questions and answers
When am I most fertile?
Ovulation usually happens 12 to 16 days before your next period, and sperm can live for up to 7 days, so your most fertile days are the few days before ovulation and the day itself. Having sex every 2 to 3 days is the best way to boost your chances.
How does the ovulation calculator work out my fertile days?
It adds your usual cycle length to the first day of your last period to find when your next period is due, then counts back 14 days, the middle of the 12 to 16 days the HSE gives, for your likely ovulation day. It shows the 5 days before ovulation and the day after as your fertile days.
Is an ovulation calculator accurate?
It is an estimate. Ovulation usually happens 12 to 16 days before your next period, so your real fertile days can move, especially if your cycle length changes from month to month. It is not a form of contraception.
How long does it usually take to get pregnant?
About 4 in 5 couples who have regular unprotected sex every 2 to 3 days get pregnant within a year.
When should I see a GP about fertility?
If you are 35 or under, after more than a year of trying. If you are 36 or over, after 6 months. See your GP sooner if your periods are irregular or absent, or you have a condition that can affect fertility.
Sources and guidance
Every health point on this page comes from the HSE pages linked below. It is general information, not medical advice, and the calculator is an estimate, not contraception. Links checked 8 October 2026.
I must say Lullagram has been a huge help in the house for the simple things. As most parents do, we take shifts at night and knowing what the baby drank and at what time helps during changeover. We both have access to the same account so I can see what my partner logged on the last feed without having to wake her. It also keeps track of everything for the PHN who thinks it's a great idea. There's loads of extras on it so it's definitely worth a punt.