Ovulation Calculator
See the handful of days each cycle that actually matter — and remember it's an estimate, not a method.
Work out your estimated ovulation date and fertile window from the first day of your last period and your average cycle length — and see the best days to try to conceive.
These are estimates from your dates for cycle awareness — not a contraceptive method and not medical advice. See your GP for fertility or contraception advice.
How your ovulation date is worked out
It's counted back from your next period
The key idea catches a lot of people out: ovulation happens about 14 days before your next period — not 14 days after your last one. The second half of the cycle, from ovulation to your period (the luteal phase), is fairly fixed at around 14 days, while the first half varies. So the calculator estimates ovulation as the first day of your last period + (cycle length − luteal phase). On a 28-day cycle that lands on about day 14; the estimate shifts with your cycle length.
Worked example
If the first day of your last period was 1 January and your cycle is 28 days, your next period is due around 29 January, so you ovulate about 14 days before that — around 15 January. Your fertile window runs roughly 10–16 January (the five days before ovulation, ovulation day, and the day after while the egg survives). If you conceive, your due date would be about 266 days after ovulation — the calculator shows that date too.
This is an estimate for cycle awareness, not a contraceptive method — and not medical advice. It's based on averages and assumes a regular cycle, so it can be wrong. If you want to avoid pregnancy, use a proven contraceptive method and speak to your GP or a sexual health clinic.
The fertile window and the best days to conceive
You're not fertile for the whole cycle — only a short window around ovulation. It's defined by how long sperm and egg survive.
- The fertile window is the ~6 days ending on ovulation day — the five days before ovulation plus ovulation itself. Sperm can survive in the reproductive tract for up to about 5 days, so intercourse before ovulation can still lead to conception.
- The egg lives only about 1 day (roughly 12–24 hours) after it's released, which is why the window doesn't extend far past ovulation.
- The best days to try are the 2–3 days before ovulation, plus ovulation day. Having sperm already present when the egg is released gives the highest chance.
For a regular 28-day cycle with ovulation around day 14, the fertile window is roughly days 9 to 14. The calculator marks the window and the peak days around your estimated ovulation date.
How cycle length changes the timing, and irregular cycles
Longer cycle, later ovulation
Because ovulation is timed from your next period, a longer cycle pushes ovulation later and a shorter cycle brings it forward. Using a ~14-day luteal phase: a 28-day cycle ovulates around day 14, a 35-day cycle around day 21, and a 24-day cycle around day 10. Set your average cycle length so the estimate matches you, and adjust the luteal phase if you know yours differs from 14 days.
Irregular cycles
If your cycle length varies a lot from month to month, a calendar estimate is less reliable — ovulation could fall earlier or later than predicted. Methods like tracking basal body temperature, cervical mucus, or using ovulation predictor kits can help pinpoint it. In the UK, your GP is the first port of call if your cycles are very irregular or you've been trying to conceive for a while — they can look into what's going on and refer you for NHS fertility investigations where appropriate.
❓ Frequently asked Frequently asked questions
When is my fertile window?
Your fertile window is roughly the six days ending on the day you ovulate — the five days before ovulation plus ovulation day itself. That span reflects biology: sperm can survive in the reproductive tract for up to about five days, while the egg lives for only about 24 hours after it's released. For a regular 28-day cycle with ovulation around day 14, the fertile window is roughly days 9 to 14. This calculator marks the window around your estimated ovulation date so you can see the days that matter.
When do I ovulate?
Ovulation happens about 14 days before your next period starts — not 14 days after your last one. This is a common mix-up: it's the luteal phase (ovulation to the next period) that stays fairly fixed at around 14 days, while the first half of the cycle varies. So on a 28-day cycle you ovulate around day 14, but on a 32-day cycle you ovulate closer to day 18. For example, a 28-day cycle starting on 1 January puts ovulation around 15 January.
What are the best days to conceive?
The best days to try are the two to three days leading up to and including ovulation. Because sperm can wait for the egg but the egg survives only about a day, having sperm already present when you ovulate gives the highest chance. That means the couple of days just before ovulation, plus ovulation day, are the peak. This calculator highlights those days within your fertile window, and shows the due date you'd expect if you conceive — ovulation plus 266 days.
Can I use an ovulation calculator as birth control?
No. This is an estimate for cycle awareness and trying to conceive, not a contraceptive method. It's based on averages and assumes a regular cycle, so it can be wrong — sperm survive for days, ovulation timing shifts, and cycles vary from month to month. If you want to avoid pregnancy, use a proven contraceptive method and speak to your GP or a sexual health clinic. This tool is not medical advice.
How does cycle length affect ovulation?
Because ovulation is timed from your next period rather than your last one, a longer cycle pushes ovulation later and a shorter cycle brings it forward. The calculator estimates ovulation as your cycle length minus your luteal phase (about 14 days). So a 28-day cycle ovulates around day 14, a 35-day cycle around day 21, and a 24-day cycle around day 10. If your cycles are irregular the estimate is less reliable, and ovulation tracking or a chat with your GP can help.
Where these figures come from
The method here is the mainstream calendar approach to estimating ovulation and the fertile window. It is an estimate for cycle awareness — not a contraceptive method and not medical advice.
- Ovulation timing — estimated as the first day of your last period + (cycle length − luteal phase), with a typical luteal phase of about 14 days.
- Fertile window — roughly the six days ending on ovulation day, reflecting sperm survival of up to ~5 days and egg survival of about 1 day.
- Best days to conceive — the 2 to 3 days before ovulation, plus ovulation day.
- Cycle-length effect — a longer cycle pushes ovulation later (a 35-day cycle ovulates around day 21); a shorter cycle brings it forward.
- Due date if you conceive — ovulation + 266 days (38 weeks).
Last checked: July 2026. This is a planning estimate for cycle awareness, not a contraceptive method and not medical advice. If you're trying to conceive, having trouble, or want to avoid pregnancy, see your GP or a sexual health clinic.
Select the question that matches where you are right now.
The headline is your estimated ovulation date — the day the egg is likely released. The breakdown shows the fertile window around it and the best days to try to conceive.
Use it to plan the days you try if you're hoping to conceive, or simply to understand your cycle. The fertile window is the short span each cycle that matters most.
It's not a contraceptive method and not a diagnosis. It's based on averages and assumes a regular cycle, so it can miss. Don't rely on it to avoid pregnancy.
Ovulation is counted back about 14 days from your next period, because the luteal phase is fairly fixed while the first half of the cycle varies.
A few things move the estimate: your cycle length, your luteal phase, and how regular your cycles are.
Ovulation is timed from your next period, so a longer cycle pushes it later — a 35-day cycle ovulates around day 21, a 24-day cycle around day 10.
The gap from ovulation to your period is usually about 14 days. If you know yours differs, set it so the estimate lines up more closely.
A calendar estimate assumes a fairly consistent cycle. If yours varies a lot, ovulation can land earlier or later than predicted — tracking methods help pinpoint it.
A few habits keep the estimate meaningful.
Enter the first day of your last period, not the day it ended. That's the anchor the calculator projects your next period and ovulation from.
Track a few cycles and enter your average length. It's the single biggest driver of when ovulation is predicted.
Basal body temperature, cervical mucus, or ovulation predictor kits can confirm ovulation, especially if your cycles are irregular. Your GP can advise.
Once you know your window, plan for what comes next — a possible pregnancy, and your own health along the way.
If you conceive, project your estimated due date from your last period or conception date.
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