Ovulation Calculator
Your estimated ovulation date, fertile window and next period from the first day of your last period and your cycle length — plus the due date if you conceived this cycle.
Ovulation is anchored to the NEXT period, not the last one: it falls about 14 days (the luteal phase) before the next period is due, so ovulation = last period + cycle length − luteal phase.
How the ovulation calculator works
Ovulation is anchored to the NEXT period, not the last one: it falls about 14 days (the luteal phase) before the next period is due, so ovulation = last period + cycle length − luteal phase. The fertile window is the five days before ovulation plus the day itself, because sperm can live up to five days and the egg about one. A pregnancy that starts at ovulation is due 266 days later.
Formula: Ovulation = LMP + cycle − luteal · fertile window = ovulation − 5 … ovulation · next period = LMP + cycle · due date = ovulation + 266 days
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 & best days to tryThe 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.
Cycle length & irregular cyclesHow 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
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).
- Further guidance — NHS — health A to Z and live well guidance.
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.
Understanding your resultSelect 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.
What to do with itUse 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.
What it is notIt'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.
Why it's before your next periodOvulation 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.
Cycle lengthOvulation 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.
Luteal phaseThe 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.
How regular your cycles areA 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.
Use the right start dateEnter 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.
Set your cycle lengthTrack a few cycles and enter your average length. It's the single biggest driver of when ovulation is predicted.
Consider trackingBasal 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.
Estimate a due dateIf you conceive, project your estimated due date from your last period or conception date.
Due date calculator → Budget for the babyEstimate the cost of a new arrival, from the nursery to the first year.
Baby cost calculator → Your maternity payYour due date sets the dates your maternity leave and pay can start from.
Maternity pay calculator → Related calculators 🤰Due DateEstimate your EDD 👶Baby CostCost of a new arrival 🤱Maternity PayLeave and pay dates 🧸Childcare CostWhat childcare costs 🍎CalorieYour daily energy needs ⚖️BMIHealthy weight range You might also be interested in 🤰Due DateEstimate your EDD 👶Baby CostPlan for a baby 🧸Childcare CostWhat childcare costs 🍎CalorieDaily energy needs ⚖️BMIHealthy weight range 🛟Emergency FundBuild a buffer Rate this calculatorQuick answers
When is my fertile window? ~6 days ending at ovulation
The fertile window is the roughly six days that end on ovulation day — the five days before it plus ovulation itself. That's because sperm can survive up to five days in the reproductive tract while the egg lives only about 24 hours after release. With a 28-day cycle and ovulation around day 14, the fertile window runs about days 9–15. Timing intercourse in this window gives the best chance of conceiving.
| Window | ~6 days |
|---|---|
| Sperm survive | Up to 5 days |
| Egg lives | ~24 hours |
- Best days Before ovulation
- 28-day cycle ~days 9–15
When do I ovulate? ~14 days before your period
Ovulation happens about 14 days before your next period starts — not 14 days after your last one. That distinction matters: the second half of the cycle (the luteal phase) is fairly constant at ~14 days, while the first half varies. So on a 28-day cycle you ovulate around day 14, but on a 32-day cycle it's around day 18, and on a 24-day cycle around day 10. This calculator counts back from your predicted next period.
| Rule | ~14 days before period |
|---|---|
| 28-day cycle | ~day 14 |
| 32-day cycle | ~day 18 |
- Luteal phase Steady ~14 days
- First half Varies
Cycle length changes everything Longer cycle, later ovulation
Your cycle length shifts ovulation. Because ovulation is anchored ~14 days before the next period, a longer cycle means later ovulation: a 35-day cycle puts it around day 21, not day 14. Assuming everyone ovulates on 'day 14' is the classic mistake that makes the window wrong for anyone whose cycle isn't 28 days. Track a few cycles to learn your real average length and the estimate gets much sharper.
| 24-day cycle | Ovulate ~day 10 |
|---|---|
| 28-day cycle | ~day 14 |
| 35-day cycle | ~day 21 |
- Assume day 14? Often wrong
- Track cycles Sharper estimate
Best days to try for a baby The 2–3 days before ovulation
The highest-odds days to conceive are the two to three days leading up to ovulation and ovulation day itself — the fertile window's peak. Having sperm already present when the egg is released works better than waiting until after. Every day or every other day across the fertile window is a common approach. If you conceive during this window, your due date is about 266 days from ovulation — the calculator shows that date too.
| Peak days | 2–3 before ovulation |
|---|---|
| Frequency | Every 1–2 days |
| Due date | Ovulation + 266 d |
- Sperm ready Beats waiting
- Conceived? See due date
Can I use this to avoid pregnancy? No — it's not reliable
Calendar-based prediction is not a reliable contraceptive. Ovulation timing shifts with stress, illness, travel and simply from cycle to cycle, and sperm can survive long enough that 'safe' days aren't as safe as they look. Fertility-awareness methods that actually work combine daily basal body temperature, cervical mucus and often ovulation tests with careful training — a calendar estimate alone isn't one of them. Use proper contraception if you're avoiding pregnancy.
| As contraception | Not reliable |
|---|---|
| Ovulation | Shifts month to month |
| Use | Proper contraception |
- Avoiding pregnancy? Don't rely on dates
- FAM methods Need training
Irregular cycles and tracking Estimates get shakier
If your cycles vary a lot in length, a calendar estimate is only a rough guide — the ovulation day genuinely moves. More reliable personal signals are basal body temperature (which rises slightly after ovulation), changes in cervical mucus (clear and stretchy near ovulation), and ovulation predictor kits that detect the LH surge 24–36 hours before. If cycles are very irregular or you've been trying to conceive for 6–12 months, see your GP.
| Irregular? | Estimate is rough |
|---|---|
| OPK detects | LH surge |
| See GP after | 6–12 months trying |
- Track Temp + mucus
- Very irregular See a doctor
Worked examples
| Inputs | Estimated ovulation | Note |
|---|---|---|
| Last period 10 March 2026, 28-day cycle | Tue 24 March 2026 | ovulation Tue 24 March 2026; fertile 19–24 March |
| A 35-day cycle from 1 January 2026 | Thu 22 January 2026 | ovulation on cycle day 21: Thu 22 January 2026 |
| A leap-day period, 28-day cycle | Thu 14 March 2024 | ovulation Thu 14 March 2024 |
FAQFrequently 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. Sources
The sources behind this calculator
- Mifflin MD et al., "A new predictive equation for resting energy expenditure in healthy individuals", Am J Clin Nutr 1990 — the BMR equation used here
- World Health Organization — Waist circumference and waist–hip ratio (2008 expert consultation) — waist-to-hip cut-offs
- Tanaka H, Monahan KD, Seals DR, "Age-predicted maximal heart rate revisited", J Am Coll Cardiol 2001 — 208 − 0.7 × age
- Nathan DM et al., "Translating the A1C assay into estimated average glucose values" (ADAG), Diabetes Care 2008 — eAG = 28.7 × A1c − 46.7
- Mosteller RD, "Simplified calculation of body-surface area", N Engl J Med 1987 — BSA = √(height × weight / 3600)
- NHS — national health guidance
Last checked: September 2026. The equations are fixed by their published papers and do not change with tax years or regulations; the thresholds quoted in verdicts name the body that set them.