Ovulation Calculator
Ovulation is estimated by counting back from your next expected period, not forward from your last one. This calculator works out the likely date and the fertile window around it.
Formula
Ovulation ≈ next period start − 14 days
- The luteal phase is the stable part, at about 14 days
- Cycle length varies, so the fertile window moves with it
A 28-day cycle starting on the 1st: ovulation around day 14
How the Date Is Estimated
The luteal phase — from ovulation to the next period — is relatively consistent at about 14 days, while the first half of the cycle varies far more.
Ovulation day ≈ Cycle length − 14
On a 28-day cycle that is day 14. On a 32-day cycle it is day 18, not day 16.
Counting forward from the last period is the common mistake, and it misplaces ovulation for anyone whose cycle is not 28 days.
The Fertile Window
The window spans roughly six days: the five before ovulation plus the day itself. Sperm survive up to five days in the reproductive tract, while an egg remains viable for only 12 to 24 hours.
That asymmetry means intercourse in the days before ovulation is more likely to result in conception than on the day after.
Calendar Estimates Have Real Limits
Cycles vary, and stress, illness, travel and weight change all shift ovulation. Studies find that only a minority of women ovulate exactly on day 14, even with a 28-day cycle.
More direct methods track the event rather than predicting it: ovulation predictor kits detect the LH surge, basal body temperature rises slightly after ovulation, and cervical mucus changes beforehand.
This is an estimate, not contraception, and not a fertility diagnosis. If you have been trying for a year, or six months if over 35, speak to your GP.
Quick Reference
| Cycle length | Ovulation day | Fertile window |
|---|---|---|
| 24 days | Day 10 | Days 5–11 |
| 28 days | Day 14 | Days 9–15 |
| 30 days | Day 16 | Days 11–17 |
| 32 days | Day 18 | Days 13–19 |
| 35 days | Day 21 | Days 16–22 |
Frequently Asked Questions
- When do I ovulate?
- Roughly 14 days before your next period, so day 14 of a 28-day cycle but day 18 of a 32-day one. Count back rather than forward.
- How long is the fertile window?
- About six days: the five before ovulation plus the day itself, because sperm survive up to five days while the egg lasts under 24 hours.
- Can I use this as contraception?
- No. Calendar estimates are unreliable because ovulation shifts with stress, illness and travel. Use proper contraception if you are avoiding pregnancy.
- What is more accurate than a calendar estimate?
- Ovulation predictor kits detecting the LH surge, basal body temperature tracking, and cervical mucus observation all measure the event rather than predicting it.