Ovulation & Fertile Window Calculator
Estimate ovulation and the fertile window from cycle length and last period date.
Formula
ovulation ≈ next period − 14 days; fertile window is the five days before through the day after
Tips
- The fertile window runs from about five days before ovulation to the day after, because sperm can survive several days while an egg is viable for roughly 24 hours.
- Ovulation timing varies between cycles even for people with regular periods. Calendar prediction alone is a rough guide, not a reliable signal.
- Ovulation predictor kits, basal body temperature tracking and cervical mucus observation all give better information than dates alone.
- **This is not a contraceptive method.** Calendar-based prediction has a high failure rate for avoiding pregnancy — pregnancies occur outside the predicted window regularly. Use proper contraception if that is the goal.
- If you have been trying to conceive for a year without success, or six months if over 35, that is the usual point to speak to a GP.
Frequently asked questions
Can I use this to avoid pregnancy?
No. Calendar-based prediction is among the least reliable approaches to avoiding pregnancy, because ovulation shifts unpredictably between cycles. If avoiding pregnancy matters, use a method with a proper effectiveness rate and speak to a healthcare provider about the options.
Why is the luteal phase adjustable?
Because the phase after ovulation is more consistent in length than the phase before it, typically 12–16 days. If you know yours from tracking, entering it gives a better estimate than assuming 14.