When does ovulation occur?

Ovulation typically occurs 14 days before your next period. For a 28-day cycle, that's day 14. For a 30-day cycle, it's day 16.

What is the fertile window?

The fertile window is about 6 days: 5 days before ovulation and the day of ovulation itself. Sperm can survive up to 5 days, while the egg is viable for 12-24 hours.

How accurate is this calculator?

This provides estimates based on average cycle patterns. Actual ovulation can vary by several days. For more accuracy, use ovulation predictor kits (OPKs) or track basal body temperature.

What if my cycle is irregular?

If your cycle varies by more than 7 days, ovulation prediction is less reliable. Track multiple cycles and use the average length, or consult a healthcare provider.

Ovulation Calculator

Calculate your ovulation date and fertile window based on your menstrual cycle length and last period.

Last Period Month

Day

Year

Cycle Length ?

days

Estimated Ovulation

—

Fertile Window Start —

Fertile Window End —

Next Period —

Extended More scenarios, charts & detailed breakdown ▾

Last Period Month

Day

Year

Cycle Length

days

Estimated Ovulation

—

Fertile Window Start —

Fertile Window End —

Next Period —

Peak Fertility Day —

Professional Full parameters & maximum detail ▾

Last Period Month

Day

Year

Average Cycle Length

days

Luteal Phase Length ?

days

Cycles to Project

Ovulation & Fertile Windows

Ovulation Date (Cycle 1) —

Fertile Window (Cycle 1) —

Ovulation Date (Cycle 2) —

Ovulation Date (Cycle 3) —

Period Predictions

Next Period —

Period After Next —

Current Cycle Status

Current Cycle Phase —

Day of Cycle —

Days to Next Ovulation —

How to Use This Calculator

Enter the first day of your last period and your average cycle length (most common is 28 days). The calculator estimates your ovulation date, fertile window, and next period date.

Formula

Ovulation Day = Cycle Length - 14
Fertile Window: 5 days before ovulation to 1 day after
Next Period = LMP + Cycle Length

Example

Example: Last period March 1, 28-day cycle: Ovulation ≈ March 15. Fertile window: March 10-16. Next period: March 29.

Frequently Asked Questions

Ovulation typically occurs 14 days before your next period. For a 28-day cycle, that's day 14. For a 30-day cycle, it's day 16.
The fertile window is about 6 days: 5 days before ovulation and the day of ovulation itself. Sperm can survive up to 5 days, while the egg is viable for 12-24 hours.
This provides estimates based on average cycle patterns. Actual ovulation can vary by several days. For more accuracy, use ovulation predictor kits (OPKs) or track basal body temperature.
If your cycle varies by more than 7 days, ovulation prediction is less reliable. Track multiple cycles and use the average length, or consult a healthcare provider.

Related Calculators

Sources & References (5) ▾

ASRM — Optimizing Natural Fertility: A Committee Opinion (2017) — American Society for Reproductive Medicine
Wilcox AJ et al. — Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby. N Engl J Med. 1995;333(23):1517-21 — NEJM
Dunson DB et al. — Day-specific probabilities of clinical pregnancy based on two studies with imperfect measures of ovulation. Hum Reprod. 1999;14(7):1835-1839 — Human Reproduction
WHO — Sexual and Reproductive Health: Natural Family Planning — World Health Organization
Mayo Clinic — Ovulation: Understanding your monthly cycle — Mayo Clinic