Ovulation and fertile window calculator to help plan a pregnancy. Predicts ovulation date based on your menstrual cycle.
Three tabs: ovulation prediction (last menstrual period + cycle length → ovulation date and 6-day fertile window), 3-month fertility calendar (visual timeline of the next three cycles), and ovulation signs checklist (symptoms and tips).
Disclaimer: Estimates only, based on average cycles. Consult a doctor for fertility planning.
Calculator information
📋 How to use this calculator
- Record the first day of your last menstrual period (LMP) accurately; use a period-tracking app if needed.
- Calculate average cycle length from one LMP to the next LMP (typically 21-35 days, average 28 days).
- Enter the LMP date and cycle length into the calculator to get an estimated ovulation date and a 6-day fertile window.
- View the fertility calendar for the next 3 months to plan or naturally avoid pregnancy.
- Confirm ovulation with physical signs: clear, egg-white cervical mucus, a basal body temperature rise of 0.3-0.5C (0.5-1F), or an ovulation predictor kit (LH surge).
- Tip: track at least 3 cycles for accurate predictions, especially if your cycle is irregular.
🧮 Calendar Method Ovulation Prediction (Ogino-Knaus)
Ovulation_date = LMP + (cycle_length - 14) | Fertile_window = [ovulation - 5, ovulation + 1] | First_fertile_day = LMP + (shortest_cycle - 18) | Last_fertile_day = LMP + (longest_cycle - 11)
- LMP = First day of Last Menstrual Period (date)
- cycle_length = days between consecutive LMPs (typically 21-35 days)
- 14 = relatively constant luteal phase duration
- Fertile window = 5 days before ovulation + day of ovulation + 1 day after
- shortest_cycle and longest_cycle = cycle range over the last 6-12 months
This method assumes a 14-day luteal phase (relatively constant), while the follicular phase varies. Accuracy is 70-80% for regular cycles, lower for irregular cycles.
💡 Worked example: Woman with LMP May 1, 2026 and 30-day cycle
Given:- LMP: May 1, 2026
- Cycle length: 30 days (average over last 3 months)
- Goal: trying to conceive
Steps:- Calculate ovulation day: 30 - 14 = 16 days after LMP
- Ovulation date: May 1 + 16 days = May 17, 2026
- Fertile window begins 5 days earlier: May 12, 2026
- Fertile window ends 1 day after ovulation: May 18, 2026
- Peak fertility (highest probability): May 15-17, 2026 (2 days before and the day of ovulation)
Result: Ovulation: May 17, 2026. Fertile window: May 12-18, 2026. Best days for intercourse: May 15-17.
❓ Frequently asked questions
What is the fertile window and when does it occur?
The fertile window is the 6-day span around ovulation (5 days before + 1 day after) when intercourse can result in pregnancy. Sperm can survive 3-5 days in the female reproductive tract, while the egg only lives 12-24 hours after ovulation. Ovulation usually occurs 14 days before the first day of the next period, so for a 28-day cycle it falls on day 14.
What are the physical signs of ovulation?
Signs include clear, stretchy, slippery cervical mucus resembling raw egg whites, a basal body temperature rise of 0.3-0.5C (0.5-1F) after ovulation, mild one-sided lower abdominal pain (mittelschmerz), increased libido, and slight breast tenderness. Ovulation predictor kits detect the surge in luteinizing hormone (LH) 24-36 hours before ovulation.
Is the calendar method effective as natural birth control?
The calendar method (Ogino-Knaus) has a typical-use failure rate of 9-25% per year, much higher than hormonal contraception (0.1-9%) or IUDs (0.2-0.8%). It only suits women with very regular cycles (variation no more than 7 days). For greater accuracy, combine it with basal body temperature tracking and cervical mucus observation (the sympto-thermal method).
Why are my menstrual cycles irregular?
Irregular cycles can result from severe stress, significant weight changes, excessive exercise, thyroid disorders, polycystic ovary syndrome (PCOS), perimenopause, or side effects of hormonal contraception. Normal cycles range from 21 to 35 days, and variation of 2-7 days is still considered normal. If cycles are shorter than 21 days, longer than 35 days, or absent for 3 consecutive months, consult a healthcare provider.
What is the chance of pregnancy from intercourse during the fertile window?
The chance of conception per cycle for a healthy couple aged 25-30 having intercourse during the fertile window is about 25-30%. The probability declines with age: about 20% at age 30-35, 10% at age 35-40, and 5% over age 40. Intercourse every 1-2 days during the fertile window gives the highest chance. Roughly 85% of couples conceive within 1 year of actively trying without contraception.
📚 Sources & references
Last updated: May 11, 2026