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Fertile Window & Ovulation Calculator
Estimate your ovulation date and fertile window from your last period and cycle length, with a calendar view. Free, no signup, runs on your device.
Important — read before acting on this
This calculator is an estimation and early-screening aid only, NOT a substitute for diagnosis or professional medical advice. For your own health, consult a doctor or qualified health professional.
Based on: WHO Asia-Pacific, Depkes RI, Mifflin-St Jeor (1990), Naegele's rule. Checked: 2026-07-16.
Your fertile window estimate
Pick the first day of your last period to see results.
The fertile window runs from 5 days before ovulation to 1 day after — sperm can survive around 5 days in the reproductive tract, while the egg lasts roughly 24 hours. Ovulation itself is estimated 14 days before the next period, not 14 days after the last one; the luteal phase is the more stable half of the cycle.
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Understanding the cycle
The luteal phase
The stretch from ovulation to the next period, relatively stable at around 14 days for most people. That's why ovulation is estimated 14 days BEFORE the next period, not 14 days after the last one — a very common error, especially for cycles that aren't 28 days.
The fertile window
A six-day span: five days before ovulation to one day after. Its length isn't because the egg lasts long — it survives about 24 hours — but because sperm can live up to roughly five days in the reproductive tract, waiting.
Limits of the calendar method
It only holds if your cycle is regular and ovulation happens as assumed. Stress, illness, weight change, and much else can shift it. That's why the calendar method is not reliable contraception.
FAQ
Frequently asked questions
How is the fertile window calculated?
First estimate ovulation: last period date plus (cycle length − 14). For a 28-day cycle that's about day 14; for a 32-day cycle, about day 18. The fertile window then runs from 5 days before ovulation to 1 day after — six days. The calculator does this and shows it on a month calendar.
Why count ovulation back from the next period?
Because the luteal phase — after ovulation — is far more stable than the phase before it, sitting near 14 days for most people regardless of total cycle length. The variable part comes before ovulation. So treating ovulation as day 14 from the last period is only right for a 28-day cycle. This calculator uses the correct method.
Why six days if the egg only lasts 24 hours?
Because the sperm do the waiting, not the egg. Sperm can survive up to about five days in the reproductive tract given supportive cervical mucus, so intercourse days before ovulation can still result in fertilisation when the egg arrives. The egg itself is viable for roughly 24 hours, which is why the window closes quickly after.
Can this be used to prevent pregnancy?
No — and there's no exception to this. This is NOT a reliable contraceptive and must not be relied on to prevent pregnancy. The calendar method assumes a regular cycle and ovulation exactly as predicted, but ovulation shifts with stress, illness, travel, weight change, and more. For effective contraception, discuss suitable methods with a doctor.
What if my cycle is irregular?
Accuracy drops sharply. The calendar method depends entirely on regularity, so a varying cycle length can put the ovulation estimate out by days. For irregular cycles, other approaches help more — basal body temperature tracking, cervical mucus observation, or ovulation prediction tests. If your cycle is very irregular or you're trying to conceive, speak with a doctor.
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