Skip to content
Allin
Sign up · Sign in

Ovulation calculator

Find your fertile window from your last period.

Related tools

All Pregnancy & baby tools →
Due date calculatorEstimate your baby's due date (Naegele's rule).Next period date calculatorEstimate the dates of your next three periods from your last period and cycle length.Pregnancy week calculatorFind your current pregnancy week and trimester from your last period.Conception date calculatorEstimate the likely conception date from a due date.Baby Growth Percentile CalculatorWeight, length and head circumference percentiles from the WHO Child Growth Standards, 0-24 months.Baby Milk Intake CalculatorBottle volumes per feed and per day by your baby's weight and age, with the feeding interval.Gestational Age CalculatorGestational age in weeks and days from your LMP, due date or conception date, with trimester and EDD.hCG Doubling Time CalculatorCompute the doubling time and percentage rise between two hCG results, read against the expected range for the level reached.

Enter First day of last period, Cycle length (days) and the Ovulation calculator works out Ovulation date, Fertile window, Next period straight away. For instance, with First day of last period = 1985-03-20 and Cycle length (days) = 28 it returns Ovulation date = 03/04/1985, Fertile window = 29/03/1985 → 04/04/1985 and Next period = 17/04/1985.

How to use it

  1. Enter your values: First day of last period, Cycle length (days).
  2. Read the result instantly: Ovulation date, Fertile window, Next period.

Frequently asked questions

How does the Ovulation calculator work?

It takes First day of last period and Cycle length (days) and derives Ovulation date, Fertile window and Next period from them. The calculation is live as you type, so the result updates on every change.

Which values does the calculator ask for?

2 values: First day of last period and Cycle length (days). Nothing else is required — no account, no file upload.

What does a typical calculation look like?

With First day of last period = 1985-03-20 and Cycle length (days) = 28, the calculator returns Ovulation date = 03/04/1985, Fertile window = 29/03/1985 → 04/04/1985 and Next period = 17/04/1985. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

When would I actually use this?

Placing a pregnancy on the calendar: the estimated due date, the current week, and when the usual appointments and screenings fall.

What is the most common mistake?

Reading the due date as a deadline. It is a median: fewer than one birth in twenty happens on it, and a normal term spans several weeks around it.

What is the difference between the Ovulation calculator and the Due date calculator?

This one returns Ovulation date and Fertile window; the Due date calculator returns Estimated due date and Estimated conception. That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

Next period date calculator is the closest one after this: Estimate the dates of your next three periods from your last period and cycle length.

What else is worth having open alongside it?

Pregnancy week calculator and Conception date calculator — they come up in the same task often enough to be worth a second tab.

Where do the figures come from, and how current are they?

Dating follows Naegele's rule (280 days from the last period) or the transfer date for assisted conception. An early ultrasound is more accurate than any calculation and takes precedence over it.

Further reading

All guides →
ExplainerOvulation and Your Fertile WindowOvulation is usually 14 days before your next period, and the fertile window spans about six days. Here's how it works and how to estimate yours.How-toHow to Calculate Your Due DateNaegele's rule adds 280 days to your last period — or subtract three months and add seven days. Here's how it works and why it's only an estimate.ExplainerPregnancy: Weeks to MonthsPregnancy is counted in weeks, but months aren't exactly four weeks. Here's why it's 40 weeks, how weeks map to months and trimesters, and a quick reference.ExplainerWhat Is a Normal Menstrual Cycle Length? The Numbers, by Life StageFIGO puts a normal cycle at 24 to 38 days; ACOG's older range is 21 to 35. Here is why two respected bodies give different numbers, how much variation is normal, and how cycles change after menarche and before menopause.How-toHow to Predict Your Next Period (And How Far Off the Estimate Can Be)Add your average cycle length to the first day of your last period. Here is how to measure that length properly, why the estimate has a range rather than a date, and when irregularity is worth a consultation.ExplainerHow Much Milk Does a Baby Need? The Rule of Thumb and Its LimitsA formula-fed baby takes roughly 2.3 fl oz per pound of body weight per day in the first months, split across the day's feeds. Here is the table by age and weight, why breastfed babies are not measured this way, and the signs that outrank any number.