Pregnancy Calculators: Due Date, Ovulation and Milestones
From planning a pregnancy to tracking its milestones — how due dates, ovulation, conception, the trimesters and healthy weight gain are estimated, in plain language.
Ovulation Calculator
Verified formula Updated Jun 2026 Private — runs on your device
Verified formula
Private
Estimated ovulation day
15 Jan 2025
- Fertile window starts
- 10 Jan 2025
- Fertile window ends
- 16 Jan 2025
For general information only — not financial, tax, legal or medical advice. Verify before you rely on it.
How to use the Ovulation Calculator
The Ovulation Calculator works out your estimated ovulation day, along with 2 related figures in an instant. Enter first day of last period and average cycle length and the result updates as you type — it is free, needs no sign-up, and runs entirely in your browser so your figures stay private.
- Pick the first day of last period.
- Enter the average cycle length.
- Read off your estimated ovulation day, together with fertile window starts and fertile window ends — the calculator updates automatically, with no button to press.
Formula
The Ovulation Calculator uses the formula:
Estimated ovulation day = First day of last period + Average cycle length - 14
Worked example
For example, with first day of last period of 2025-01-01 and average cycle length of 28 days, the estimated ovulation day is 15 Jan 2025.
| First day of last period | 2025-01-01 |
|---|---|
| Average cycle length | 28 days |
| Estimated ovulation day | 15 Jan 2025 |
|---|---|
| Fertile window starts | 10 Jan 2025 |
| Fertile window ends | 16 Jan 2025 |
Results are estimates for educational use, not professional advice.
Key terms explained
- Mean
- The average of a set of numbers — their sum divided by how many there are.
Frequently asked questions
Ovulation is estimated about 14 days before your next period, found by adding your cycle length to the last period and subtracting 14.
It is roughly the five days before ovulation plus the ovulation day, when conception is most likely.
It assumes regular cycles and a 14-day luteal phase. Cycles vary, so use it as a guide alongside other ovulation signs.
It is not reliable for contraception, since ovulation can shift. Use a proven method if you are trying to prevent pregnancy.
The Ovulation Calculator uses the formula: Estimated ovulation day = First day of last period + Average cycle length - 14. For example, with first day of last period of 2025-01-01 and average cycle length of 28 days, the estimated ovulation day is 15 Jan 2025.
Pick the first day of last period. Enter the average cycle length. Read off your estimated ovulation day, together with fertile window starts and fertile window ends — the calculator updates automatically, with no button to press.
Related guides
Ovulation Table: Estimated ovulation day by Average cycle length
Reference table of estimated ovulation day for Ovulation across a range of average cycle length values — exact, engine-computed figures you can read off at a glance.
How to Calculate Ovulation: Formula, Steps & Examples
Learn how to calculate Ovulation — the formula explained step by step, with worked examples and a free calculator to check your answer.