Ovulation Calculator
Estimate your ovulation day and fertile window for the next 3–12 cycles from your last period and cycle length, plus your next period and due date.
This ovulation calculator projects your likely ovulation day and fertile window for the next several cycles, starting from the first day of your last period. Ovulation usually happens about 12–16 days before the next period begins, so the calculator counts back from each expected period using your luteal phase length — 14 days unless you know your own.
How to use this tool
- Enter the first day of your last period (the first day of real bleeding, not spotting).
- Enter your average cycle length — the days from the start of one period to the start of the next.
- Leave the luteal phase at 14 days unless you have measured it with ovulation tests or temperature charting.
- Choose how many cycles to plan ahead and read the table.
Formula
Ovulation = first day of last period + cycle length − luteal phase length
The fertile window is ovulation day and the five days before it: sperm can survive in the body for up to about five days, while the egg lasts around a day. The next period is the last period + cycle length, and the due date if you conceive is ovulation + 266 days (38 weeks).
Example
With a last period starting on 1 January 2026, a 28-day cycle and a 14-day luteal phase, ovulation is estimated for 15 January 2026 (cycle day 15). The fertile window runs from 10 to 15 January, with 13–14 January the most fertile days. The next period is due on 29 January, and a pregnancy conceived that cycle would have an estimated due date of 8 October 2026.
Frequently asked questions
How accurate is an ovulation calculator?
Calendar dates can be wrong even when cycles seem regular. LH tests detect a surge that can precede ovulation; temperature changes can support retrospective interpretation. Neither makes this projected calendar an exact observation of ovulation.
Can I use this to avoid pregnancy?
No. A projected calendar cannot establish safe days. Fertility-awareness contraceptive methods require their own instruction and rules, and even regular cycles vary. This page is only a planning estimate.
What if my cycles are irregular?
Predictions become less useful when cycle lengths vary. A shortest/longest-cycle calculation is also only a calendar estimate, and it does not reliably cover every fertile day in irregular cycles. Seek clinical advice for new or persistent irregularity.
When should I take a pregnancy test?
Most tests can be used from the first missed period. If you do not know when the period is due, NHS advice is to test at least 21 days after unprotected sex. Repeat an early negative if pregnancy remains possible.
Sources
This tool gives an estimate for general information only. It is not financial, tax, legal or medical advice — check with a qualified professional before you rely on the result.
Last updated: 2026-10-30