EstimatorOvulation Calculator
Your fertile window, ovulation day and next period from two inputs — plus a forward calendar covering the next three cycles so you can plan ahead, not just this month.
Your next three cycles
| Cycle | Period starts | Fertile window | Ovulation | Next period |
|---|
The fertile window runs from five days before ovulation through the day after — sperm can survive up to five days, the egg about 24 hours.
How it works
Ovulation is driven by the luteal phase — the stretch from ovulation to the next period — which stays near 14 days for most people even when total cycle length varies. So ovulation day ≈ period start + (cycle length − 14). The fertile window opens five days earlier (sperm survival) and closes a day after ovulation (egg lifespan). If a cycle leads to pregnancy, the due-date estimate is ovulation + 266 days.
Formula & worked example
Formula: Ovulation day = the first day of your last period + cycle length − 14 days, treating the luteal phase as a fixed 14 days. The fertile window runs from 5 days before ovulation to 1 day after; the next period is due one full cycle after the last one started, and an estimated due date is ovulation + 266 days.
Worked example: A period starting Thursday, 1 January 2026 with a 28-day cycle: ovulation falls on Thursday, 15 January, the fertile window runs 10–16 January, the next period is due 29 January, and conception in that window points to a due date of 8 October 2026.
When to use this
- You are trying to conceive and want this cycle's best days marked, not just the textbook day 14.
- Your cycle is longer or shorter than 28 days and you have realised ovulation moves with it.
- You want the next three cycles laid out to plan around work, travel or treatment dates.
- You conceived recently and want the rough due date that follows from this cycle's ovulation.
How to use it, step by step
- Enter the first day of your last period — day one, not the day it ended.
- Set your real average cycle length, counted from the first day of one period to the first day of the next.
- Read the ovulation day and the fertile window around it; the two days before ovulation and the day itself are the strongest.
- Scroll to the three-cycle table if you are planning further ahead than this month.
- Remember the dates are calendar estimates — tests and symptoms on the day outrank them.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| First day of last period | The anchor date every calculation counts from | A date within your current or most recent cycle |
| Average cycle length | Days from day one of one period to day one of the next | The tool accepts 21 to 45 days; most cycles run 21 to 35 |
Another worked example
For a period starting Monday, October 5, 2026 with a 32-day cycle, ovulation is not day 14 — it falls on Friday, October 23, 2026, because the count works back 14 days from the next period. The fertile window runs Sunday, October 18 to Saturday, October 24, with the best days Wednesday, October 21 to Friday, October 23. The next period is expected Friday, November 6, 2026, and if this cycle conceives, the estimated due date is Friday, July 16, 2027. Estimates only — real ovulation shifts, and this is not a contraception method.
Frequently asked questions
How is ovulation calculated from my period?
Ovulation happens about 14 days before the NEXT period, so with a 28-day cycle it lands on day 14, but with a 32-day cycle it lands on day 18. Enter your real average cycle length — the count matters more than the textbook 28.
What exactly is the fertile window?
The six days ending just after ovulation: sperm can live up to five days in the reproductive tract, and the egg survives roughly 24 hours. Conception is most likely in the two days before ovulation and on the day itself.
My cycles are irregular. Will this still work?
Only roughly. The calculator assumes every cycle matches your average length; irregular cycles can ovulate a week earlier or later than the estimate. For irregular cycles, treat the window as a guide and consider ovulation predictor kits.
If I conceive this cycle, when would the due date be?
About 266 days after ovulation (or 280 days after the period that started the cycle). The Pregnancy Due Date Calculator gives the full picture with trimester milestones.
🤰 Already pregnant?
The Due Date Calculator works from your last period, conception date or an ultrasound.
Can I use this calculator to avoid pregnancy?
No, and the page says so plainly. The dates assume your cycle matches its average exactly; stress, illness and ordinary variation move real ovulation by days, and sperm survive up to five days, which widens the risky stretch well beyond the printed window. A calendar estimate is a planning aid for conceiving, not a reliable barrier to it.
Should an ovulation test result override these dates?
Yes. The calculator predicts from last cycle's pattern; an ovulation predictor kit reacts to the hormone surge in this cycle, in your body, right now. When the two disagree, trust the test and treat the printed window as the rough map it is — especially if your cycles vary by more than a couple of days.