Clearblue Fertility Calculator
Clearblue fertility calculator works out Estimated ovulation and Ovulation falls on from First day of your last period, Average cycle length and Luteal phase length. The default figures on this page give estimated ovulation of 2026-09-24; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
5 steps
- The next period is expected 28 days after 2026-09-10, on 2026-10-08.
- Ovulation is one luteal phase earlier: 2026-10-08 - 14 days = 2026-09-24 (cycle day 15).
- The fertile window is the six days ending on ovulation: 2026-09-19 to 2026-09-24.
- Intercourse on the two days ending at ovulation carries the highest per-cycle chance, so 2026-09-23 and 2026-09-24 are the peak days.
- A pregnancy conceived this cycle would implant between 2026-09-30 and 2026-10-06 and be due on 2027-06-17.
Projected cyclestable
| cycle | periodStart | fertileWindowStart | ovulationDate | nextPeriodStart |
|---|---|---|---|---|
| 1 | 2026-09-10 | 2026-09-19 | 2026-09-24 | 2026-10-08 |
| 2 | 2026-10-08 | 2026-10-17 | 2026-10-22 | 2026-11-05 |
| 3 | 2026-11-05 | 2026-11-14 | 2026-11-19 | 2026-12-03 |
| 4 | 2026-12-03 | 2026-12-12 | 2026-12-17 | 2026-12-31 |
| 5 | 2026-12-31 | 2027-01-09 | 2027-01-14 | 2027-01-28 |
| 6 | 2027-01-28 | 2027-02-06 | 2027-02-11 | 2027-02-25 |
Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
4 sources
What the clearblue fertility calculator calculates
Predicts ovulation, the six-day fertile window, the peak fertility days and the implantation window from the last period and cycle length, and projects them across the next several cycles.
How to use this clearblue fertility calculator
Enter First day of your last period, Average cycle length, Luteal phase length and Today. Estimated ovulation updates as you type. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Estimated ovulation, Ovulation falls on and Fertile window opens.
The method follows Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle, Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation, Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy and ACOG, Fertility Awareness-Based Methods of Family Planning.
What you can compare
Change one input at a time to see how it affects Estimated ovulation, Ovulation falls on and Fertile window opens. The inputs you can vary are First day of your last period, Average cycle length, Luteal phase length and Today.
Understanding Estimated ovulation, Ovulation falls on and Fertile window opens
An estimate from calendar arithmetic, not a clinical assessment. Only about 4% of births occur on the estimated due date; ultrasound dating by a clinician supersedes menstrual dating when they disagree.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How much LH does Clearblue detect?
On the figures this page opens with, the estimated ovulation is 2026-09-24. No single number fits everyone: it follows First day of your last period, Average cycle length and Luteal phase length, so enter your own above and the answer updates as you type.
What information do I need for the clearblue fertility calculator?
You need First day of your last period, Average cycle length, Luteal phase length and Today. The page opens with default figures that give estimated ovulation of 2026-09-24, so you can see the scale before entering your own.
What does this clearblue fertility calculator show?
It shows Estimated ovulation, Ovulation falls on, Fertile window opens, Fertile window closes, Fertile window, Days in the fertile window, Peak fertility begins and Peak fertility ends, with the working and any warnings for your entries.
Can I check how the clearblue fertility calculator result was calculated?
Yes. The “How this is worked out” section shows the calculation steps, and the sources section identifies the references used for the method.