Ovulation Calculator
It estimates your ovulation date and fertile window from your last period and cycle length, projected across the next three cycles.
Estimated ovulation
Aug 9, 2026
Ovulation date
Aug 9, 2026
Fertile window opens
Aug 4, 2026
Fertile window closes
Aug 10, 2026
Next period due
Aug 23, 2026
Cycle length used
28 days
Luteal phase used
14 days
| Cycle | Fertile window | Ovulation |
|---|---|---|
| 1 | Aug 4, 2026 — Aug 10, 2026 | Aug 9, 2026 |
| 2 | Sep 1, 2026 — Sep 7, 2026 | Sep 6, 2026 |
| 3 | Sep 29, 2026 — Oct 5, 2026 | Oct 4, 2026 |
This is an estimate, not contraception. Cycle length varies month to month and ovulation can shift, so calendar methods are not a reliable way to avoid pregnancy. Speak to a healthcare provider about contraception, or about persistently irregular cycles.
What it calculates
It estimates your ovulation date and fertile window from your last period and cycle length, projected across the next three cycles.
Why it matters
The fertile window is only about six days long, and it does not sit in the middle of the cycle for anyone whose cycle is not 28 days.
Who it's for
Anyone trying to conceive, tracking a cycle, or wanting to understand their own fertility timing.
Formula
- LMP
- First day of the last menstrual period
- C
- Cycle length in days
- L
- Luteal phase, typically 14 days
Worked example
Last period 1 January, 28-day cycle
- 1Next period = 1 Jan + 28 days = 29 Jan
- 2Ovulation = 29 Jan − 14 days = 15 Jan
- 3Fertile window = 10–16 Jan
Ovulation around 15 January, fertile from 10 to 16 January
How the ovulation calculator works
The luteal phase — from ovulation to the next period — is relatively constant at about 14 days regardless of cycle length. So ovulation is calculated backwards from the expected next period rather than forwards from the last one. That is why a 35-day cycle ovulates around day 21, not day 17. The fertile window opens five days before ovulation because sperm survive that long, and closes about a day after, which is the egg's viable lifespan.
The luteal phase — from ovulation to the next period — stays fairly constant at about 14 days regardless of cycle length. So ovulation is calculated backwards from the expected next period, not forwards from the last one.
That is why a 35-day cycle ovulates around day 21 rather than day 17. The fertile window opens five days before ovulation because sperm survive that long, and closes about a day after, which is roughly the egg's viable lifespan.
Common mistakes
- Assuming ovulation is always on day 14 — that only holds for a 28-day cycle.
- Counting from the end of the period instead of the first day.
- Treating the estimate as reliable contraception; it is not.
Tips and best practice
- Ovulation predictor kits and basal body temperature tracking confirm the estimate against your actual cycle.
- Cervical mucus becoming clear and stretchy is a useful real-time sign of the fertile window.
- If cycles are irregular, track several before relying on the estimate.
Frequently asked questions
When do I ovulate?
About 14 days before your next period, not 14 days after your last one. On a 28-day cycle those coincide; on a 35-day cycle ovulation falls around day 21.
How long is the fertile window?
About six days: the five days before ovulation, since sperm survive that long, plus roughly a day afterwards while the egg remains viable.
Can I use this as contraception?
No. Cycle length varies month to month and ovulation can shift, so calendar-based estimates are not reliable contraception. Speak to a healthcare provider about proper methods.
What if my cycles are irregular?
The estimate becomes much less reliable. Ovulation predictor kits, basal body temperature charting or cervical mucus tracking give far better information, and persistent irregularity is worth discussing with a doctor.
What is the luteal phase?
The stretch between ovulation and your next period. It is fairly consistent at 12–16 days for most people, which is why ovulation is calculated backwards from the next period.
Related calculators
Methodology & trust
- Formula source
- Standard calendar method using a fixed luteal phase and a six-day fertile window.
- Last updated
- 2026-07-28
- Privacy
- Every calculation runs in your browser. No inputs are sent to a server or stored.
- Accessibility
- Keyboard navigable, labeled inputs and WCAG AA color contrast.