What this calculator does
Ovulation typically occurs about 14 days before the next period begins, not 14 days after the last one. That distinction matters: the luteal phase after ovulation is fairly consistent at around 14 days, while the phase before it varies considerably between people and between cycles.
The fertile window spans roughly six days, the five days before ovulation plus the day itself, because sperm can survive several days in the reproductive tract while the egg is viable for around 24 hours. This calculator works forward from your last period and cycle length to place that window.
The formula
The next period is projected by adding your cycle length to the first day of the last one. Ovulation is placed 14 days before that projected date, and the fertile window opens five days earlier, closing the day after ovulation.
| Term | Meaning |
|---|---|
| Cycle length | Days from the first day of one period to the first day of the next. |
| Luteal phase | The interval from ovulation to the next period, typically around 14 days. |
| Fertile window | The days on which intercourse can result in conception. |
The inputs explained
| Field | What to enter |
|---|---|
| First day of last period | The first day of your most recent period: the first day of bleeding, not the last. |
| Cycle length (days) | Your average cycle length in days. If it varies, use the average of your recent cycles and treat the result as approximate. |
When to use it
Trying to conceive
The fertile window shows the days with the highest chance of conception. Calendar estimates are a starting point; ovulation predictor kits and basal body temperature tracking give more direct evidence of when ovulation actually occurs.
Understanding an irregular cycle
Run your shortest and longest recent cycles separately. The spread between the two fertile windows shows how much uncertainty irregularity introduces: often more than people expect.
Planning around your cycle
The projected period dates are useful for scheduling travel, events or medical appointments, quite apart from any fertility question.
Knowing the limits of calendar methods
Calendar-based estimates assume a consistent luteal phase and a predictable cycle. They are not a reliable contraceptive method on their own: failure rates for calendar-based approaches are substantially higher than for most other methods.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How cycle length shifts the fertile window
The same starting date with different average cycle lengths. Dates are generated relative to the day this page was built.
| Cycle length | Estimated ovulation | Fertile window | Next period expected |
|---|---|---|---|
| 24 days | 4 Aug 2026 | 30 Jul 2026 → 5 Aug 2026 | 18 Aug 2026 |
| 26 days | 6 Aug 2026 | 1 Aug 2026 → 7 Aug 2026 | 20 Aug 2026 |
| 28 days | 8 Aug 2026 | 3 Aug 2026 → 9 Aug 2026 | 22 Aug 2026 |
| 30 days | 10 Aug 2026 | 5 Aug 2026 → 11 Aug 2026 | 24 Aug 2026 |
| 32 days | 12 Aug 2026 | 7 Aug 2026 → 13 Aug 2026 | 26 Aug 2026 |
| 35 days | 15 Aug 2026 | 10 Aug 2026 → 16 Aug 2026 | 29 Aug 2026 |
Questions
When am I most fertile?
The two or three days immediately before ovulation, and the day of ovulation itself, carry the highest chance of conception. The window opens earlier because sperm can survive up to about five days.
Is ovulation always 14 days before the period?
Approximately, but not exactly. The luteal phase is more consistent than the follicular phase, but it still varies between roughly 12 and 16 days between individuals, and can vary within one person from cycle to cycle.
How accurate is a calendar estimate?
Reasonable for regular cycles, poor for irregular ones. Ovulation predictor kits detect the hormone surge directly, and basal body temperature confirms ovulation after it has happened: both are more reliable than the calendar alone.
Can I use this as contraception?
No. Calendar-based methods have considerably higher failure rates than most other forms of contraception, particularly with irregular cycles. Speak to a doctor or family planning clinic about options that suit you.
What if my cycles are very irregular?
Calendar prediction becomes unreliable, and persistent irregularity is worth discussing with a doctor: it can have treatable underlying causes.
If you are already pregnant, the pregnancy due date calculator works forward from the same starting date.