Core answer: Ovulation day ≈ 14 days before the first day of your NEXT period. In a 28-day cycle that is about day 14, with a fertile window of days 9–15. If cycles are irregular the calendar method fails — confirm with basal body temperature or ovulation test strips.
The ovulation formula
A cycle has two phases: the follicular phase (variable length) and the luteal phase (ovulation → next period, constant at ~14 days). Therefore:
Ovulation day = first day of next period − 14 days
Fertile window = 5 days before ovulation → 1 day after
Sperm survive 3–5 days inside the female body, while the egg stays fertilizable for only 12–24 hours — that is where "5 before, 1 after" comes from.
Quick table for 21–35 day cycles
Counted from the first day of the current period:
| Cycle length | Ovulation (day #) | Fertile window (days #) |
|---|---|---|
| 21 days | day 7 | days 2–8 |
| 25 days | day 11 | days 6–12 |
| 28 days | day 14 | days 9–15 |
| 30 days | day 16 | days 11–17 |
| 35 days | day 21 | days 16–22 |
Prerequisite: regular cycles (variation ≤7 days between cycles). With bigger swings this table is only a rough reference.
Basal body temperature (BBT)
After ovulation, progesterone raises resting temperature by 0.3–0.5 °C until the next period:
- Measure orally every morning before getting up or eating, at a fixed time
- Plot the curve: low phase → ovulation (the dip) → high phase (+0.3–0.5 °C)
- Three consecutive elevated days confirm ovulation has occurred
Limitation: the rise is retrospective confirmation — great for learning your own pattern, useless for advance prediction; colds, late nights and alcohol all interfere.
Ovulation test strips (LH)
Luteinizing hormone (LH) surges 24–48 hours before ovulation — that is what strips detect:
- Start testing 5 days before the expected ovulation day, at a fixed time daily (10 am–8 pm; do NOT use first-morning urine)
- Test line ≥ control line = positive → ovulation within 24–48 hours
- Limit fluids for 2 hours before testing to avoid diluted false negatives
Worked examples: 28-day and 35-day cycles
Example 1 (28 days): period starts on the 1st, next expected on the 29th. Ovulation = 29 − 14 = the 15th; fertile window the 10th–16th. For conception, the 12th, 14th and 16th — every other day is enough.
Example 2 (35 days): period starts on the 1st, next expected on the 6th of next month. Ovulation ≈ the 22nd; window the 17th–23rd. If cycles swing between 30–40 days, add LH strips from day 10 onward.
Optimal timing for conception
- Every 1–2 days during the fertile window is enough — daily sex actually lowers sperm concentration
- The 1–2 days BEFORE ovulation carry the highest probability (~25–30% per cycle)
- On the day the strip turns positive and the day after, plan once each
- ~85% of healthy couples conceive within a year; under 35, seek help after 12 months of trying; over 35, after 6 months
Common mistakes and myths
- "The rhythm method is reliable" — ovulation shifts with stress, illness and travel; sperm survive up to 5 days. Annual failure rate of "safe period" contraception is ~15–25%, and the WHO does not recognize it as effective birth control.
- "You can always feel ovulation" — only ~20% of women notice ovulation pain or stretchy discharge; most feel nothing, so going by feel is unreliable.
- "Right after the period is safe" — with short cycles (21–25 days) the fertile window can start almost immediately; days 2–8 are already risky.
- "Confirmed ovulation guarantees pregnancy" — sperm quality, fallopian tubes and the uterine lining all matter too; ovulation is only one necessary condition.
Use the [Ovulation Calculator](/c/health/ovulation) to generate your ovulation calendar from your last period and cycle length, and the [Period Calculator](/c/health/period) to track cycle regularity.