CodeKitHubWhat Is This Tool?
An ovulation calculator estimates when you release an egg each cycle. The key biological fact behind it: the luteal phase — the stretch from ovulation to the next period — is relatively stable at about 14 days, while the phase before ovulation varies from person to person. So ovulation is estimated as your next period's date minus 14 days: in a 28-day cycle that's day 14, in a 35-day cycle it's day 21.
The fertile window spans about 6 days: the 5 days before ovulation plus the day after. Sperm survive up to 5 days in the reproductive tract, while the egg is viable for only about 12–24 hours after release — which is why the days *before* ovulation matter as much as the day itself. A calendar can only estimate this window from averages; ovulation test strips (LH tests) and basal body temperature tracking detect it directly.
Why Use It?
- Estimates ovulation day as next period minus 14 days — the standard luteal-phase method — adjusted to your cycle length from 21 to 35 days.
- Shows the full ~6-day fertile window (5 days before ovulation through 1 day after), not just a single date.
- Forecasts ovulation, fertile window and period dates for the next 3 cycles, so you can plan ahead.
- Free, instant and private — the dates you enter are processed in your browser and never uploaded or stored.
- Explicit about what it can't do: it cannot detect ovulation and is clearly labeled as unsuitable for contraception.
How to Use
- Pick the first day of your last period in the date field.
- Set your average cycle length in days (default 28; adjustable from 21 to 35).
- Press the calculate button.
- Read your estimated ovulation day, fertile window and next period, plus the 3-cycle forecast table.
Example
Input
Last period started May 1, 2026, cycle length 28 daysOutput
Estimated ovulation: May 15, 2026; fertile window May 10–16; next period May 29Ovulation = next period (May 29) − 14 days = May 15. With a 35-day cycle, the next period would be June 5 and ovulation May 22 — a full week later, from the same start date.
Ovulation day by cycle length
Day 1 is the first day of your period. The table shows why the popular "day 14" rule of thumb only works for 28-day cycles — in a short 21-day cycle, ovulation can happen while you'd still assume you're far from fertile days. That asymmetry is one of the main reasons calendar-based contraception fails.
| Cycle length | Estimated ovulation | Fertile window |
|---|---|---|
| 21 days | Day 7 | Days 2–8 |
| 24 days | Day 10 | Days 5–11 |
| 28 days | Day 14 | Days 9–15 |
| 32 days | Day 18 | Days 13–19 |
| 35 days | Day 21 | Days 16–22 |
Signs that ovulation is approaching
- LH surge — ovulation test strips detect the luteinizing hormone spike 24–36 hours before ovulation; this is the most accessible direct method.
- Cervical mucus becomes clear and stretchy, similar to raw egg white, in the days before ovulation.
- Basal body temperature rises by about 0.3–0.5 °C after ovulation — useful to confirm it happened, too late to time conception that cycle.
- Some people feel one-sided lower abdominal pain (mittelschmerz) around ovulation.
- This tool is for general reference only, and none of these signs are certain either — if you've been trying to conceive for over a year (or 6 months over age 35), see a doctor.
Where this fits in real workflows
- Once a period is late, the Pregnancy Due Date Calculator turns the same LMP date into a due date and week-by-week timeline.
- The Period Calculator predicts your next 6 periods — useful for spotting irregularity before relying on any fertility math.
- Preparing your body for pregnancy often starts with baseline numbers: the BMI Calculator and Calorie Calculator provide reference values to discuss with your doctor.
→ Pregnancy Due Date Calculator · Period Calculator · BMI Calculator · Calorie Calculator
Frequently Asked Questions
When do I ovulate?
On average, about 14 days before your next period starts. With a 28-day cycle that's around day 14 of the cycle; with a 35-day cycle around day 21; with a 24-day cycle around day 10. The part of the cycle before ovulation is what varies between people — the 14 days after it are relatively constant.
What is the fertile window?
The roughly 6 days per cycle when intercourse can lead to pregnancy: the 5 days before ovulation plus about one day after. Sperm can survive up to 5 days waiting for the egg, but the egg itself lives only 12–24 hours — so the highest-probability days are the 2–3 days right before ovulation.
Can I use this calculator to avoid pregnancy?
No — and this matters. Calendar-based methods have a high real-world failure rate (typically quoted around 12–24% per year), because ovulation shifts with stress, illness, travel and sleep, and sperm survive for days. If you want to prevent pregnancy, talk to a doctor about reliable contraception. Treat every day as potentially fertile as far as contraception is concerned.
My cycles are irregular. Does the estimate still work?
Poorly. The method assumes your next cycle will match your average length; if your cycles vary by more than a few days, the predicted ovulation day can miss by a week or more. Ovulation test strips (LH tests), basal body temperature or cervical mucus tracking measure your actual cycle instead of a calendar average — and a doctor can help investigate persistently irregular cycles.
Is my data stored anywhere?
No. The calculation runs in your browser with JavaScript; the dates you enter are never sent to a server, stored or logged.