Ovulation Calculator
Ovulation day, fertile window and next period from your last period.
Quick answer
- Formula: Ovulation = last period + (cycle length − 14) days · Fertile window = ovulation − 5 days to ovulation + 1 day
- Worked example — Estimated ovulation date: January 15, 2026 (full example below).
Ovulation Calculator
| Estimated ovulation date | January 15, 2026 |
| Fertile window | January 10, 2026 to January 16, 2026 |
| Next period expected | January 29, 2026 |
With a last period starting January 1, 2026 and a 28-day cycle, ovulation is estimated on January 15, 2026, the fertile window runs January 10–16, and the next period is due January 29.
Ovulation typically occurs about 14 days before the next period starts, whatever the total cycle length — so the estimated ovulation day is your cycle length minus 14 days after the first day of your last period. The fertile window covers the five days before ovulation plus the day after, because sperm can survive up to five days while the egg survives about a day.
Cycle-based estimates assume regular cycles and are educational only — they are not reliable contraception and not medical advice. Hormonal methods or a clinician can confirm ovulation more precisely.
The formula
Worked example
With a last period starting January 1, 2026 and a 28-day cycle, ovulation is estimated on January 15, 2026, the fertile window runs January 10–16, and the next period is due January 29.
The example is pre-loaded in the result panel — press Calculate with your own numbers to replace it.
Frequently asked questions
Why is ovulation counted backwards from the next period?
The luteal phase (ovulation to period) is fairly constant at about 14 days, while the first phase varies. Counting back from the next period is therefore more reliable than counting forward from the last one.
Can I rely on this to avoid pregnancy?
No. Calendar estimates fail when cycles vary even slightly, and sperm survival extends the risk window. Use a proven contraceptive method and discuss options with a clinician.
Category: Health & Fitness calculators
Sources & methodology
This calculator applies the formula shown on this page exactly as written, using the standard definitions for its inputs. Results are computed from the values you enter — no data is stored — and are rounded only for display.
Content last reviewed by CalcHub Editorial.
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