How does the menstrual cycle calculator work?
The calculator uses two inputs — the first day of your last period and your average cycle length — to apply standardized medical formulas. It does not guess: it calculates.
- Next period date: First day of last period + cycle length (default 28 days).
- Ovulation day: Cycle length minus 14 days (fixed luteal phase — Ogino-Knaus method).
- Fertile window: 5 days before ovulation + ovulation day = 6 days of peak fertility.
- Estimated due date: First day of last period + 280 days (40 weeks) — Naegele's Rule.
All calculations run instantly, directly in your browser, with no data sent to any server. The calculator works offline after the first page load.
The Ogino-Knaus Method — the basis of fertility calculation
The Ogino-Knaus method, independently discovered by Japanese gynecologist Kyusaku Ogino and Austrian gynecologist Hermann Knaus, is based on a fundamental biological principle: the luteal phase (the interval between ovulation and menstruation) has a relatively constant duration of 14 days, regardless of total cycle length.
This means ovulation consistently occurs 14 days before the first day of the next period. The variable is the follicular phase (first half of the cycle), which may last 7 days in a 21-day cycle, or 21 days in a 35-day cycle.
Naegele's Rule — calculating your due date
Naegele's Rule is the standard method for calculating the estimated due date (EDD), used by gynecologists, midwives, and digital health platforms worldwide. Formulated by Franz Karl Naegele in 1812, it remains the clinical baseline today.
The estimated date may vary by ±2 weeks from the actual birth. A first-trimester ultrasound (weeks 11–14) provides the most accurate dating and may revise the due date based on fetal measurements.
Frequently asked questions
How accurate is the ovulation calculation?
The calculator is accurate within the mathematical limits of the Ogino-Knaus method. Accuracy depends on the regularity of your cycle. For regular cycles (±1–2 day variation), the margin of error is small. For irregular cycles, the actual fertile window may be a few days earlier or later than the estimate.
Can I use this calculator for contraception?
No. This calculator is an informational tool, not a contraceptive method. The calendar method has a significant failure rate. For contraception, always consult a licensed gynecologist.
Why do my data disappear when I reload the page?
Your data is saved in your browser's local storage (localStorage). If you cleared your browser data or are using incognito/private mode, the data will be gone. This is a privacy feature, not a bug.
Does the calculator work without an internet connection?
Yes. After the first page load, the calculator works completely offline. All calculations are performed locally, with no server connection required.
The algorithms are fully documented with peer-reviewed references. Read the scientific methodology →