Family and cycle · Cycle
Estimate ovulation day from the last period
One date: the first day of the last period. The tool places ovulation on day 14 and a fertile window from day 10 to day 17.
Calculate
How the calculator works
On a 28-day cycle, ovulation is conventionally placed on day 14, which is 13 days after the first day of the period. The fertile window shown covers days 10–17: a few days before ovulation, the day itself, and a little after. A 35-day cycle shifts everything; this page does not replace an ovulation test or an ultrasound.
Formula and method
ovulation = LMP + 13 days
Cycle day 14 if LMP is day 1.
window = LMP + 9 to LMP + 16
Days 10–17, a calendar convention, not a hormone reading.
The model assumes a 28-day ovulatory cycle. It does not use LH, basal temperature, or your real cycle lengths. The same day-14 hypothesis underlies Naegele’s due date. If cycles are irregular, this calendar is only a ballpark.
Worked example
LMP on 1 January 2026
lmp = 2026-01-01.
- Ovulation on day 14 → 14 January.
- Window days 10–17 → 10 to 17 January.
Estimated ovulation on 14 January 2026.
Input notes
- First day of last period
- First day of the last period. If the date is uncertain, so is the estimate.
Assumptions and limits
Assumptions
- 28-day cycle.
- A single ovulation on day 14.
- Civil UTC calendar.
Limits
- No LH test.
- No correction for a 35-day cycle.
- Not medical advice and not conception counseling.
How to read the result
The fertile window is not a promise of conception, and later ovulation is common. To date a pregnancy that has already started, the due-date calculator and ultrasound are more relevant.
Common mistakes
Entering the day of intercourse.
The field wants the first day of the period.
Treating day 14 as a measurement.
It is a 28-day-cycle convention.
Ignoring an irregular cycle.
Your real cycle length moves ovulation.
Comparison
| Cycle day | Event |
|---|---|
| Day 1 | First day of the period |
| Days 10–17 | Fertile window shown |
| Day 14 | Model ovulation |
Related calculations
- Pregnancy weekThe LMP and, if you want, a date. The tool counts weeks of amenorrhea and shows the trimester.
- Pregnancy due dateOne date: the first day of the last period. The tool adds 280 days for the due date and 14 days for a model conception.
- Age calculatorThe age shown is civil: completed years, then months, then leftover days. It is not the number of days divided by 365.25. An empty “Age on” field means today.
Frequently asked questions
How do I estimate the ovulation day?
Day 14 of a 28-day cycle.
If the first day of the period is day 1, ovulation is placed 13 days later. A longer cycle pushes it later.
Is this an ovulation test?
No.
There is no LH assay. This is a calendar. A urine test or clinical follow-up measures something else.
What is the fertile window for?
To see the days around day 14.
It covers a few days before and after the model ovulation. It is not a promise of conception.
Author and update
Written by Rédaction HexaCalc (editorial team). Content last updated: August 25, 2026. No third-party medical or financial review is claimed.
This calculation uses a standard mathematical identity. See also the methodology.
Already dating a pregnancy?
Naegele’s due date uses the same LMP and adds 280 days.
Due date calculatorCategory: Family and cycle