Ovulation Calculator
Estimate your ovulation date and fertile window from your cycle length.
Written by Golam Rabbani, Founder & Lead Engineer — Last updated 2026-05-01
How to use this ovulation calculator
- Enter the first day of your last menstrual period using the date picker.
- Set your cycle length in the "Cycle length (21–35 days)" field (default is 28).
- Press Calculate to see your ovulation date, fertile window, and next period date.
- Read the Fertile Days calendar — the pink tile marks ovulation day.
- Use Copy to save the result to your clipboard, or Reset to start over.
About this ovulation calculator
The ovulation calculator estimates when you are most likely to ovulate and which days fall within your fertile window, based on the first day of your last menstrual period and your typical cycle length.
The tool uses the standard luteal-phase method: ovulation is placed at cycle day (cycle length − 14), counting from the first day of your last period. The fertile window spans the five days before ovulation through the day after, giving a seven-day range when conception is most likely. Next period is estimated by adding your full cycle length to the start of your last period. This is an estimate for general information and is not a substitute for advice from a qualified clinician. Predictions are based on a regular cycle, and natural variation in cycle length or ovulation timing is normal.
For example, if your last period started on 2026-05-01 and your cycle is 28 days, the tool places ovulation on 2026-05-15 (day 14), the fertile window from 2026-05-10 through 2026-05-16, and the next period on 2026-05-29. This is useful for anyone tracking their cycle to plan or understand their reproductive health.
FAQ
- How does the ovulation calculator estimate my fertile window?
- It applies the standard luteal-phase rule: ovulation is estimated at cycle length minus 14 days from the first day of your last period. The fertile window then spans the five days before ovulation through the day after, since sperm can survive several days in the reproductive tract.
- What formula does the tool use to find the ovulation date?
- Ovulation date = first day of last period + (cycle length − 14 days). For a 28-day cycle that is day 14; for a 30-day cycle it is day 16. The fertile window starts 5 days before that date and ends 1 day after it.
- How accurate are the predictions?
- The dates are estimates based on an assumed regular cycle. Actual ovulation can shift by several days due to stress, illness, or natural variation. The tool cannot confirm ovulation or diagnose any medical condition; consult a healthcare provider for clinical guidance.
- What cycle lengths does the tool accept?
- The calculator accepts cycle lengths between 21 and 35 days. If you enter a value outside that range, it shows an error and will not calculate. Cycle lengths shorter than 21 or longer than 35 days may indicate irregular cycles worth discussing with a clinician.
- Does the ovulation calculator store my data?
- No. The ovulation calculator runs entirely in your browser; nothing is sent to a server. Closing or refreshing the page clears all inputs.
- Is the ovulation calculator free?
- Yes, it is completely free with no signup or account required.