HEALTH · PERIOD PREDICTOR
Period Predictor Calculator
Predict your next period and fertile window from your last menstrual period date and average cycle length. Estimates ovulation date and days until your next cycle.
Not for contraception or fertility treatment — consult a healthcare provider.
About This Calculator
Enter the first day of your last period and your average cycle length to see when your next period is expected, your estimated ovulation date, and your fertile window. Predictions are based on the standard assumption that ovulation occurs 14 days before the next period — individual cycles vary.
How It Works
The calculator adds one cycle length to your last menstrual period date to estimate when the next period will begin. Ovulation is estimated as cycle_length − 14 days after your LMP (constant luteal phase assumption). The fertile window spans 5 days before ovulation to 1 day after, reflecting typical sperm viability (~5 days) and egg viability (~24 hours).
The Formula
Next period = LMP + cycle_length Ovulation = LMP + (cycle_length − 14)
- LMP
- first day of last menstrual period
- cycle_length
- average cycle length in days (typically 21–35)
Frequently Asked Questions
- How accurate is a period predictor?
- Predictions based on average cycle length are most reliable for people with regular cycles (21–35 days). For cycles that vary significantly month-to-month — such as with PCOS, perimenopause, or high stress — a single cycle-length average may over- or under-predict. Using several months of data to find your average improves accuracy.
- Can I use this to avoid pregnancy?
- No. Estimating your period or fertile window from cycle averages is not a reliable method of contraception. Actual ovulation can shift significantly due to illness, stress, travel, or hormonal changes. Consult a healthcare provider for contraceptive guidance.
- What is the fertile window?
- The fertile window is the days each cycle when pregnancy is most possible — sperm can survive in the reproductive tract for up to 5 days, and an egg is viable for roughly 24 hours after ovulation. That creates a window of about 6 days ending one day after ovulation. Timing intercourse within this window increases the chance of conception.
- Does this work for irregular cycles?
- Less reliably. The formula assumes ovulation always occurs 14 days before the next period (a fixed luteal phase). If your cycle length varies by more than a few days, use the midpoint of your shortest and longest recent cycles as your estimate. An irregular warning is shown for cycles outside the 21–45 day range.