๐ธ Cycle Predictor
Enter your last period start date to calculate
This is an estimate only. Cycles vary between individuals โ consult a doctor for medical guidance.
Enter your last period date and average cycle length to get a predicted date for your next period and an estimated fertile window. Use it as a statistical guide for tracking your cycle or planning ahead, not a guarantee.
How to use
- Enter the start date of your last period.
- Enter your average cycle length and period length.
- See your predicted next period date and estimated fertile window.
How the calculation works
This tool uses the start date of your last period and your average cycle length to estimate upcoming period and ovulation dates. The next period is last period + cycle length, and each later one adds the cycle length again; four cycles are shown. Ovulation is estimated as 14 days before the next period. The time from ovulation to the next period (the luteal phase) varies relatively little between people and is often put at about 14 days, so counting back from the next period is the usual approach whatever the cycle length. The fertile window runs from five days before ovulation to the day after, because sperm can survive in the body for several days and an egg for about a day after ovulation.
Worked example
Last period starting 1 September with a 28-day cycle Next period: 1 September + 28 days = 29 September Estimated ovulation: 29 September โ 14 days = 15 September Fertile window: 10โ16 September With a 32-day cycle, the next period would be 3 October and ovulation around 19 September โ a longer cycle puts ovulation further from the start of the period.
Things to be aware of
- These are estimates based on an average cycle. Stress and health can shift your cycle and ovulation.
- Do not use this as a method of contraception.
- Irregular cycles make predictions less reliable. Basal body temperature tracking or ovulation tests can help.
- If you are concerned about irregular periods or severe pain, see a gynaecologist.
FAQ
How is the fertile window calculated?
It's estimated by working backward from your predicted next period, based on the general window in which ovulation is thought to occur.
Is it accurate if my cycle is irregular?
If your cycle is irregular, predictions from this formula may drift from your actual cycle. Treat it as a general guide and consult a gynecologist if needed.
How many cycles ahead can I see predictions for?
You can see predictions for several upcoming cycles, not just the very next one.
Is this useful for fertility planning?
It can serve as a general reference for your fertile window, but it's based purely on statistical calculation. If you're actively planning, combining it with basal body temperature tracking and a doctor's guidance is recommended.
What is the period length field used for?
It's used to show the length of your period on the cycle calendar and schedule view.