Period Calculator — Predict Your Next Period & Fertile Window
Enter the first day of your last period and we'll estimate your next periods, your likely ovulation day and your fertile window — so you can plan travel, events, appointments or a pregnancy with confidence. It takes about 10 seconds and nothing you enter is stored.
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Period Calculator
Enter your details below. Your predicted dates appear instantly — nothing is stored or sent anywhere.
How This Period Calculator Works
This calculator uses a simple, well-established model of the menstrual cycle. Ovulation typically happens about 14 days before your next period starts, and your cycle length is the number of days from day 1 of one period to day 1 of the next. From the information you enter, we estimate:
Your Next Periods
Your next three estimated period start dates, based on your average cycle length. Each prediction assumes your cycle stays regular — which it often doesn't, so treat later dates as increasingly approximate.
Ovulation Day
Estimated as 14 days before your next predicted period. For a 28-day cycle, that's around day 14. For shorter or longer cycles, ovulation shifts earlier or later accordingly.
Fertile Window
The roughly six days when pregnancy is possible: the five days before ovulation plus ovulation day itself, because sperm can survive up to five days in the reproductive tract.
What can make your cycle irregular?
Stress, illness, travel, significant weight changes, intense exercise, conditions like PCOS and thyroid disorders, breastfeeding, perimenopause and some medications can all shift your cycle. If your cycles regularly fall outside 21–35 days or change suddenly, it's worth speaking with a doctor.
Get Better Predictions
Simple habits that make cycle tracking and predictions more accurate.
Track Day 1 Every Month
Always record the first day of actual bleeding, not spotting. This is the single most important data point for accurate predictions.
Use Your 3-Month Average
Cycles naturally vary. Using the average of your last three cycles gives a more reliable cycle length than a single month.
Note Other Symptoms
Tracking flow, pain, mood and cervical mucus alongside dates gives you and your GP a much clearer picture of your cycle health.
Irregular, Painful or Unexpected Periods?
Speak with an AHPRA-registered Australian doctor by phone or video, from anywhere in Australia. Get advice, prescriptions and referrals where clinically appropriate.
Book an Online GP ($49)⚠️ When to See a Doctor About Your Period
- Your periods stop suddenly, or become less than 21 or more than 35 days apart
- Your bleeding is very heavy (soaking through a pad or tampon every 1–2 hours) or lasts longer than 7 days
- Your period pain is severe or stops you doing normal activities
- You have bleeding between periods or after sex
- Your periods suddenly change after being regular
- You think you may be pregnant or have missed periods without an obvious cause
Frequently Asked Questions
Common questions about period prediction and cycle health.
It gives an estimate based on the numbers you enter and an average-length luteal phase. Cycles vary from month to month, so real dates can differ — predictions further in the future are less reliable. Don't use it as contraception or to avoid pregnancy.
For most adults, a cycle between 21 and 35 days is considered normal, counting from day 1 of one period to day 1 of the next. A single "textbook" 28-day cycle is common but not required — anywhere in that range with some month-to-month variation is typical.
The calculator estimates ovulation as 14 days before your next predicted period — the average length of the luteal (post-ovulation) phase. Ovulation timing can shift between cycles, so treat this as a guide rather than a certainty.
No. This calculator is not a contraceptive method. Ovulation can shift between cycles and sperm can survive for several days, so calendar-based predictions alone are not reliable for avoiding pregnancy. Speak with a doctor about effective contraception.
No. All calculations happen locally in your browser — the dates you enter are never sent to or stored on our servers.
NewTeleDoc Medical is an Australian telehealth provider. You speak with an AHPRA-registered doctor by phone or video, wherever you are in Australia. An online GP consultation is $49 — see our pricing & fees page for the full list.
See a doctor if your periods stop, come less than 21 or more than 35 days apart, are very heavy or painful, or suddenly change. Online consultations are not suitable for emergencies — call 000 in an emergency.
Where it is clinically appropriate, a doctor can discuss contraception with you and provide a prescription electronically. Read more on our contraception page.
Questions About Your Cycle?
Book a private telehealth consultation with an AHPRA-registered GP — for period concerns, contraception, prescriptions or referrals, from anywhere in Australia.