User Guide
- Enter the first day of your last period — the day bleeding started, not ended.
- Set your average cycle length (first day of one period to first day of the next; 28 is average, 21–35 is common).
- Set how many days your period usually lasts.
- Read your next three estimated periods, each with the predicted ovulation day and fertile window, plus a days-until countdown.
About the Period Calculator
A period calculator (menstrual cycle calculator) predicts when your next periods will start, based on two numbers you already know: the first day of your last period and your average cycle length. This one forecasts your next three cycles — each with its estimated ovulation day and fertile window — and counts down the days to the next one. Everything is calculated in your browser; no dates you enter are stored or sent anywhere.
How the calculation works — with an example
If your last period started on 1 July and your cycle averages 28 days, your next period is estimated for 29 July. Ovulation typically happens about 14 days before the next period — around 15 July — and because sperm survive up to five days, the fertile window runs roughly 10–16 July. Note the direction of that rule: ovulation counts back from the next period, which is why people with 35-day cycles ovulate around day 21, not day 14.
What people actually use it for
Planning: knowing your period dates before booking a trip, a trek, an exam date or a wedding outfit is the everyday use. Spotting irregularity: tracking predictions against reality for a few months shows whether your cycle is consistent — cycles regularly shorter than 21 days, longer than 35, or swinging widely are worth a gynaecologist visit. Trying to conceive: the fertile window is the target window; pair this with the dedicated Ovulation Calculator for a conception-focused view, and the Due Date Calculator once there’s good news.
The honest limits
These are calendar estimates around your average — stress, illness, travel, weight changes and conditions like PCOS all shift real cycles. Two rules to keep: the fertile-window estimate is not reliable contraception (calendar methods have high real-world failure rates), and no calculator replaces medical advice. Use the predictions as a planning aid and a pattern-spotter, not a guarantee.
Frequently Asked Questions
How do I calculate my next period date?
Add your average cycle length to the first day of your last period. Last period 1 July + 28-day cycle = next period around 29 July. The calculator does this for your next three cycles.
What is a normal cycle length?
Anything from 21 to 35 days is common in adults, and cycles vary by a few days month to month. Consistently outside that range, or suddenly irregular, is worth discussing with a gynaecologist.
When do I ovulate?
Typically about 14 days before your NEXT period — not 14 days after the last one. With a 28-day cycle that’s around day 14; with a 35-day cycle, around day 21.
Can I use the fertile window as contraception?
No. Cycle predictions are estimates around an average, and calendar-based methods have high real-world failure rates. Use the fertile window for planning and awareness, not birth control.
Is my data private?
Yes — all calculations happen in your browser. The dates you enter are never stored or sent to any server.