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.
What counts as a normal cycle — and when to get it checked
A typical cycle runs 21 to 35 days in adults (up to 45 in the first years after menarche), with bleeding lasting 2 to 7 days. Variation of a few days month to month is normal. Worth raising with a doctor: cycles consistently shorter than 21 or longer than 35 days, bleeding heavier than soaking a pad or tampon every hour for several hours, periods lasting more than 7 days, bleeding between periods or after sex, no period for three months when not pregnant, or pain that stops you doing normal activities. These can point to conditions such as PCOS, fibroids, endometriosis or thyroid problems — all common and all treatable.
Sources & further reading
- NHS — Periods — normal cycle ranges, irregular periods and when to seek help.
- American College of Obstetricians and Gynecologists — clinical guidance on menstrual cycles and abnormal bleeding.
- US Office on Women’s Health — plain-language menstrual health information.
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.