Period Calculator
Predict your next period, ovulation day and fertile window from the first day of your last period and your usual cycle length.
Next period starts
29 May 2026
- Next period date range
- 29 May 2026 → 2 June 2026
- Estimated ovulation day
- 15 May 2026
- Fertile window (6 days, ending on ovulation)
- 10–15 May 2026
- Cycle 2 — period start
- 26–30 June 2026
- Cycle 2 — ovulation
- 12 June 2026
- Cycle 3 — period start
- 24–28 July 2026
- Cycle 3 — ovulation
- 10 July 2026
Your next period is estimated as the first day of your last period plus your average cycle length. Ovulation is dated 14 days before the next period — the luteal phase is biologically near-constant at ~14 days, while the follicular phase varies and is what makes cycles longer or shorter (ACOG Committee Opinion 651, NHS 2023). The fertile window is the six days ending on ovulation day: sperm survive up to five days in fertile cervical mucus and the egg is viable for about 24 hours (Wilcox, NEJM 1995). This is an educational estimate, not contraception or a diagnostic tool — real cycles vary, and stress, illness, travel, breastfeeding, perimenopause and PCOS can shift any of these dates.
How to use this calculator
Enter the first day of your most recent period (the first day of bleeding, not the last), your average cycle length in days (count from day 1 of one period to day 1 of the next — typical range 21–35, default 28), and how many days your bleeding usually lasts. The calculator returns the start date and date range of your next three periods, the estimated ovulation day for each cycle, and the 6-day fertile window that ends on ovulation day.
How the calculation works
Period dates are projected forward by adding your average cycle length to the first day of your last period (LMP), repeated for cycles 2 and 3. Ovulation is dated 14 days before the next period — the luteal phase between ovulation and menstruation is biologically near-constant at about 14 days, per ACOG Committee Opinion 651 (Menstruation as a Vital Sign, 2015, reaffirmed 2024) and the NHS guidance on the menstrual cycle (2023). It is the follicular phase before ovulation that varies and makes individual cycles longer or shorter. The fertile window is the six days ending on ovulation day: this comes from Wilcox, Weinberg and Baird's landmark 1995 New England Journal of Medicine study which showed that almost all pregnancies result from intercourse during this six-day interval, because sperm can survive up to five days in fertile cervical mucus and the ovum is viable for about 24 hours after release. All date arithmetic is done in UTC so that summer-time changes never shift the answer.
Worked example
LMP = 1 May 2026, cycle length = 28 days, period length = 5 days. Next period starts 29 May 2026 (1 May + 28 days) and runs 29 May – 2 June. Ovulation = 29 May − 14 = 15 May. Fertile window = 10–15 May (the five days before ovulation plus ovulation day itself). The cycle after that starts 26 June, ovulation 12 June, fertile window 7–12 June; the third cycle starts 24 July. For a longer 35-day cycle starting on the same LMP, the next period falls on 5 June and ovulation on 22 May — the extra week lengthens the follicular phase, not the luteal phase, which is why the gap from ovulation to next period stays at 14 days.
Frequently asked questions
How accurate is a period calculator?
It is as accurate as your inputs and how regular your cycle is. For someone with a consistently 28-day cycle, the next-period prediction is usually correct within a day or two. Real cycles routinely vary by 2–7 days from month to month even in healthy adults; ACOG considers cycles regular if they fall in the 21–35 day range and vary by less than 7–9 days. Stress, illness, travel, hard training, weight change, hormonal contraception and life events like breastfeeding or perimenopause can shift dates by a week or more. Tracking a few cycles in a row gives a much better personal average than guessing.
Why is ovulation dated 14 days before the next period rather than 14 days after the last one?
Because the luteal phase — from ovulation to the start of menstruation — is biologically near-constant at about 14 days, while the follicular phase that runs from menstruation to ovulation varies. Cycle-to-cycle differences in length are almost entirely follicular-phase differences. For a textbook 28-day cycle the two phases each happen to be 14 days, so "14 days after LMP" and "14 days before next period" land on the same day. But for a 35-day cycle, ovulation is on day 21 (= 35 − 14), not day 14. Dating ovulation back from the next period is therefore the more accurate convention, used by ACOG, NHS, NICE and most cycle-tracking apps.
When am I most fertile in my cycle?
The fertile window is the six days ending on the day of ovulation — five days before ovulation, plus ovulation day itself. This was established by Wilcox, Weinberg and Baird in their 1995 NEJM study of 221 healthy women trying to conceive: virtually no pregnancies resulted from intercourse outside this window. The probability of conception is highest in the two to three days immediately before ovulation, when fertile cervical mucus is at its peak and sperm have time to reach the egg. After ovulation, the egg is only viable for about 24 hours, so fertility drops sharply.
Can I use this calculator to avoid pregnancy?
No — this is an educational tool, not contraception. Fertility-awareness-based methods (FABMs) can be effective but rely on more than a calendar: they combine basal body temperature, cervical mucus observation and (often) urinary LH testing, plus typically several months of training. Calendar-only methods like the rhythm method have a typical-use failure rate of around 13–24% per year (Trussell, Contraception 2011), because ovulation timing is variable and sperm survival is long. If you are trying to avoid pregnancy, use a contraceptive method with a measured failure rate that you find acceptable, and talk to a clinician.
What if my cycles are irregular?
If your cycles regularly fall outside the 21–35 day range, vary by more than 7–9 days from month to month, or you go more than 90 days without a period after menarche-stabilisation, ACOG considers that worth a medical conversation — common causes include PCOS, thyroid disorders, perimenopause, eating disorders, very high training loads, and stress, all of which are treatable. A calendar-based prediction will not be reliable in that situation: better tools are basal body temperature charting, ovulation-predictor (LH) sticks, or seeing a gynaecologist for hormone testing. Enter your typical cycle if it helps you plan, but treat the dates as a rough guide.
Why ask for cycle length instead of using a default 28 days?
Because only about 13% of women have a 28-day cycle (Chiazze et al., JAMA 1968, and confirmed by larger app-based studies since). The most common cycle length is 27 days, and the healthy range is 21–35 days. Using your own average — rather than the textbook 28 — typically improves the next-period prediction by 3–10 days for anyone with a longer or shorter cycle. If you do not know your cycle length, count the days from the first day of one period to the first day of the next, ideally averaged over three or more cycles.