Predict next periods, ovulation dates and fertile windows for the upcoming cycles.
Ovulation = cycle length − 14 days (luteal phase is the more constant of the two phases). Fertile window = ovulation − 5 days to ovulation + 1 day (sperm survival + ovum lifespan). This is a planning tool, not a contraceptive method.
The menstrual cycle is one of the most consequential biological rhythms in a woman's life — it governs fertility, can influence mood and energy, and intersects with travel, athletic peak performance, and family planning. Yet most people track it imprecisely or not at all. A simple calculator that takes the first day of the last period, the average cycle length, and the period duration can predict the next 6 cycles plus their fertile windows and ovulation dates, with reasonable accuracy for women whose cycles are regular. This is not a contraceptive method — fertility-awareness methods require additional signals (basal body temperature, cervical mucus, LH strips) for true effectiveness — but it's a useful planning tool for non-contraceptive purposes (knowing when the next period might fall during a vacation, planning conception attempts, anticipating PMS).
The cycle is divided into two phases: follicular (period start to ovulation, variable length) and luteal (ovulation to next period, ~14 days, much more constant across women). Therefore:
The calc projects 6 cycles forward from the entered last-period start. It then determines the user's current cycle phase by comparing today's date to the boundaries: period (during the bleeding days), fertile window (during the 5+1 day fertile span), ovulation day, follicular (between period end and ovulation start), or luteal (after ovulation, before next period).
Pick the first day of your last period from the date input. Set the average cycle length in days (21 to 45 — most women are 26–32, with 28 as the textbook average). Set the period length in days (typically 3–7). The calc projects the next 6 cycles, identifies the current phase, computes the next ovulation and fertile window, and shows a 3-cycle calendar visualization with periods (red), fertile windows (light green), and ovulation markers (dot).
Last period started 2026-04-15, average cycle 28 days, period length 5 days.
If today is 2026-05-03 and the last period started 2026-04-15, today is in the luteal phase of cycle 1 (ovulation was 4 days ago).
Not a contraceptive method. The fertile window predicted here is a planning estimate, not a contraceptive. True fertility-awareness methods (FAM) like the Symptothermal method combine cycle tracking with daily basal body temperature, cervical mucus observation, and sometimes LH-test strips — and even then, perfect-use failure rates are 0.4 % per year while typical-use rates are 6–24 %. The calculator assumes a textbook cycle and does not adapt to individual variation.
Cycle length variability. Even women with "regular" cycles have ±3-day variation between cycles. PCOS, perimenopause, breastfeeding, hormonal contraceptives just discontinued, stress, travel, and major weight changes can shift cycle length by a week or more. The calculator uses your average; the actual next period can differ.
Ovulation isn't always 14 days before next period. The classic luteal phase is 12–16 days; a 14-day fixed value is a population mean. Some women have an 11-day luteal phase; others 16-day. Use BBT or LH strips to nail this down personally.
Anovulatory cycles. ~10 % of cycles in adolescents and ~5 % in healthy adults are anovulatory — bleeding occurs without preceding ovulation. The calc doesn't detect this; if conception attempts repeatedly fail despite tracking, get medical evaluation.
First post-pill cycles. After stopping hormonal contraception, the first 1–3 cycles can be irregular as the hypothalamic-pituitary-ovarian axis re-establishes its rhythm. Don't extrapolate from a post-pill cycle.
Perimenopause distorts everything. From mid-40s onward, cycles can shorten, lengthen, or skip. The 28-day model fails progressively over a 10-year transition. Use shorter trailing averages (last 3 cycles, not 12).
Cycle 1 of a teenager. The first 2–3 years after menarche are typically anovulatory and irregular; predictions are unreliable.
Contraceptive interference. The calc assumes natural cycles. Hormonal contraception (pill, ring, patch, hormonal IUD) suppresses ovulation; the "withdrawal bleed" is not a true period. This calc has no meaning during hormonal contraception.
Day-1 ambiguity. "First day of period" means the first day of bleeding — not spotting. Some women have 1–2 days of pre-period spotting that should not count as day 1.