What the period calculator does
The Period Calculator predicts when your next period will start based on three inputs: the first day of your last period, your typical cycle length (in days), and your typical period length (also in days). From that, it projects the next 4 expected periods, an estimated ovulation date, and the fertile window where pregnancy is most likely.
The math is simple calendar arithmetic:
- Next period start = last period start + cycle length
- Next ovulation = next period - 14 days (the 'luteal phase' is consistently around 14 days regardless of total cycle length)
- Fertile window = ovulation - 5 days to ovulation + 1 day (sperm can live ~5 days in the reproductive tract; egg is viable ~24 hours after ovulation)
The accuracy of the prediction is entirely about how regular your cycle is. If you're a textbook 28-day-every-month person, the dates will be close. If your cycle varies 3-5 days month to month (which is normal), treat them as best estimates.
How to use the calculator
- Pick the first day of your last period from the date picker. (If you bled lightly the day before what you considered "the start," count that as day 1 — flow can be light at the very beginning.)
- Enter your cycle length — the days from start of one period to start of the next. Average is 28; normal range is 21-45.
- Enter your period length — how many days bleeding lasts. Most periods are 3-7 days.
- The calculator shows your next expected period (start and end), your next 4 periods, and the estimated next ovulation/fertile window.
- Tap Copy to grab a 4-cycle schedule for your calendar app or planner.
What's a normal cycle?
The "28-day cycle" you've heard quoted forever is just an average. Real cycles vary widely between people and across a person's lifetime:
- 21–45 days is considered normal in adults. Below or above that range warrants a check-in with a doctor.
- Teenagers often have irregular cycles for the first 2-3 years after their first period as the hypothalamic-pituitary-ovarian axis matures.
- 20s and 30s tend to be the most regular years for most people.
- 40s and approaching menopause often bring shorter cycles (or longer ones, or both, in alternating months) as ovarian function changes.
- Postpartum and breastfeeding: cycles often pause entirely while exclusively breastfeeding (lactational amenorrhea), then resume irregularly for a few months.
- Hormonal contraception: combined oral contraceptives, the pill, the patch, the ring, and the implant suppress natural cycles. The "period" you have on the placebo week of combined pills is a withdrawal bleed, not a true menstruation. The calculator doesn't apply if you're on hormonal contraception.
What matters more than the exact length is consistency. A regular 35-day cycle is healthier (in the sense of being more predictable and probably reflective of regular ovulation) than an erratic 28-day-then-22-day-then-40-day pattern.
How ovulation timing actually works
The menstrual cycle has two halves separated by ovulation:
- Follicular phase: from period start to ovulation. VARIABLE in length — this is what makes cycles different lengths. A short follicular phase = a 21-day cycle; a long one = a 40-day cycle.
- Luteal phase: from ovulation to next period. CONSISTENT at ~14 days (give or take 1-2 days) for most people.
This is why the "ovulation = 14 days before next period" rule works for any cycle length, and is more accurate than "ovulation = day 14" (which only works for textbook 28-day cycles).
The calculator uses this rule. For a 35-day cycle, your ovulation is around day 21. For a 24-day cycle, it's around day 10. For a textbook 28-day cycle, it's around day 14.
The fertile window in detail
Pregnancy is possible only when sperm meet a recently-released egg. The biology dictates a window of about 6 days:
- 5 days BEFORE ovulation: sperm can survive in the female reproductive tract (especially in cervical mucus) for up to 5 days, waiting for the egg.
- The day of ovulation: the egg is released.
- 1 day after ovulation: the egg is viable for about 24 hours; if not fertilized, it dies and is reabsorbed.
So the realistic fertile window is from 5 days before ovulation to 1 day after — 6 days total. The most likely conception days are the 2-3 days right before ovulation, when sperm are already present and waiting. The day of ovulation itself is also high-fertility but slightly less so (sperm have to travel up to meet the egg, and the egg's window is short).
Worked example
Say your last period started on March 5, your cycle is 30 days, and your periods last 5 days.
- Next period: March 5 + 30 = April 4 to April 8.
- Next ovulation: April 4 - 14 = March 21.
- Fertile window: March 16 to March 22.
- Period 2: April 4 + 30 = May 4–8.
- Period 3: June 3–7.
- Period 4: July 3–7.
The "first" prediction (next period) is the most reliable; later predictions compound any cycle variation, so periods 3 and 4 should be treated as "approximate."
Why your cycle might shift unexpectedly
Cycles vary even in healthy people. Common causes of a "late" or "early" period:
- Stress — high cortisol can suppress ovulation, delaying the next period.
- Travel and time zones — disrupts circadian rhythms that regulate hormones.
- Illness — especially viral infections, which the immune system can interpret as a "stop reproducing" signal.
- Weight changes — especially rapid loss; very low body fat can stop ovulation entirely.
- Intense exercise — common in competitive athletes; "athlete amenorrhea" is well-documented.
- Sleep disruption — chronic short sleep affects hormonal regulation.
- Hormonal birth control changes — starting, stopping, or switching methods all affect cycles for several months.
- Hormonal IUD — periods often become very light or stop entirely.
- Pregnancy — if your period is more than a week late and there's any chance of pregnancy, take a test.
- Approaching menopause — typically starts with subtle changes in cycle length, then increased irregularity.
- Polycystic ovary syndrome (PCOS), thyroid issues, hyperprolactinemia — medical conditions worth investigating if irregularity is persistent.
Period tracking vs period predicting
This calculator predicts based on YOUR INPUT. Most period-tracking apps (Clue, Flo, Glow, Apple Health, Cycles) do the same prediction PLUS log your actual cycles over time, refining their predictions with your specific patterns and any symptoms you report (bloating, mood, cervical mucus, basal body temperature).
If you've been tracking for several cycles, the apps' predictions will usually beat this calendar calculator because they've learned your individual variation. If you're new to tracking or want a quick what-if without installing anything, the calculator is the right tool.
Calendar method as contraception — proceed with caution
Calendar-based fertility awareness ("the rhythm method") has a real-world failure rate of about 24% per year — meaning 24 out of 100 couples relying on it become pregnant within a year. Compare to:
- Condoms (typical use): 13% failure / year
- Combined oral contraceptives (typical): 7% failure / year
- IUDs (hormonal or copper): < 1% failure / year
The high failure rate of calendar methods reflects: (1) cycles aren't perfectly regular, (2) ovulation can shift unexpectedly, (3) sperm survive longer than people often expect, and (4) the math is unforgiving — one slip in a "safe" day is all it takes.
Combined fertility-awareness methods (calendar + cervical mucus + basal body temperature) can do better — sometimes as good as 5-10% failure per year — but still nowhere near hormonal or barrier methods. If pregnancy prevention is critical, use a more effective method.
When to talk to a doctor
- Periods that are very heavy (soaking through more than one pad/tampon per hour for several hours).
- Severe pain that interferes with daily life (could indicate endometriosis, fibroids, or other conditions).
- Cycles consistently shorter than 21 days or longer than 45 days.
- Sudden persistent change in your usual pattern (3+ months of significant irregularity after years of consistency).
- Bleeding between periods or after intercourse.
- Periods that stop unexpectedly (and pregnancy is ruled out) for 3+ months.
- Trying to conceive for 12+ months without success (or 6+ months if over 35).
What the calculator gives you, summarized
- Next 4 expected periods — start and end dates, useful for trip planning, scheduling, and stocking up on supplies.
- Next ovulation date — based on the 14-day-before-period rule.
- Fertile window — the 6-day stretch where pregnancy is most likely.
- Privacy by design — runs entirely in your browser; no data leaves your device.
- Honest framing — the calculator is upfront that predictions are estimates, not guarantees, and that calendar methods aren't reliable contraception.
Three inputs (last period start, cycle length, period length), four cycles of predictions plus ovulation/fertility info. Useful for planning; not a substitute for medical advice or reliable contraception.