Predict your next 6 menstrual periods with start dates, fertile windows, ovulation dates, and PMS windows side-by-side. Customisable cycle length and period duration; tracks cycle regularity.
Reviewed by the CalculatorKosh Editorial TeamUpdated June 2026Free · No sign-up
Period Calculator
Predict your next 6 menstrual periods with start dates, fertile windows, ovulation dates, and PMS windows side-by-side. Customisable cycle length and period duration; tracks cycle regularity.
Your Cycle
The first day of bleeding from your most recent period.
Normal range 21-35; average 28.
Bleeding days; normal 2-7.
Symptoms start ~5-7 days before period.
Predict the next 1-12 cycles.
Next 6 cycles
| Cycle | Period | Fertile Window | Ovulation | PMS Window |
|---|---|---|---|---|
| #1 | May 29 – Jun 2 | May 10 – May 16 | May 15 | May 22 – May 28 |
| #2 | Jun 26 – Jun 30 | Jun 7 – Jun 13 | Jun 12 | Jun 19 – Jun 25 |
| #3 | Jul 24 – Jul 28 | Jul 5 – Jul 11 | Jul 10 | Jul 17 – Jul 23 |
| #4 | Aug 21 – Aug 25 | Aug 2 – Aug 8 | Aug 7 | Aug 14 – Aug 20 |
| #5 | Sep 18 – Sep 22 | Aug 30 – Sep 5 | Sep 4 | Sep 11 – Sep 17 |
| #6 | Oct 16 – Oct 20 | Sep 27 – Oct 3 | Oct 2 | Oct 9 – Oct 15 |
Cycle Phases
Day 1-5 — the bleeding window. Uterine lining sheds.
Period end to ovulation. Estrogen rises; a follicle matures.
~14 days before next period. Egg is released — peak fertility.
Ovulation to next period (~14 days). Progesterone rises then drops.
The last 5-7 days of the luteal phase. Bloating, mood changes, cramps as progesterone falls.
Next 60 days at a glance
From May 1
When to see a doctor
Consult a gynecologist if your cycle is consistently shorter than 21 days, longer than 45 days, or varies by more than 7-9 days between cycles. Also if periods last longer than 7 days, are extremely heavy (changing pad or tampon every hour), or if you have severe pain that affects daily life — these can indicate underlying conditions like PCOD, endometriosis, or fibroids.
How It Works
This period calculator projects the next several menstrual cycles from two simple inputs — the first day of your last period (your LMP, or last menstrual period) and your average cycle length. For each predicted cycle it lays out the period window, the ovulation date, the 6-day fertile window, and the 5-7 day PMS window on a single screen, plus a 60-day calendar strip that overlays every phase. It is built for anyone who wants to plan ahead: to anticipate the next period before a trip or event, to know which days are most and least fertile, or simply to understand the rhythm of their own body. It works as a calendar-based screening and planning tool — not as a diagnostic test and not as a contraceptive method.
How the math works
The prediction rests on three well-established rules. First, each future period starts cycle length × n days after your LMP, so a 28-day cycle simply repeats every 28 days. Second, ovulation in any given cycle falls 14 days before the next period, because the luteal phase (ovulation to the next bleed) is biologically near-constant at roughly 14 days regardless of how long the overall cycle is. Third, the fertile window spans the 5 days before ovulation — sperm can survive that long in the reproductive tract — through 1 day after, when the released egg is still viable, giving a 6-day window in which conception is possible. The PMS window is drawn from period start − your PMS days through the day before bleeding begins, because premenstrual symptoms are driven by the sharp late-luteal drop in progesterone.
A worked example
Suppose your last period began on 1 May and your average cycle is 28 days. The calculator adds 28 days to predict the next period on 29 May. Ovulation is then 29 May minus 14 days, which lands on 15 May. The fertile window runs from 10 May (ovulation minus 5) through 16 May (ovulation plus 1) — these are the days to target if you are trying to conceive, or to avoid if you are not. The PMS window, with a 7-day setting, runs from 22 May through 28 May. Change the cycle length to 32 days and every downstream date shifts: the next period moves to 2 June and ovulation moves to 19 May, because the extra days all land in the follicular phase before ovulation, not after it.
Why short cycles ovulate earlier
Because the luteal phase is fixed near 14 days, all the variation in cycle length comes from the follicular phase — the stretch from period start to ovulation. A 21-day cycle ovulates around day 7, while a 35-day cycle ovulates around day 21. This is the single most misunderstood point about fertility: women with shorter cycles can ovulate, and therefore conceive, much sooner after their period ends than the common "day 14" rule of thumb suggests. Always anchor ovulation to the next period, never to a calendar day-number.
Tips to get the most accurate prediction
Track at least three to six cycles and use your averagecycle length rather than a single month, which can be unusually long or short. Always count day 1 as the first day of full bleeding, not spotting the day before. If your cycles swing by only a day or two, calendar predictions like these are quite reliable; if they swing widely, pair this tool with ovulation predictor kits or basal body temperature tracking, which read the body directly rather than from a calendar. Re-enter your real LMP each month so the projection re-anchors to what actually happened.
Common mistakes to avoid
The most frequent errors are entering the date a period endedinstead of the day it started, confusing cycle length (first day of one period to the first day of the next) with period length (the number of bleeding days), and treating the fertile window as a guarantee — neither the presence nor the absence of fertile days here should be used for contraception, since real ovulation can shift with stress, illness, travel, sleep, weight change, or hormonal contraceptives. Remember this is a screening and planning aid only. If your cycles are consistently shorter than 21 days, longer than 45 days, vary by more than 7-9 days between cycles, or are accompanied by very heavy bleeding or severe pain, treat the result as a prompt to consult a gynecologist rather than a diagnosis — irregular cycles can reflect PCOS, thyroid issues, or other conditions that need proper evaluation.
Frequently Asked Questions
Calendar-based calculators are most useful when your cycle is regular. They project future periods by adding your average cycle length to your last period date — actual dates can still vary by 2-7 days even for women with very regular cycles, because the body responds to stress, illness, sleep, travel, weight changes, and exercise. For irregular cycles (where individual cycles vary by more than 7-9 days from each other), calendar predictions become unreliable and tracking apps that use basal body temperature or ovulation tests give better results.
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