Predict your next six period start dates, see a live view of where you are in your current cycle, and download the forecast to your calendar. Pair with the Ovulation Calculator for a detailed fertile-window estimate.
Period Calculator
LiveHow this is calculated
Last period started June 1, 2026, with a 28-day cycle: next period ≈ June 29, 2026, then July 27, August 24, and three more cycles beyond that -- all shown on the timeline above, with the dial showing exactly where today falls in the current cycle.
The four phases, in full
| Phase | Typical days (28-day cycle) | What's happening |
|---|---|---|
| Menstrual | 1–5 | The uterine lining sheds (your period) as hormone levels drop, triggering the start of a new cycle. |
| Follicular | 1–13 | Overlaps with menstruation at the start. Follicle-stimulating hormone (FSH) prompts ovarian follicles to develop; the uterine lining begins rebuilding. This phase's length is the main source of cycle-to-cycle variation. |
| Ovulatory | ~14 | A surge in luteinizing hormone (LH) triggers release of a mature egg, viable for fertilization for roughly 12–24 hours. |
| Luteal | 15–28 | The ruptured follicle (corpus luteum) produces progesterone, preparing the uterine lining for possible implantation. Comparatively steady — typically 10–17 days — across different people. |
What can affect cycle regularity
| Factor | Typical effect |
|---|---|
| Stress | Can delay ovulation, pushing the whole cycle later that month |
| Significant weight change | Both notable weight loss and weight gain can disrupt regular ovulation — the BMI Calculator can help track this alongside cycle changes |
| Thyroid function | Both an underactive and overactive thyroid commonly affect cycle regularity — the TSH (Thyroid) Calculator is a starting reference point, not a diagnosis |
| Hormonal birth control changes | Starting, stopping, or switching methods can shift timing for several months while the body adjusts |
| Breastfeeding / postpartum | Cycles often remain irregular or absent for a variable period after childbirth |
| Perimenopause | Cycle length and regularity commonly change in the years leading up to menopause |
| PCOS or other hormonal conditions | Can cause persistently irregular or absent cycles — worth a healthcare provider's evaluation, not just a calculator adjustment |
The "14-day luteal phase" is an average, not a constant
Nearly every period calculator treats the luteal phase as a fixed 14 days. The actual clinical range, per an American Society for Reproductive Medicine committee opinion on luteal phase deficiency, is roughly 10 to 17 days across different people. Most of the natural variation in when a period actually arrives traces back to the follicular phase -- the luteal phase is the comparatively steady half, which is exactly why it's used as the anchor for backward-counting ovulation estimates.
When cycle changes are worth a doctor's visit
- Periods consistently shorter than 21 days or longer than 35 days apart
- A period absent for 3 or more months without pregnancy or a known cause
- Sudden, unexplained changes to a previously regular cycle
- Bleeding heavy enough to soak through a pad or tampon every hour for several hours
- Severe pain that interferes with daily activities
Common mistakes
Frequently asked questions
How accurate are these predictions?
Most reliable for your very next period if your cycle is fairly regular, and less certain further out since cycle length can naturally vary. Tracking 3-6 cycles and using the average improves accuracy meaningfully.
What does the dial actually show?
Your position in the current cycle today, computed from your entered last period date and cycle length -- correctly accounting for however many full cycles have passed since then, not just the most recent one.
What if my period is late?
Occasional variation is common -- stress, travel, illness, or routine changes are frequent causes. Regularly missed or irregular periods are worth discussing with a healthcare provider.
Can I use this to track ovulation too?
The dashboard above shows a quick fertile-window and estimated ovulation summary alongside your period prediction. For a more detailed ovulation estimate with additional factors, use the dedicated Ovulation Calculator.
What's in the downloaded calendar file?
A standard .ics file with all 6 predicted period dates as calendar events, importable into Google Calendar, Apple Calendar, Outlook, or any app that accepts the standard calendar format. Generated entirely in your browser -- nothing is uploaded anywhere.
Is the luteal phase really always 14 days?
No -- 14 days is a population average. The documented clinical range is roughly 10 to 17 days across different people, though it's the more stable half of the cycle compared to the follicular phase.
What is a normal cycle length?
21 to 35 days is commonly cited as typical (ACOG), with 28 days as a well-known average that not everyone actually has.
Does this tool store or send my dates anywhere?
No -- everything, including the calendar file, is calculated and generated entirely in your browser. Nothing is transmitted or stored.
Sources
- Typical cycle length range (21-35 days). American College of Obstetricians and Gynecologists (ACOG).
- Luteal phase variability (10-17 days). ASRM Practice Committee opinion, "Current clinical irrelevance of luteal phase deficiency."
- Fertile window timing. Wilcox AJ et al., New England Journal of Medicine, 1995, and BMJ, 2000.
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