Period Calculator
Predict your next six menstrual cycles, fertile window, and estimated ovulation date based on your last period and average cycle length.
Results from this calculator are estimates provided for general informational purposes only, based on formulas, rates, and standards commonly accepted as of 2026. Figures may differ slightly from other calculators or professional sources due to rounding methods, differing assumptions, or regional regulations, and rules may change over time. Always consult a qualified professional — such as a financial advisor, healthcare provider, or other relevant specialist — before making decisions based on these results.
Next Period Expected
Nov 2, 2026
Summary
Based on a last period starting Oct 5, 2026 with a 28-day cycle, your next period is expected around Nov 2, 2026, with a fertile window from Nov 11, 2026 to Nov 17, 2026 and estimated ovulation around Nov 16, 2026.
Estimated Ovulation
Nov 16, 2026
Fertile Window
Nov 11, 2026 – Nov 17, 2026
Period and fertile window across the next 6 cycles (days from cycle start)
What is a Period Calculator?
A period calculator estimates the dates of your upcoming menstrual cycles based on the start date of your last period and your typical cycle length. It projects forward using simple averages, giving you a heads-up on when to expect your next several periods, your likely fertile window, and your estimated ovulation date each cycle.
Because cycle length naturally varies from month to month — even for people with very regular cycles — these are estimates, not guarantees. The further out a prediction is, the more it can drift from your actual cycle, so treat predictions for cycle 4, 5, or 6 as rough guidance rather than precise dates.
Next 6 Cycles
| Cycle | Period Dates | Fertile Window | Est. Ovulation |
|---|---|---|---|
| 1 | Oct 5, 2026 – Oct 9, 2026 | Oct 14, 2026 – Oct 20, 2026 | Oct 19, 2026 |
| 2 | Nov 2, 2026 – Nov 6, 2026 | Nov 11, 2026 – Nov 17, 2026 | Nov 16, 2026 |
| 3 | Nov 30, 2026 – Dec 4, 2026 | Dec 9, 2026 – Dec 15, 2026 | Dec 14, 2026 |
| 4 | Dec 28, 2026 – Jan 1, 2027 | Jan 6, 2027 – Jan 12, 2027 | Jan 11, 2027 |
| 5 | Jan 25, 2027 – Jan 29, 2027 | Feb 3, 2027 – Feb 9, 2027 | Feb 8, 2027 |
| 6 | Feb 22, 2027 – Feb 26, 2027 | Mar 3, 2027 – Mar 9, 2027 | Mar 8, 2027 |
How Predictions Are Calculated
Each predicted period start is simply the previous period's start date plus your average cycle length. Ovulation is estimated using the luteal phase, which is far more consistent across individuals (typically 12-16 days, averaging 14) than the first half of the cycle:
The fertile window covers the 5 days before ovulation plus the day of ovulation itself, reflecting that sperm can survive up to about 5 days while an egg is viable for roughly 24 hours after release.
Why Predictions Get Less Reliable Over Time
This calculator assumes every future cycle is exactly the same length as your average — but real cycles vary due to stress, illness, travel, sleep, exercise changes, and normal biological variability. A "regular" 28-day cycle might really run anywhere from 25 to 31 days month to month. Because each cycle's prediction builds on the last, small variations compound, so cycle 5 or 6's predicted dates carry meaningfully more uncertainty than cycle 2's.
Tracking Your Actual Cycle Improves Accuracy
The single best way to improve these predictions is to log your actual period start dates over several cycles and recalculate your average cycle length from real data, rather than relying on a single guess. If your cycles vary widely (for example, ranging from 24 to 38 days), fertility awareness methods based on cycle length alone become much less reliable, and tracking additional signs like basal body temperature or cervical mucus can help narrow down ovulation more precisely.
When to See a Doctor
Cycles are generally considered irregular if they're consistently shorter than 21 days, longer than 35 days, or vary by more than about 7-9 days from cycle to cycle. Irregular cycles, very heavy or very light bleeding, severe pain, or a missed period alongside a negative pregnancy test are all reasons to check in with a healthcare provider rather than rely on estimates from a calculator.
Example — Your Current Inputs
Based on a last period starting Oct 5, 2026 with a 28-day cycle, your next period is expected around Nov 2, 2026, with a fertile window from Nov 11, 2026 to Nov 17, 2026 and estimated ovulation around Nov 16, 2026.
Additional Example — Shorter Cycle
Someone with a last period starting January 1 and a 24-day cycle would expect their next period around January 25, with ovulation around January 11 and a fertile window from about January 6 to January 12 — noticeably earlier in the cycle than someone with a 28- or 30-day cycle.
About These Parameters
- First Day of Last Period
- The date your most recent period began — day 1 of your current cycle. All future predictions are anchored to this date.
- Average Cycle Length
- The number of days from the start of one period to the start of the next. If you don't know your average, 28 days is a commonly used default, though anywhere from 21 to 35 days is considered typical.
- Average Period Length
- How many days your period itself typically lasts, usually 3 to 7 days. This only affects the displayed period end date, not the cycle-to-cycle predictions.
Frequently Asked Questions
How accurate is a period calculator?
It's only as accurate as your average cycle length, and even a very regular cycle naturally varies by a few days month to month. Treat the next 1-2 predicted cycles as reasonably reliable and later ones as rougher estimates.
What is the fertile window?
The days in your cycle when pregnancy is possible — roughly the 5 days before ovulation plus the day of ovulation itself, since sperm can survive several days while the egg is only viable for about 24 hours after release.
Can this calculator be used for birth control?
No. This calculator is for general awareness and planning only. Fertility awareness methods used for contraception require careful, consistent tracking of multiple fertility signs and guidance from a healthcare provider — a simple date-based estimate is not reliable enough on its own to prevent pregnancy.
Why is my actual period different from the prediction?
Cycle length is affected by stress, illness, travel, weight changes, exercise, certain medications, and normal biological variability — any of these can shift your actual period earlier or later than a simple average-based prediction.