Period Calculator
Predict your next periods and fertile windows from your last period and cycle length, with the day of your cycle today and what counts as normal variation.
Period Calculator: with the default inputs, next period starts is November 16, 2026.
First day of one period to the first day of the next. Average your last three to six cycles.
- Next period ends
- November 20, 2026
- Days until it starts
Negative means it is overdue against this prediction.
- 21
- Day of your cycle today
- 8
- Phase today
- Follicular
- Next ovulation
- November 2, 2026
- Next fertile window
The five days before ovulation plus ovulation day.
- Oct 28 to Nov 2
Assumptions
- Every future cycle is assumed to be exactly the length you entered; real cycles vary by several days even when they are considered regular.
- Ovulation is placed a fixed luteal phase before the next period (default 14 days), because the luteal phase is the more constant half of the cycle.
- The fertile window is the six days ending on ovulation day (Wilcox et al., 1995).
- Cycle day and days-until are measured against today's date on your device.
- This is a planning estimate, not contraception, and not a diagnosis. Cycle changes that persist are worth a clinician's attention.
| # | Period starts | Period ends | Fertile window | Ovulation |
|---|---|---|---|---|
| 1 | November 16, 2026 | November 20, 2026 | Nov 25 – Nov 30 | November 30, 2026 |
| 2 | December 14, 2026 | December 18, 2026 | Dec 23 – Dec 28 | December 28, 2026 |
| 3 | January 11, 2027 | January 15, 2027 | Jan 20 – Jan 25 | January 25, 2027 |
| 4 | February 8, 2027 | February 12, 2027 | Feb 17 – Feb 22 | February 22, 2027 |
| 5 | March 8, 2027 | March 12, 2027 | Mar 17 – Mar 22 | March 22, 2027 |
| 6 | April 5, 2027 | April 9, 2027 | Apr 14 – Apr 19 | April 19, 2027 |
Every row assumes a cycle of exactly the length you entered. Real cycles drift; treat later rows as rougher than earlier ones.
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How this is worked out
The formula
Next period = first day of last period + cycle length Ovulation = cycle start + (cycle length − luteal phase) Fertile window = ovulation − 5 days … ovulation Cycle day today = days since the current cycle began + 1
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- First day of your last period
- A calendar date.defaults to today-7
- Average cycle length
- First day of one period to the first day of the next. Average your last three to six cycles.from 15 to 60 · whole numbers only · defaults to 28
- Days of bleeding
- A number.from 1 to 14 · whole numbers only · defaults to 5
- Cycles to predict
- A number.from 1 to 24 · whole numbers only · defaults to 6
- Luteal phase length(under More options)
- Days from ovulation to the next period. Fairly constant within a person; 12–14 is typical.from 8 to 20 · whole numbers only · defaults to 14
What you get back
- Next period startsmain answer
- Next period ends
- Days until it starts
- Negative means it is overdue against this prediction.
- Day of your cycle today
- Phase today
- Next ovulation
- Next fertile window
- The five days before ovulation plus ovulation day.
What this assumes
- Every future cycle is assumed to be exactly the length you entered; real cycles vary by several days even when they are considered regular.
- Ovulation is placed a fixed luteal phase before the next period (default 14 days), because the luteal phase is the more constant half of the cycle.
- The fertile window is the six days ending on ovulation day (Wilcox et al., 1995).
- Cycle day and days-until are measured against today's date on your device.
- This is a planning estimate, not contraception, and not a diagnosis. Cycle changes that persist are worth a clinician's attention.
About this calculator
Give this your last period and your usual cycle length and it maps out the next several months: when each period starts and ends, when you ovulate, and which days are fertile. It also tells you what day of your cycle you are on right now, which is the thing most people are actually trying to work out.
How to use it
Enter the first day of bleeding of your last period — not the last day, and not the day spotting started. Cycle length is measured first-day-to-first-day, so a 28-day cycle means twenty-eight days from one period starting to the next one starting. If your cycles vary, average your last three to six rather than picking a typical one.
Under More options you can set the luteal phase — the gap between ovulation and your next period. Fourteen days is the default and is close for most people. The luteal phase is the stable part of the cycle; when a cycle runs long or short, it is almost always the follicular phase before ovulation that stretched or shrank. That is why ovulation is calculated backwards from the next period rather than forwards from the last one.
How to read the results
- Cycle day counts from day one, the first day of bleeding.
- The fertile window is the five days before ovulation plus ovulation day itself. Sperm survive up to five days; the egg survives about a day.
- The table projects forward for as many cycles as you ask. Early rows are decent; the sixth is a guess stacked on five other guesses, because every cycle's error carries into the next.
What counts as normal variation
FIGO's standard puts normal cycle frequency at 24 to 38 days, and normal cycle-to-cycle variation at about a week — if your shortest and longest cycles in a year differ by more than seven to nine days, that is classed as irregular. ACOG makes a related point in the other direction: the menstrual cycle is a vital sign, and cycles that stop, become very irregular, or bring bleeding for more than about eight days are worth investigating rather than tolerating.
Ordinary things move a cycle: travel, illness, hard training blocks, weight change, stress, a new medication. One late period after a stressful month is usually just that. A pattern of them is information.
What this cannot do
It cannot detect a pregnancy, and it cannot be used as contraception. Calendar methods have real-world failure rates in the region of 12–15 pregnancies per 100 users a year — better than nothing and far worse than most alternatives — precisely because ovulation moves. If you are avoiding pregnancy, use a method that does not depend on the future looking like the past. If you are trying to conceive, ovulation predictor kits and basal temperature will beat this calendar every time.
Frequently asked questions
▸When is my next period?
Add your cycle length to the first day of your last period. On a 28-day cycle, a period starting on the 3rd means the next one is due around the 31st.
▸How late is too late?
A period that arrives up to a week either side of the prediction is within normal variation for many people. If you are more than a week late and could be pregnant, take a test; if periods stop for three months without pregnancy, see a clinician.
▸What is a normal cycle length?
FIGO puts the normal range at 24 to 38 days, with variation of no more than about a week between your shortest and longest cycles. Adolescents in the first few years after their first period vary far more than that and usually settle.
▸Can I use this to avoid pregnancy?
No. Calendar-based methods fail for roughly 12–15 in 100 users a year in typical use, because ovulation timing shifts. If avoiding pregnancy matters, use a method that does not rely on predicting the future.
▸Why did my cycle suddenly change?
Travel, illness, stress, weight change, a new training load and many medications all shift ovulation, which shifts the whole cycle. One odd cycle is usually nothing; a run of them is worth a check-up.
Related calculators
The questions people ask next to a period.
Estimate ovulation and the six-day fertile window from your last period and cycle length, with the next three cycles mapped and an honest note on accuracy.
Work back to the estimated conception date from a due date, a last period or a birth date, with the fertile window that produced it and an honest error range.
Due date from your last period, conception date, IVF transfer or a scan date, with how many weeks pregnant you are today, trimester and key milestone dates.
Days, weeks, months and business days between two dates, or add and subtract days, weeks, months or years from any date — leap years handled exactly.