Period Calculator
Predict your next periods, ovulation date, and fertile window from your last period and average cycle length, with a six-cycle calendar.
How to use this calculator
- 1Enter the first day of your last period, which is day one of the cycle.
- 2Enter your average cycle length — first day of one period to first day of the next.
- 3Enter how many days your period usually lasts.
- 4If you know your luteal phase length from tracking, enter it; otherwise 14 days is the standard assumption.
How it works
Cycle prediction
next period = last period + cycle length ovulation = next period − luteal phase length fertile window = ovulation − 5 days … ovulation + 1 day due date = last period + 280 days + (cycle length − 28)
Ovulation is calculated backwards from the next expected period rather than forwards from the last one. This is because the luteal phase — ovulation to period — is remarkably consistent at 12 to 16 days for a given person, while the follicular phase before ovulation is what varies and is what makes one cycle longer than another. Counting forwards from the last period assumes ovulation on day 14, which is wrong for anyone whose cycle is not 28 days.
Worked example
With a last period starting 8 July and a 28-day cycle, the next period is expected on 5 August. Ovulation is 14 days before that — 22 July — so the fertile window runs from 17 to 23 July.
Period Calculator: the complete guide
The four phases of a cycle
The cycle begins on the first day of bleeding. Menstruation typically lasts 3 to 7 days as the uterine lining sheds. The follicular phase overlaps and continues after, as follicle-stimulating hormone matures an egg and rising oestrogen rebuilds the lining.
Ovulation is a single event: a surge of luteinising hormone releases the egg, usually 12 to 16 days before the next period. The luteal phase follows, during which progesterone maintains the lining. If no implantation occurs, progesterone falls and the next period begins.
The key asymmetry is that only the follicular phase varies much. Someone with a 35-day cycle does not have a longer luteal phase — they have a longer follicular phase, and they ovulate around day 21 rather than day 14.
Why the fertile window is six days, not one
The egg survives only about 12 to 24 hours after release. Sperm, however, can survive in fertile cervical mucus for up to five days. That makes the fertile window roughly five days before ovulation plus the day of ovulation itself.
Probability is not uniform across it. The highest chance of conception is in the two days immediately before ovulation and on the day itself; the day after drops sharply. Intercourse every one to two days across the window is generally more effective than trying to time a single day precisely.
This is not a contraceptive method
Calendar-based prediction is one of the least reliable forms of contraception, with typical-use failure rates around 12 to 24% per year. Cycle length varies month to month, ovulation can shift by several days with stress, illness, travel, or disrupted sleep, and sperm survival extends the risk window considerably.
Proper fertility awareness methods track basal body temperature, cervical mucus, and often ovulation predictor kits together, and require training to use. A calendar estimate alone is not one of them.
When irregularity is worth a conversation
A typical adult cycle runs 21 to 35 days, and it is normal for individual cycles to vary by a few days. Adolescents in the first years after menarche and people approaching menopause both see wider natural variation.
Worth raising with a clinician: cycles consistently shorter than 21 or longer than 35 days, variation of more than 7 to 9 days between cycles, bleeding heavier than a pad or tampon per hour for several hours, periods lasting more than seven days, bleeding between periods, absence of periods for three months without pregnancy, or pain that stops you functioning. Conditions such as PCOS, thyroid disorders, endometriosis, and fibroids are common and treatable.
Frequently asked questions
How do I calculate my cycle length?
Count from the first day of one period to the day before the next period starts. That is one full cycle. Track three or four cycles and average them — a single measurement will not reflect your normal variation.
When am I most likely to conceive?
In the two days before ovulation and on the day of ovulation itself. The full fertile window is about six days because sperm survive up to five days, but the probability is heavily concentrated at the end of it.
Can I rely on this to avoid pregnancy?
No. Calendar prediction is among the least reliable contraceptive approaches, with typical-use failure rates around 12–24% per year. Ovulation timing shifts with stress, illness, and travel, and sperm survive for days. Use a proper contraceptive method.
Why does the calculator ask for a luteal phase length?
Because it is the more reliable half of the cycle. The luteal phase is consistently 12–16 days for a given person, while the follicular phase varies. Counting ovulation backwards from the next period is therefore far more accurate than assuming day 14, especially for cycles that are not 28 days.