Period Calculator

How this calculator works

Day 1 is the first day of real bleeding, not the spotting before it; your cycle length runs from that day 1 to the day before the next one. The calculator adds your cycle length to the start of your last period to predict the next, then repeats the step twice more.

Ovulation is estimated by counting back, because the second half of the cycle holds fairly steady at 12 to 14 days while the first half stretches and shrinks. The tool places ovulation 14 days before each predicted period and marks the fertile window as the five days before it plus ovulation day. “Days until next period” is the gap between today and the next predicted start; past that date, the line flips to “days late.”

The defaults, 28 and 5, are placeholders. In the largest published dataset, 612,613 cycles from 124,648 app users, the average cycle was 29.3 days and only about 13 percent of cycles were exactly 28 days. Ovulation also landed later than the textbook day 14 on average, which is why this calculator anchors it to the next period instead. Enter your own average from the last three to six cycles if you know it.

How to read your result

How long is a normal menstrual cycle? ACOG puts the typical range at 21 to 35 days, with 2 to 7 days of bleeding. A swing of up to 7 to 9 days between your shortest and longest cycle over a year still counts as regular, and it is the main reason a period calendar drifts.

Late period calculator: what “days late” means

Two days late against a 28-day assumption usually means your cycle is not 28 days. Late ovulation is the most common reason a real period arrives late: if a cold pushed ovulation back four days, the period follows four days later. If pregnancy is possible, a home test on the first day you are late is reliable for most brands. One late cycle is normal; three missed months is not.

Where the number misleads

Apps and calendars guess; they do not measure. A period tracker knows your past start dates, nothing more. Only urine LH strips, waking temperature and cervical mucus changes actually track ovulation; if you are trying to conceive or avoid conceiving, use those.

Stress, travel, illness and weight change

Each can delay or skip ovulation, and the period moves with it. A bad flu or losing 10 lb quickly can add a week to one cycle without meaning anything is wrong.

Hormonal contraception

On the pill, patch or ring, the monthly bleed is a withdrawal bleed timed by the pack, not a period timed by ovulation; the calculator describes your pill schedule, not your cycle. A hormonal IUD or implant often makes bleeding lighter, irregular or absent,.

Periods in your 40s: shorter, skipped, heavier

For most women the calendar starts changing in the early to mid 40s. First the cycles shorten: a lifelong 29-day cycle becomes 25 or 26. Then they scatter, a 24-day cycle followed by a 38-day one, and eventually months are skipped. The STRAW+10 staging system draws two lines: the early menopausal transition begins when consecutive cycles persistently differ by 7 days or more, the late transition when a gap of 60 days or more appears. Menopause is named in hindsight, 12 months after the final period.

Flow often gets heavier along the way: in cycles without ovulation the lining builds without the progesterone signal that normally limits it, and the eventual bleed can be long and heavy. That is common in perimenopause and still worth a check, because heavy bleeding at this age can also come from fibroids, polyps or thyroid changes, and because it quietly drains iron. The table above does not relax with age: hourly soaking, bleeding past 7 days, bleeding between periods or after sex, and any bleeding after 12 months of none deserve an appointment.

What to do with it

1. Track three cycles, then re-enter your real average. Note day 1, the last bleeding day and one word for flow; your own average beats the 28-day default.
2. Count changes on your heaviest day. If it is hourly, that number and the length of your longest recent bleed are what to bring to a clinician.
3. In your 40s, add a column for gaps. A cycle 7 or more days different from the last, or a 60-day gap, tells you where you are. Heavier periods at this stage are why iron sits high in our list of supplements worth taking after 50, and why a ferritin test should come before the bottle.
4. Book the visit when the table says so. An annual well-woman visit belongs on your checklist for your body regardless.
5. Watch sleep in the second half of the cycle. Progesterone raises core temperature after ovulation and night sweats arrive with the transition, two common reasons for waking up at 3 a.m. before a period.