How the period calculator works
Your next period is due one cycle length after the first day of your last period. With a 28-day cycle, that’s 4 weeks later.
The calculator repeats that step to forecast six periods ahead. It also estimates ovulation — about 14 days before each period — so you can see your fertile window. For a more detailed fertility view, open the ovulation calculator.
Tip: track three to six cycles and use the average. If your cycle varies by more than a week, treat the dates as a rough guide.
What is a normal menstrual cycle?
| Typical range (adults) | Talk to a doctor if… | |
|---|---|---|
| Cycle length | 21–35 days (24–38 in newer definitions) | Regularly shorter than 21 or longer than 35 days |
| Period length | 2–7 days | Bleeding lasts more than 7 days |
| Variation | Up to 7–9 days between cycles | Cycles suddenly change, or you miss 3 in a row |
| Flow | Changing a pad or tampon every 3–6 hours on heavy days | Soaking through one every hour for several hours, or passing large clots |
In teenagers, cycles of 21–45 days are normal for the first few years. ACOG encourages treating the menstrual cycle as a “vital sign” — a useful window into overall health.
The four phases of your menstrual cycle
- Menstruation (day 1 to about day 5): the uterine lining sheds — your period.
- Follicular phase (day 1 to ovulation): estrogen rises as an egg matures. This phase varies most in length.
- Ovulation (about 14 days before your next period): the egg is released; your fertile window ends.
- Luteal phase (after ovulation, about 12–14 days): progesterone rises. PMS symptoms such as bloating, breast tenderness or mood changes may appear before your period.
Irregular or late periods
Cycles naturally vary by a few days. Common reasons for a late or irregular period include pregnancy, stress, illness, travel, intense exercise, big weight changes, breastfeeding, hormonal birth control, thyroid problems, PCOS and approaching menopause.
If your period is late and there’s a chance you’re pregnant, check when to take a pregnancy test for the most reliable day to test. If you’re already pregnant, the pregnancy calculator shows how far along you are.
Frequently asked questions
How do I calculate when my next period is due?
Add your average cycle length to the first day of your last period. If your last period started on March 3 and your cycle is 28 days, your next period should start around March 31. Count cycle length from day 1 of one period to day 1 of the next.
What is a normal menstrual cycle length?
For adults, ACOG describes a normal cycle as about 21 to 35 days (some newer definitions use 24 to 38 days), and a period usually lasts 2 to 7 days. In the first few years after periods start, cycles of 21 to 45 days are common. Only a minority of people have an exact 28-day cycle.
Why is my period late?
Pregnancy is the most common reason, so take a test if you could be pregnant. Other causes include stress, illness, travel, big changes in weight or exercise, breastfeeding, thyroid problems, polycystic ovary syndrome (PCOS) and perimenopause.
How many days late can a period be before I worry?
A few days either way is normal. If your period is more than a week late and a pregnancy test is negative, test again in a few days. See a doctor if you miss three periods in a row, or your cycles are regularly shorter than 21 or longer than 35 days.
Can a period calculator help me get pregnant?
Yes — knowing your cycle length lets you estimate when you ovulate. Use our ovulation calculator to see your fertile window, which ends about 14 days before your next period.