Predict upcoming period dates, track your cycle length, and understand your menstrual health. Based on your cycle history.
A period calculator predicts your next period by adding your cycle length to the start of your last period. For an average 28-day cycle, that means your next period is expected about 28 days after the last one began. Tracking a few cycles improves accuracy, since cycle length naturally varies from month to month.
| # | Start date | End date | Days until |
|---|
Predicting your period is simple arithmetic once you know your average cycle length — the number of days from the start of one period to the start of the next.
A period is generally considered late once it is more than about 5 days past your predicted date, though normal cycles do vary. Occasional lateness is common and not usually a concern.
| Situation | What It May Mean |
|---|---|
| 1–4 days late | Normal cycle variation |
| 5–7 days late | Often still normal; could take a pregnancy test if applicable |
| Over 1 week late | Consider a pregnancy test; note stress, illness or travel |
| Frequently late or missed | Worth discussing with a doctor |
Many things besides pregnancy can delay a period, including stress, illness, weight change, intense exercise, travel and hormonal conditions.
Your cycle has four phases driven by changing hormones. The calculator tracks where you are across them.
| Phase | Typical Days | What Happens |
|---|---|---|
| Menstrual | 1–5 | The period — the uterine lining sheds |
| Follicular | 1–13 | An egg matures; lining rebuilds |
| Ovulation | ~14 | The egg is released |
| Luteal | 15–28 | Body prepares for a possible pregnancy |
Cycle length varies from person to person and can change with age. Knowing your normal helps you spot changes worth mentioning to a doctor.
| Cycle Length | Category |
|---|---|
| 21–35 days | Normal for adults |
| Under 21 days | Short — worth monitoring |
| Over 35 days | Long — worth monitoring |
| Very irregular | Common in teens and around menopause; discuss if persistent |
A typical menstrual cycle is 21–35 days; periods last 2–7 days. Cycles vary naturally. The Jean Hailes Foundation for Women's Health (Australia) recommends tracking your cycle to notice changes.
Consult your GP if periods are very heavy, very painful, irregular, or absent. In Australia, telehealth GP consultations are available via Medicare for women's health concerns.
The menstrual cycle is the monthly series of hormonal changes that prepare the body for potential pregnancy. Day 1 of the cycle is the first day of menstrual bleeding. The cycle length is counted from day 1 of one period to day 1 of the next. While 28 days is cited as "average," normal cycles range from 21 to 35 days and vary month to month for most people.
| Phase | Days (28-day cycle) | What Happens |
|---|---|---|
| Menstruation | Days 1–5 | Uterine lining sheds; oestrogen and progesterone are low |
| Follicular phase | Days 1–13 | FSH stimulates follicle development; oestrogen rises |
| Ovulation | ~Day 14 | LH surge triggers egg release |
| Luteal phase | Days 15–28 | Progesterone rises; body prepares for potential implantation |
| Feature | Normal Range | Seek Medical Advice If |
|---|---|---|
| Cycle length | 21–35 days | Under 21 or over 35 days consistently |
| Period duration | 3–7 days | Over 7 days, or under 2 days |
| Flow volume | 30–80 mL total | Soaking a pad/tampon per hour for 2+ hours |
| Cramping | Mild–moderate day 1-2 | Severe pain affecting daily activities |
| Colour | Dark red to bright red | Grey or unusual discharge |
What is a normal menstrual cycle length?
Normal menstrual cycles range from 21 to 35 days, counted from day 1 of one period to day 1 of the next. The commonly cited 28-day average applies to only a minority of women. Cycles naturally vary by 2-3 days from month to month. Consistently shorter than 21 days or longer than 35 days may indicate an underlying condition worth discussing with a GP.
How many days should a period last?
Normal periods last 3-7 days. A period under 2 days may indicate low oestrogen or other issues. A period lasting over 7 days (menorrhagia) is considered prolonged and may cause iron deficiency anaemia with heavy flow. The Jean Hailes Foundation recommends seeing a doctor if periods consistently last over 7 days or are associated with heavy bleeding.
What causes irregular periods?
Common causes of irregular cycles include: stress (most common), significant weight loss or gain, excessive exercise, polycystic ovary syndrome (PCOS — the most common hormonal condition in Australian women of reproductive age), thyroid disorders, perimenopause, and certain medications. A single irregular cycle is usually normal; consistently irregular cycles warrant GP review.
How do I calculate my next period?
Add your average cycle length to the first day of your last period. For a 30-day cycle starting 3 June, your next period is expected around 3 July. Tracking several cycles gives a more accurate personal average, since cycle length naturally varies. The calculator above does this for several months ahead.
How do I know if my period is late?
A period is usually considered late once it is more than about five days past your predicted date. Enter your last period date and cycle length in the calculator to see your expected date. Occasional lateness is common; frequent lateness or missed periods are worth discussing with a doctor.
Why is my period late if I am not pregnant?
Many things besides pregnancy can delay a period, including stress, illness, significant weight change, intense exercise, travel across time zones, hormonal conditions like PCOS, thyroid problems, and approaching menopause. An occasional late period is normal. If lateness becomes a pattern, it is worth seeing a doctor.
How do I calculate my menstrual cycle length?
Count from the first day of one period to the day before your next period starts — that total is your cycle length. Do this over a few months and take the average, since it varies. A normal adult cycle is anywhere from 21 to 35 days.
What is a normal period cycle?
A normal menstrual cycle for adults ranges from 21 to 35 days, with bleeding lasting about 3 to 7 days. Teenagers and those approaching menopause often have more variable cycles. What matters most is what is normal and consistent for you, and noticing significant changes.
Can I get pregnant on my period?
It is uncommon but possible, especially with shorter cycles, because sperm can survive up to about five days. If you ovulate early, sperm from sex during a period could still be present. A period calculator is not a reliable contraceptive; use a proven method if you want to avoid pregnancy.
When should I see a doctor about my periods?
See a doctor if your periods are very heavy, very painful, stop unexpectedly, come more often than every 21 days or less often than every 35 days, or if you bleed between periods. Persistent irregularity is also worth checking. These can have manageable causes a doctor can help with.