Period Calculator

How to use the period calculator

1. Enter the first day of your last period – the first day of real bleeding, not spotting.
2. Enter your usual cycle length: the number of days from the first day of one period to the first day of the next. If you are not sure, enter several past start dates and the calculator will work it out.
3. Enter how many days your period usually lasts.
4. Read your predicted dates, and add them to your calendar if you like.

Example: if your last period began on 1 October 2026 and your cycles are 28 days long, your next period is predicted to start on 29 October. Ovulation is most likely around 15 October (somewhere between about 13 and 17 October), and your most fertile days are roughly 10 to 16 October.

What is a normal menstrual cycle?

In the first few years after periods start, cycles are often longer and less regular. The American College of Obstetricians and Gynecologists describes cycles of 21 to 45 days as typical for adolescents. Cycles also tend to become irregular again in the years leading up to menopause (perimenopause).

The phases of the menstrual cycle

• Menstruation (day 1 to about day 5): the lining of the womb is shed as a period.
• Follicular phase (day 1 to ovulation): hormones prompt the ovaries to mature an egg and the womb lining to thicken. This phase varies most in length, which is why cycle lengths differ.
• Ovulation: an egg is released, usually about 12 to 16 days before the next period begins.
• Luteal phase (after ovulation to the next period): the body prepares for a possible pregnancy. If the egg is not fertilised, hormone levels fall and the next period starts.

Ovulation and the fertile window

Because the time from ovulation to the next period is fairly steady, the calculator estimates ovulation by counting back from your next predicted period – about 14 days, within a range of roughly 12 to 16. An egg survives for about a day after it is released, but sperm can survive in the body for up to 7 days, so pregnancy is possible from sex in the days before ovulation as well. Most fertility guidance treats the five days before ovulation and the day of ovulation as the most fertile time.

If you are trying to get pregnant, the NHS advises having sex every two to three days throughout the cycle rather than trying to time it exactly, because ovulation can vary. Ovulation predictor kits, which detect the hormone surge before ovulation, and tracking basal body temperature or cervical mucus can give extra clues.

This is not a method of contraception

Calendar predictions are not reliable enough to prevent pregnancy. Ovulation can happen earlier or later than expected – especially with irregular cycles, after stopping hormonal contraception, after having a baby, while breastfeeding or during perimenopause. If you want to avoid pregnancy, talk to a doctor, nurse or sexual health clinic about contraception.

Irregular periods

It is normal for cycle length to vary by a few days. Periods are generally considered irregular when the length between them keeps changing a lot. Common reasons include stress, big changes in weight, intense exercise, polycystic ovary syndrome (PCOS), thyroid problems, starting or stopping hormonal contraception, breastfeeding and perimenopause. Pregnancy is a common reason for a missed period, so take a pregnancy test if you might be pregnant. With irregular cycles, enter as many past start dates as you can – the calculator shows a range for your next period instead of a single date.

When to see a doctor

• Bleeding between periods, after sex or after menopause.
• Periods that suddenly change – becoming much heavier, longer, lighter or more painful.
• Heavy periods: soaking through pads or tampons every 1–2 hours, passing large clots or bleeding for more than 7 days.
• Periods that stop for 3 months or more (and you are not pregnant), or that have not started by age 16.
• Severe period pain that stops you doing everyday activities.
• Cycles consistently shorter than 21 days or longer than 35 days (outside the teenage years).

Tips for tracking your cycle

• Record the first day of every period – a calendar or app is enough.
• Note the number of bleeding days, how heavy the flow is and any symptoms such as cramps, headaches, mood changes or breast tenderness.
• Track at least three to six cycles to see your normal pattern.
• Bring your record to medical appointments – it helps doctors spot problems quickly.

Working out your average cycle

A single cycle can be misleading, so the calculator averages as many cycles as you enter. Suppose your last six periods started 28, 30, 27, 31, 29 and 29 days apart. The average is 29 days and the range is 27 to 31 days, so instead of a single date, the calculator shows your next period as most likely in a window of about four days around day 29. The wider your range, the wider the predicted window.

Period symptoms and PMS

Many people notice symptoms in the days before a period – bloating, breast tenderness, headaches, mood swings, tiredness or spotty skin. This is premenstrual syndrome (PMS), and symptoms usually ease once the period starts. Regular exercise, sleep and limiting caffeine, alcohol and salt help some people; over-the-counter painkillers can help with cramps. If symptoms are severe or affect your daily life, a doctor can suggest treatments. Recording symptoms in the calculator's notes helps you spot patterns.

Using period products safely

Change pads regularly and tampons at least every 4 to 8 hours, and use the lowest absorbency tampon you need. Leaving a tampon in too long increases the risk of toxic shock syndrome (TSS), a rare but serious infection – seek urgent medical help if you develop a sudden high temperature, a rash or flu-like symptoms while using a tampon or menstrual cup. Menstrual cups and period underwear are reusable options; follow the cleaning instructions.

Periods after pregnancy and with contraception

After giving birth, periods may return within weeks if you are not breastfeeding, or not for many months if you are. You can ovulate – and become pregnant – before your first period returns. Hormonal contraception changes bleeding patterns: the bleed in the pill-free week is a withdrawal bleed rather than a true period, and some methods make periods lighter, irregular or absent. When you stop hormonal contraception, cycles can take a few months to settle, so predictions will be less reliable at first.

Sources

1. NHS – Periods — nhs.uk/conditions/periods
2. NHS – Fertility in the menstrual cycle — nhs.uk
3. ACOG Committee Opinion 651 – Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign — acog.org