period tools

Menstrual Cycle Calculator

A complete view of your cycle — periods, fertile window, ovulation, and the four hormonal phases. Enter your last period date and cycle length, and the calculator does the rest.

Pick the date your most recent period started.

21 days45 days

Days from one period's first day to the next. Most cycles are 21–35 days.

1 days10 days

How many days bleeding usually lasts.

Pick the first day of your last period to see your next 6 cycles.

The four phases of the menstrual cycle

The menstrual cycle is not one event but a rolling sequence of hormonal phases. Knowing where you are in the cycle helps explain why your energy, mood, sleep, and appetite shift through the month. The four phases below describe a typical 28-day cycle — the day numbers shift up or down for longer or shorter cycles.

1. Menstrual phase

Days 1–5

The lining of the uterus sheds and you bleed. Hormones (oestrogen, progesterone) are at their lowest. Energy is often low — be kind to yourself.

2. Follicular phase

Days 1–13

Overlaps with menstruation. The ovaries prepare to release an egg, oestrogen rises, the uterine lining starts rebuilding. Energy and mood usually climb through this phase.

3. Ovulation

Around day 14

A mature egg is released from an ovary. This is your most fertile time. Many people notice a brief temperature dip then rise, clearer cervical fluid, and a small twinge of pain on one side.

4. Luteal phase

Days 15–28

The body prepares for a possible pregnancy. Progesterone peaks then falls if no fertilisation occurs, triggering the next period. PMS symptoms — bloating, breast tenderness, mood shifts — show up here.

What counts as a normal cycle?

Most adult cycles run 21 to 35 days. The textbook 28-day cycle is one specific shape — many people are perfectly regular at 25 or 33 days. What matters more than the exact length is consistency: if your cycle stays within a few days of its own average month to month, that’s considered regular.

Variation of more than seven days between cycles, or sudden new irregularity after years of regular cycles, is worth bringing up with a healthcare provider. Common reasons include thyroid imbalance, PCOS, perimenopause, hormonal contraception transitions, and significant lifestyle changes.

Why each phase affects how you feel

  • Menstrual: low hormone levels can mean lower energy and mood. Many people prefer gentler activities, more sleep, and warmth.
  • Follicular: rising oestrogen lifts mood, energy, and motivation. Often the best window for harder workouts and cognitively demanding work.
  • Ovulation: peak oestrogen + a small testosterone bump often brings sharper energy, better verbal fluency, and higher libido — for just one to two days.
  • Luteal: progesterone dominates, which can mean slower mornings, more cravings, and PMS symptoms in the final days. Sleep and gentle movement help.

Frequently asked questions

What is a menstrual cycle?
The menstrual cycle is the monthly hormonal process that prepares the body for pregnancy. It runs from the first day of one period to the first day of the next, and includes four overlapping phases: menstrual, follicular, ovulation, and luteal. A typical cycle lasts 21 to 35 days.
What are the four phases of the menstrual cycle?
The four phases are: (1) Menstrual — days 1–5, when bleeding occurs; (2) Follicular — days 1–13, when the body prepares to release an egg and oestrogen rises; (3) Ovulation — around day 14, when an egg is released; (4) Luteal — days 15–28, when the body prepares for possible pregnancy or sheds the lining if not pregnant.
Which phase am I in right now?
It depends on how many days have passed since the first day of your last period. Days 1–5: menstrual. Days 6–13: follicular. Around day 14: ovulation. Days 15–28: luteal. The exact day numbers shift based on your cycle length — for a 30-day cycle, ovulation moves to around day 16.
How long is a normal menstrual cycle?
Anywhere between 21 and 35 days is considered normal for adults. The 28-day cycle is an average, not a strict rule. Cycles shorter than 21 days, longer than 35 days, or that vary by more than a week between cycles are worth tracking and discussing with a healthcare provider.
How can tracking my menstrual cycle help me?
Tracking helps you predict periods, plan around energy and mood patterns, time conception attempts or contraception use, and spot changes that could indicate a health issue. Most cycle apps and our Period Calculator do the math for you — you just enter your start dates.

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