How the prediction works
Your cycle length is counted from the first day of one period to the first day of the next — not from when bleeding stops. The calculator adds your cycle length repeatedly to your last start date to project the coming months.
Worked example: a period starting on 3 March with a 30-day cycle predicts the next start around 2 April, then 2 May, then 1 June. The prediction is only as good as the regularity of your cycles, and error compounds with each month projected forward — the third prediction is meaningfully less reliable than the first.
What counts as normal
The textbook 28-day cycle is an average, not a standard, and most people do not run to it. A cycle anywhere between 21 and 35 days is considered within the normal adult range, and variation of a few days from month to month is entirely typical.
Some further context worth having. Cycles are commonly irregular for the first year or two after periods begin, and again during perimenopause. Bleeding usually lasts two to seven days. Typical total blood loss is around 30–40 ml across a period — far less than most people estimate, which is part of why "heavy" is hard to judge by eye.
Why cycles shift
The phase before ovulation is where most variability lives, so anything that delays ovulation delays the whole cycle. Common causes:
- Stress — significant psychological or physical stress can delay or suppress ovulation.
- Illness, particularly with fever.
- Travel and disrupted sleep, especially across time zones.
- Substantial weight change in either direction, and very low body fat.
- Large increases in training volume.
- Hormonal contraception, starting or stopping it.
- Perimenopause, typically from the mid-forties onward.
- Breastfeeding, which commonly suppresses cycles for months.
A single late or early period after a stressful month is usually unremarkable. A persistent change in your own established pattern is the thing worth paying attention to.
Tracking is worth more than predicting
The genuine value of logging your cycles is not the forecast — it is building a record of what is normal for you. Averages across the population tell you little; three or four months of your own data tells you a great deal.
Alongside dates, it is worth noting flow, pain, mood, energy, and any symptoms that recur at the same point each month. Patterns that are invisible month to month become obvious across six, and if you ever do need to see a doctor about your cycle, arriving with a few months of records makes the conversation considerably more productive.
Cycle phases, briefly
- Menstrual (days 1–5ish) — the lining sheds. Hormones are at their lowest; energy often follows.
- Follicular (until ovulation) — oestrogen rises, energy and mood typically improve.
- Ovulation (around 14 days before the next period) — an egg is released; some people feel a brief one-sided twinge.
- Luteal (after ovulation) — progesterone rises, then falls if there is no pregnancy. Premenstrual symptoms cluster here.
For more detail, see our guide to understanding your menstrual cycle, and the ovulation calculator for fertile-window estimates.
When to speak to a doctor
Worth getting checked if your cycles are consistently shorter than 21 days or longer than 35; if they suddenly become irregular after being regular; if you miss three or more periods and are not pregnant; if bleeding is very heavy — soaking through a pad or tampon hourly for several hours, or passing clots larger than a 10p coin; if periods last longer than seven days; if pain is severe enough to interfere with daily life; or if you bleed between periods or after sex.
Period pain that stops you functioning is not something to endure quietly. Conditions such as endometriosis, fibroids, PCOS and thyroid dysfunction are common, and several are frequently diagnosed years later than they could have been because the symptoms were normalised.