The menstrual cycle, ovulation and your fertile window
7 min read · Updated 2026-10-12 · FitCalc
The menstrual cycle is often described as a fixed 28-day loop with ovulation in the middle, and almost nobody's cycle behaves that way. Predictions work well as a rough guide because the underlying pattern is regular even when the length is not, but the margin of error matters more than the estimate itself.
The phases, and why ovulation timing varies
The cycle has two halves. The follicular phase runs from the first day of a period to ovulation, and the luteal phase runs from ovulation to the next period. The luteal phase is remarkably consistent — typically 12 to 14 days — while the follicular phase varies considerably between people and from cycle to cycle.
This is the single most useful fact for prediction: because the luteal half is stable, ovulation can be estimated backwards from the next period almost as well as forwards from the last one. Where a prediction is unreliable is the irregular cycles, where the assumptions break down entirely.
The fertile window and how long it really is
The egg survives for roughly 24 hours and viable sperm for up to five, so the theoretical fertile window runs from about five days before ovulation to the day after. Six days is the figure most estimators use, and it is a real biological window rather than a single day.
An ovulation estimate calculated from a period start date is a projection, not an observation. Basal body temperature and cervical mucus changes give much better confirmation, and both require consistent daily recording before they become reliable.
What makes cycle length change
The luteal phase shifts least. The follicular phase responds to everything that affects the body's energy availability and hormone signalling — significant weight change, intense training, poor sleep, illness and travel across time zones can all lengthen it noticeably.
Cycles also vary naturally with age, becoming more irregular through the perimenopause. A sudden large change in a previously predictable pattern is worth discussing with a clinician rather than trying to explain with a calculator.
Using the predictions you have
A period calculator is genuinely helpful for forward planning — knowing roughly when to expect the next period, and which days fall in the fertile window, is useful for a range of reasons. Use it for planning and for recording, not as contraception.
Keep a simple log of actual start dates. After three or four cycles you have a better sense of your own typical length than any estimator will, because the estimator only knows what you tell it and the log remembers everything.
Run your own numbers
Free calculators that answer the “how much” version of this question. They run entirely in your browser — nothing you type is uploaded or stored.
Common questions
Can I predict ovulation accurately?
Estimates from cycle dates are projections and typically fall a few days either side of the true ovulation day. Basal body temperature and cervical mucus tracking give better confirmation when recorded consistently.
How many days is the fertile window?
About six days, running from roughly five days before ovulation to the day after. The egg survives about 24 hours and viable sperm up to five days.
Why does my cycle length change from month to month?
The luteal phase is stable while the follicular phase varies with energy availability, training load, sleep, illness and travel. Large changes in an otherwise predictable pattern are worth raising with a clinician.
FitCalc provides general information only and is not medical advice, diagnosis or treatment. Figures are illustrative and depend on your own circumstances. Always consult a qualified health professional about your situation. See our full disclaimer.