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Menstrual Cycle Length Calculator

Average cycle length, shortest, longest and regularity from your recent period start dates.

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Menstrual Cycle CalculatorPredict your next periods, ovulation day, fertile window and current cycle phase from your last period.Sleep cycle calculatorFind the best bedtimes to wake up refreshed, based on 90-minute sleep cycles.Baby Growth Percentile CalculatorWeight, length and head circumference percentiles from the WHO Child Growth Standards, 0-24 months.Baby Milk Intake CalculatorBottle volumes per feed and per day by your baby's weight and age, with the feeding interval.Gestational Age CalculatorGestational age in weeks and days from your LMP, due date or conception date, with trimester and EDD.hCG Doubling Time CalculatorCompute the doubling time and percentage rise between two hCG results, read against the expected range for the level reached.Implantation CalculatorDate the 6-to-12-day implantation window from your last period or a known ovulation date, with the peak days, the earliest reliable test date and the expected period.IVF Due Date CalculatorDate an IVF pregnancy from the transfer date and the embryo's age, with gestational age today, the trimester and term milestones, and the equivalent LMP date.

Menstrual Cycle Length Calculator works straight from this page — free, instant, nothing to install. It sits under Pregnancy & baby in our catalogue, alongside Menstrual Cycle Calculator and Sleep cycle calculator.

How to use it

  1. Open the tool — no signup or install needed.
  2. Enter your input or adjust the available options.
  3. Get your result instantly, then copy or download it.

Frequently asked questions

What is Menstrual Cycle Length Calculator?

Average cycle length, shortest, longest and regularity from your recent period start dates.

When would I actually use this?

Placing a pregnancy on the calendar: the estimated due date, the current week, and when the usual appointments and screenings fall.

What is the most common mistake?

Reading the due date as a deadline. It is a median: fewer than one birth in twenty happens on it, and a normal term spans several weeks around it.

How is Menstrual Cycle Length Calculator different from Menstrual Cycle Calculator?

They sit next to each other but answer different questions: Menstrual Cycle Calculator is the one to open when you need it to predict your next periods, ovulation day, fertile window and current cycle phase from your last period. Pick whichever matches what you're starting from — both are free.

Is there a tool for the next step?

Sleep cycle calculator is the closest one after this: Find the best bedtimes to wake up refreshed, based on 90-minute sleep cycles.

What else is worth having open alongside it?

Baby Growth Percentile Calculator and Baby Milk Intake Calculator — they come up in the same task often enough to be worth a second tab.

Where do the figures come from?

Dating follows Naegele's rule (280 days from the last period) or the transfer date for assisted conception. An early ultrasound is more accurate than any calculation and takes precedence over it.

Further reading

All guides →
ExplainerWhat Is a Normal Menstrual Cycle Length? The Numbers, by Life StageFIGO puts a normal cycle at 24 to 38 days; ACOG's older range is 21 to 35. Here is why two respected bodies give different numbers, how much variation is normal, and how cycles change after menarche and before menopause.How-toHow to Predict Your Next Period (And How Far Off the Estimate Can Be)Add your average cycle length to the first day of your last period. Here is how to measure that length properly, why the estimate has a range rather than a date, and when irregularity is worth a consultation.ExplainerCycle Length, Ovulation, and What a Fertile Window Really IsOvulation is estimated far better by counting backwards from the next period than forwards from the last one, because the phase after ovulation is the stable one. Here is the inversion worked for every cycle length, the fertile window from the study that defined it, and the failure rates that make this unusable as contraception.ExplainerHow Much Milk Does a Baby Need? The Rule of Thumb and Its LimitsA formula-fed baby takes roughly 2.3 fl oz per pound of body weight per day in the first months, split across the day's feeds. Here is the table by age and weight, why breastfed babies are not measured this way, and the signs that outrank any number.ExplainerIVF Dating Is the One Case Where the Date Is Actually KnownEvery other pregnancy is dated from an estimate. After IVF, fertilisation is timed to the hour, so gestational age is arithmetic: 266 days from fertilisation, minus the embryo's age at transfer. Here is the derivation for day-3, day-5 and day-6 embryos, why frozen and fresh transfers use the same rule, why an early scan rarely moves an IVF date, and what the two-week offset on a scan report means.ExplainerhCG Doubling Time, and What a Single Number Cannot Tell YouDoubling time is ln(2) times the interval divided by the natural log of the ratio. Here it is derived and worked on real pairs, with the two things most often got wrong online: the expected rate slows as the level rises, and a single value is meaningless because the reference range at one gestational week can span more than two hundredfold.