Period Calculator
Predict your next several period dates, with the fertile window and estimated ovulation day for each cycle, from your last period and typical cycle length.
How to use this calculator
- 1Enter the first day of your last period, how long it typically lasts, and your average cycle length.
- 2Choose how many future cycles to predict.
- 3Read the calendar table for a run of upcoming periods, ovulation days and fertile windows at once.
How the calculation works
Next period = Last period + cycle length Ovulation = Next period − 14 days- Last period
- The first day of your most recent period
- Cycle length
- Days from the start of one period to the start of the next
Each subsequent predicted period simply adds the cycle length again — cycle 2 is last period + 2 × cycle length, and so on.
The fertile window extends 5 days before ovulation (the outer limit of sperm survival) through about 1 day after (the egg's viable window).
Worked example
Last period 1 July 2026, 5-day period, 28-day cycle
- 1.Next period: 1 July + 28 days = 29 July 2026, lasting 5 days through 2 August.
- 2.Estimated ovulation for that cycle: 29 July − 14 days = 15 July 2026.
- 3.Fertile window: 10 July (5 days before ovulation) through 16 July (1 day after).
Result: Next period 29 July – 2 August 2026
What determines cycle length
A menstrual cycle is governed by a feedback loop between the brain and the ovaries: hormones signal the ovaries to prepare and release an egg, and the ovaries in turn produce hormones that shape the rest of the cycle, including when the next period starts. A cycle of 21 to 35 days is generally considered within the typical range for adults, with 28 days often quoted only as a population average rather than a norm every individual matches.
Why calendar-based prediction works, up to a point
This calculator projects future periods by repeating the same cycle length forward from the last known one. For someone whose cycles run fairly consistently, that approach predicts upcoming dates reasonably well a cycle or two out. But real cycles carry natural month-to-month variation even when they are broadly "regular," and small variations compound the further into the future a prediction reaches — which is why a calendar projected several cycles ahead should be read as a general guide rather than a fixed schedule.
What can shift a cycle from its usual pattern
Cycle length is sensitive to more than just its own internal rhythm.
- Stress and illness — both can delay or advance ovulation, shifting the whole cycle length that month.
- Travel and disrupted routines — changes in sleep schedule and time zones are a common, temporary cause of a shifted cycle.
- Significant weight change or intense exercise — can affect the hormonal signals that regulate the cycle.
- Starting or stopping hormonal contraception — cycles often take a few months to settle into a new pattern afterward.
- Perimenopause and underlying conditions — conditions such as PCOS or thyroid disorders, and the approach of menopause, are well-documented causes of less predictable cycles.
Common everyday uses for cycle tracking
Beyond fertility planning, knowing roughly when a next period is expected is useful for everyday logistics — timing travel, planning for supplies, or simply anticipating a week when premenstrual symptoms are more likely. It is also useful context to bring to a healthcare provider: a consistent record of actual cycle lengths over time, compared against a simple average-based projection like this one, is often the first thing a clinician asks about when investigating irregular cycles.
What this assumes, and where it stops
Assumptions
- Cycles are reasonably regular — this is a statistical projection, not a measurement of any specific future cycle.
Limitations
- Accuracy drops the further into the future a prediction runs, since small month-to-month variations compound.
- Irregular cycles, recent hormonal contraception use, perimenopause, or conditions like PCOS make calendar-based prediction substantially less reliable.
- Does not track symptoms, flow, or actual observed data — it is a projection from averages, not a log of what actually happened.
Common questions
How is this different from the Ovulation Calculator?
This tool is built around predicting a run of upcoming periods — useful for planning travel, events, or supplies — and shows ovulation and fertile windows for each of them at once using a standard 14-day luteal phase assumption. The Ovulation Calculator focuses on a single upcoming cycle in more depth and lets you enter your own measured luteal phase length if you know it differs from 14 days.
Why might my actual period not match the prediction?
Cycle length naturally varies cycle to cycle even in people with regular periods — stress, illness, travel, weight changes, and exercise can all shift ovulation timing. This calculator projects forward from a fixed average, so the further ahead a prediction is, the more room there is for it to drift from what actually happens.
Sources
- Menstrual cycle: What's normal, what's not — American College of Obstetricians and Gynecologists
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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