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Period Calculator

Health

Predict your next period date, cycle day, and upcoming periods for the next 3 months. Track your menstrual cycle with ease, privacy, and accuracy.

Reviewed by the thecalcu.com team · Last updated June 15, 2026

July 28th, 2026
Average Cycle Length
days
20 days45 days
Period Duration
days
2 days10 days

Next Period

August 4th, 2026

Starting today · ends August 4th, 2026

Days Away

0

Cycle Day

Day 1

Period Duration

5 days

Cycles Tracked

0

Day 1Day 1Day 28

Upcoming Periods

August 4th, 2026

ends August 8th, 2026

Today

September 1st, 2026

ends September 5th, 2026

28d

September 29th, 2026

ends October 3rd, 2026

56d

What is a Period?

A period calculator predicts the date of your next menstrual period, shows which day of your current cycle you're on, and projects upcoming periods over the next several months, all by applying your average cycle length forward from your last period's start date. Menstrual cycle tracking is the foundation of a lot of everyday reproductive health awareness: it helps you plan around a period for travel, sports, or exams, spot irregularities that might signal something worth checking with a doctor, and understand where PMS symptoms fit relative to your cycle. The calculator outputs days until your next period as a number you add to today's date, your current cycle day, and days until the current or next period ends, giving you a full picture of your cycle position at a glance.

Why Use a Period Calculator?

Most people track their cycle mentally by approximate date, and that accuracy degrades fast, a single early or late period is enough to throw off memory-based tracking for months. This calculator anchors its predictions in your actual data, your last period date and average cycle length, and updates instantly rather than relying on mental math that drifts over time. Planning around a period for travel, athletic events, or an exam is a genuinely practical everyday need, and having a specific projected date beats a vague sense of "sometime next week."

Who Should Use This Calculator?

Anyone planning ahead for travel, sports, or events where a period would be inconvenient to work around.

Anyone tracking cycle regularity, to catch a pattern shift early rather than only noticing after several irregular months.

Couples using natural family planning, combining cycle data with other indicators like basal body temperature to identify fertile and non-fertile windows.

People managing PCOS, thyroid conditions, or other hormonal factors who need to bring accurate cycle data to a doctor's appointment rather than a rough recollection.

Anyone newer to tracking their cycle who wants a straightforward way to understand cycle phases and timing, pair it with the ovulation-calculator and pregnancy-due-date-calculator for a fuller reproductive health toolkit, and the bmi-calculator since weight changes are a common factor behind cycle irregularity.

How to use this Period calculator

  1. Enter the first day of your last period, the day bleeding started, even if it was light.
  2. Enter your average cycle length in days; if you're not sure, count the days between the start of your last two periods.
  3. Enter your typical period duration in days.
  4. Days until next period is the headline result, add it to today's date to get the calendar date.
  5. Current day in cycle shows exactly where you are right now.
  6. Period ends tells you how many days from today the next period will wrap up.
  7. Use this alongside the ovulation-calculator to track your period and fertile window together.

What Insights Does the Period Calculator Give You?

Days Until Next Period is the number most people are looking for, a specific count to plan travel, sports, or events around rather than a vague guess.

Current Cycle Day puts today in context relative to the four cycle phases, which is useful for understanding current symptoms or energy levels.

Period Ends projects forward past the start date to give you the full window a period will occupy, useful for planning around it rather than just the start.

Cycles Since Last Period gives you a running count that's handy for spotting drift in your average cycle length over time, if this number climbs while your predicted dates start missing consistently, it's worth recalculating your average from more recent cycles.

Formula & Methodology

Days since LMP          = Today − Last period date Cycles elapsed          = Floor(days since LMP ÷ cycle length) Current day in cycle    = (Days since LMP mod cycle length) + 1 Next period start       = LMP + (cycles elapsed + 1) × cycle length Days until next period  = Next period start − Today Period end              = Days until next period + period duration − 1

Worked example, last period May 15th, cycle 30 days, duration 5 days, today is June 10th: Days since LMP = 26 days. Cycles elapsed = Floor(26 ÷ 30) = 0 full cycles. Current cycle day = (26 mod 30) + 1 = Day 27. Next period start = May 15 + 30 days = June 14. Days until next period = June 14 − June 10 = 4 days. Period ends = 4 + 5 − 1 = 8 days away (June 19).

Frequently Asked Questions

A menstrual cycle is the monthly hormonal cycle that prepares the body for a potential pregnancy, counted from the first day of one period to the first day of the next. The average cycle runs 28 days, though anywhere from 21 to 35 days is considered normal, and the period itself, the bleeding phase, typically lasts 3-7 days.
It takes your last period's start date and adds your average cycle length: Next period start = Last period date + Cycle length. Enter a 30-day cycle starting May 1st and it predicts the next period for May 31st. It's a statistical projection, so actual timing can vary by a few days even in a fairly regular cycle.
It tells you where you are right now within your cycle. Day 1 is the first day of your period; roughly days 1-5 are the menstrual phase, days 6-13 the follicular phase, day 14 (in a 28-day cycle) is close to ovulation, and days 15-28 make up the luteal phase.
Some month-to-month variation, usually 1-5 days, is completely normal. Cycles that swing by more than 7 days are generally considered irregular, and common causes include PCOS, thyroid conditions, stress, significant weight changes, heavy training loads, and breastfeeding; it's worth mentioning to a doctor if it persists.
You can, using your average cycle length calculated over 3-6 months, but expect lower prediction accuracy. If your cycle varies by more than 7 days, the next period date could be off by a week or more, adaptive tracking apps built on your own history tend to do better in that situation, and a doctor can investigate if the irregularity is significant.
Count the days from the first day of one period to the first day of the next, that's one full cycle. Track 3-6 cycles and average them: for example, 28 + 30 + 27 + 29 = 114, divided by 4 gives 28.5, which you'd round to 29. More cycles tracked means a more reliable average.
The menstrual phase (roughly days 1-5) is the bleeding phase. The follicular phase (days 1-13) overlaps with it, as follicles develop and estrogen rises. Ovulation happens around day 14 in a 28-day cycle, marking peak fertility. The luteal phase (days 15-28) follows, with progesterone rising to prepare the uterus, dropping again if no pregnancy occurs to trigger the next period.
Enter the first day of your most recent period, your average cycle length (default 28 days), and your typical period duration (default 5 days). The results show days until your next period, your current cycle day, and when the next period is expected to end, add the 'days until' figure to today's date for the actual calendar date.
Calendar-based tracking alone isn't a reliable contraceptive method, the rhythm method has a typical-use failure rate of roughly 24% per year. Combining calendar tracking with basal body temperature and cervical mucus observation (the symptothermal method) is more effective but still less reliable than hormonal options, so talk to a doctor about what fits your situation.
A period is generally considered late once it hasn't started within 5 days of the expected date. Most pregnancy tests are reliable 1-2 days after a missed period, and common non-pregnancy causes include stress, illness, exercise or weight changes, travel across time zones, and hormonal shifts, a doctor can check for thyroid issues or PCOS if it's a recurring pattern.
Premenstrual syndrome refers to physical and emotional symptoms in the 1-2 weeks before a period, during the luteal phase, mood changes, irritability, bloating, breast tenderness, headaches, and fatigue are common. Symptoms usually resolve within a couple of days of the period starting; severe cases with significant mood disruption are sometimes diagnosed as PMDD.
Also known as
menstrual cycle calculatorperiod trackernext period dateperiod due date calculatormenstruation calculator