Period calculator: your next three periods
Enter the first day of your last period and your average cycle length. We'll map out your next three period dates right here in your browser — nothing to install, nothing stored.
The short answer: add your average cycle length to the first day of your last period. On a 28-day cycle, a period that started on 1 June means the next one is due around 29 June. The calculator below does the counting — and the next three months — for you.
Why these dates? The math we use
Same counting method as the MoodSwings app (algorithm v2.0.0): your next period is your last start plus your cycle length; ovulation is estimated by counting 15 days back from that next period, because the luteal phase is more consistent than the first half of the cycle. The fertile window shown is the deliberately-wide estimate we give a brand-new user (6 days before ovulation to 4 after) — with only one cycle to go on, honesty beats false precision. As the app learns your real pattern it narrows the window. For cycles under 20 days a meaningful ovulation estimate isn't possible from dates alone, so we don't show one.
Predictions that update themselves
These dates assume a perfectly steady cycle. MoodSwings re-predicts from every period you log — including irregular patterns — so the calendar stays honest.
Estimates for education, not medical advice — and not a contraception method. Cycles shift with stress, sleep, travel and health.
How to calculate your next period
The whole method fits in one sentence: add your average cycle length to the first day of your last period. Everything else is refinement. Here is the count, step by step.
- Find the first day of your last period. Day 1 is the first day of real bleeding, not the spotting that can come before it. That date is what every period prediction is built on, so it is worth getting right.
- Know your average cycle length. Count from the first day of one period to the day before the next one started. Most people land somewhere between 24 and 35 days. If you are not sure yet, start with 28 and refine it once you have logged a couple of real cycles.
- Add your cycle length to your last start date. That sum is your estimated next period. If your last period began on 1 June and your cycle is 28 days, your next is due around 29 June.
- Repeat to map the months ahead. Add your cycle length again for each cycle after that. The calculator above does all three at once and marks the dates for you.
A worked example. Your last period started on Monday 1 June and your cycle usually runs 28 days. Add 28 to 1 June and your next period is due around 29 June; add another 28 and you are looking at 27 July. If your cycle is closer to 31 days, those dates become 2 July and 2 August instead — which is why the cycle-length number matters more than people expect. Three days of drift compounds into more than a week by the third month.
Whether you call it a period calculator, a period estimator, a next period calculator or a menstrual predictor, this is the same arithmetic underneath. If you want the reasoning behind why the estimate sharpens over time and how accurate it really gets, our period predictor guide covers that in depth.
How period prediction works
The simplest period prediction adds your average cycle length to the first day of your last period. If your last period started on the 1st and your cycle is 28 days, your next is estimated around the 29th. This calculator repeats that for the next three cycles so you can see the months ahead at a glance.
Real cycles vary, so a one-time calculation is a starting point rather than a promise. An app that learns from each cycle you log — like MoodSwings — adapts to your actual pattern, including irregular cycles, and gets more accurate over time.
How to work out your PMS window
Once you know your next period date, your PMS window is the stretch immediately before it. Premenstrual symptoms cluster in the last five to seven days before bleeding starts — the late luteal phase, when the hormones that rose after ovulation drop away again. Take the date the calculator gives you and count back about a week: that is the window where mood dips, irritability, cramps, sore breasts, bloating, cravings and flat energy are most likely to turn up.
Two honest caveats. First, this is a population pattern, not your pattern — some people feel it for two days, others for ten, and some barely notice it at all. Second, PMS timing follows ovulation, so in a month where ovulation runs late, your PMS window slides later with it and your period follows. That is why a fixed calendar estimate can feel right for months and then abruptly feel wrong.
Knowing roughly when it lands is genuinely useful — it reframes a rough Tuesday as a predictable part of the month rather than something inexplicable, and it lets you put the demanding week somewhere else. But a calculator can only ever give you the average shape. Working out your actual pattern means noticing which symptoms show up, how strongly, and how many days out — which is the whole point of logging them.
When is my next period? Reading the dates you get
The calculator returns your next three period start dates and roughly how long each should last. Treat the first one as the most trustworthy and the third as the loosest — every cycle of drift compounds into the next, so a prediction three months out inherits all the variation in between.
If your period has not shown up by the predicted date, do not panic at day one: a few days either side is ordinary. The late period calculator tells you exactly how many days overdue you are against your own cycle length rather than a generic 28-day assumption, which is a far more useful answer than "it's late."
How to find your average cycle length
Your cycle length is the number of days from the first day of one period to the first day of the next — day 1 is the first day of real flow, not spotting. To get a reliable average, count across two or three cycles and take the middle value rather than relying on a single month, which can be thrown off by a one-off late or early period.
- 24–35 days is the typical range, and a 28-day cycle is just the average — not a target.
- A cycle that varies by up to about 7 days month to month is generally still considered regular.
- If you genuinely do not know your length yet, start with 28, then refine it once you have logged a couple of real cycles.
Why your real period rarely lands on the exact date
A calendar prediction assumes every cycle is identical, but bodies are not calendars. Plenty of ordinary things nudge ovulation — and therefore your period — earlier or later:
- Stress, poor sleep, or illness
- Travel and time-zone changes
- Big shifts in exercise or weight
- Coming off or starting hormonal birth control
- Conditions like PCOS or thyroid issues
This is exactly why a tracker that learns beats a one-time sum: instead of repeating the same fixed number, it watches your actual cycles and tightens the prediction around your real range. If you want to see your fertile days too, the ovulation calculator maps your fertile window, and the late-period calculator tells you whether you are actually overdue.
When a late or irregular period is worth a check-up
A few days of variation is normal, and one late period is rarely a concern. It is worth talking to a clinician, though, if your period is more than a week late and pregnancy is possible, if it stops for three or more months when you are not pregnant, if your cycles suddenly turn very irregular, or if periods become extremely heavy, very painful, or come with bleeding in between. A calculator is for awareness, not diagnosis — these patterns deserve a professional look.
Never get caught off guard — let MoodSwings do it
Re-running a calculator every month is easy to forget, and it can never learn your cycle. MoodSwings predicts your next period, PMS window, and fertile window automatically from your real history, adapts to irregular cycles, and gives you a gentle heads-up before each one — alongside quick mood and symptom tracking so the whole month makes more sense. Get MoodSwings free on iPhone and stop doing the math.
Questions people ask
How does the period calculator work?
It adds your average cycle length to your last period start date to predict your next period, then repeats that for the following cycles. The more regular your cycle, the more accurate the estimate.
When is my next period?
Add your average cycle length to the first day of your last period. On a 28-day cycle, a period that started on 1 June puts the next one around 29 June. Enter those two details above and the calculator gives you your next three dates instantly.
How can I predict my next period without an app?
You only need two things: the first day of your last period and your usual cycle length. Add one to the other and that is your estimate. It works fine when your cycle is steady — the arithmetic only falls apart when your cycle length keeps moving, which is where tracking several cycles beats a one-time sum.
How do I calculate my PMS window?
PMS symptoms usually show up in the last few days before bleeding starts — roughly the 5 to 7 days leading into your predicted date. Take the next period date from the calculator above and count back about a week; that stretch is when mood shifts, cramps, sore breasts and low energy most commonly land. It is an estimate, and everyone's pattern is different — which is exactly the thing MoodSwings learns from what you actually log.
Is a period estimator the same as a period calculator?
Yes — period calculator, period estimator and next period calculator all describe this same tool: working out your upcoming period dates from your last period and your cycle length. The wording changes, the arithmetic does not. What differs is a period predictor inside an app, which keeps learning from every cycle you log instead of doing the sum once; our period predictor guide covers how accurate that gets.
How do I know my cycle length?
Count from the first day of one period to the first day of the next — day 1 is the first day of full flow. The average is around 28 days, but anywhere from 24 to 35 is common and still normal. If it varies, use a typical value and track a few cycles to find your real average.
What counts as a regular vs. irregular cycle?
A cycle that lands within about 7 days of the same length each time is generally considered regular. If your cycle length swings by more than that, or your periods are unpredictable, a one-time calculation will be rougher — tracking a few cycles in an app is the better way to see your real pattern.
Why is my period prediction off?
Predictions are estimates. Stress, illness, travel, big changes in sleep or exercise, weight changes, and natural month-to-month variation can all shift your cycle. Tracking several cycles in an app gives a far more personal and adaptive prediction than a one-time calculation.
How late is too late for a period?
A period is generally considered late once it is more than about 5–7 days past your predicted date, though a few days of variation is completely normal. If you could be pregnant, a test is most reliable from the day your period is due. If your period is regularly very late or absent and pregnancy is ruled out, it is worth talking to a clinician.
Can I use a period calculator as birth control?
No. Period and fertile-window estimates are not reliable contraception — ovulation timing shifts, especially with irregular cycles. If preventing pregnancy is your goal, talk to a clinician about appropriate methods.
When should I see a doctor about my cycle?
Check in with a clinician if your periods suddenly become very irregular, stop for three or more months (and you are not pregnant), are extremely heavy or painful, or if you have bleeding between periods. These are worth a professional look rather than guessing from a calendar.
Does MoodSwings predict my period automatically?
Yes — MoodSwings learns your cycle from what you log and predicts your next period, fertile window, and PMS window, updating as it goes, and can gently remind you before each one. It is free to try on iPhone.
Stop re-running the math every month
MoodSwings learns your real cycle from the periods you log, predicts every upcoming period, and tracks the moods and symptoms that show up before it — so your PMS window is your actual pattern, not a population average.