Period Calculator

A start date is only half of what you need

Most cycle tools will tell you when your next period starts. That is useful right up until the moment you are actually trying to plan something, at which point the question is never just "when does it start". It is "will it be done by the wedding", "does it land on the flight", "do I need to pack for this".

That is what this one is for. It asks a question the others skip: how many days does your period usually last. Because it knows that, it can hand you a start date and an end date, and then do the same for the three periods after that. Below is how the dates come out, what a normal period length looks like, some honest fine print about the fertile window it also shows, and the bleeding patterns that are worth raising with a doctor rather than planning around.

Four periods, mapped out

You give it three things: how long your cycle runs, how many days your period lasts, and when your last period started. The arithmetic is not complicated, and it helps to see it land.

Say your last period started on 1 June 2026, your cycle runs 28 days, and you bleed for 6 days:

Next period starts = 1 June + 28 days = 29 June
Next period ends = 29 June + 6 days − 1 = 4 July

The minus one is there because the first day counts as day one of the bleed, not day zero. Then it repeats the step for the three after that, giving you 27 July to 1 August, 24 August to 29 August, and 21 September to 26 September. Four months of dates from three inputs.

Every row after the first assumes your next cycle is exactly as long as the one you described, and the one after that, and the one after that. Bodies are not that obedient. Treat the first period as a decent estimate and the later ones as pencil marks that will need moving.

How long should a period actually last?

The tool offers you 3 to 8 days, which covers essentially everyone, and the honest answer is that the normal range is wide. Most people bleed for somewhere between 3 and 7 days. Around 5 is the most common, which is why so many calculators quietly assume it.

The internationally used definition draws the line at 8: bleeding that lasts 8 days or less is considered normal duration, and bleeding beyond 8 days is called prolonged. Within that, your flow will usually start light, get heavier for a day or two, then taper. Typical total blood loss across a whole period is only about 2 to 3 tablespoons, which surprises most people, since it always looks like considerably more.

Enter your own usual number rather than the average. The whole point of this tool is that it uses yours.

The fertile window it also shows, and its fine print

Alongside your period dates, the tool marks an ovulation day and a fertile window. This is worth being straight about, because it takes a shortcut that the rest of our cycle tools do not.

It places ovulation exactly 14 days before your next period is due, which is the textbook average rather than your own luteal phase. If your luteal phase runs shorter or longer than 14 days, and plenty do, that date will be off. It also marks a wider span of days around ovulation than the research supports, so read it as a rough "this is roughly the zone" rather than a precise target.

So use it for a general sense of where in the month you are. If you are actually tracking ovulation, our Ovulation Calculator lets you enter your real luteal phase instead of assuming 14, and the Fertility Calculator covers what the evidence actually says about which days count. And none of these are contraception, this one included.

When your period is worth mentioning to a doctor

Planning around your period is one thing. Putting up with one that is genuinely too heavy is another, and a lot of people do the second for years because they have nothing to compare against. Their period is the only one they have ever had, so it seems normal by default.

Any of these is worth a conversation:

• Bleeding that lasts more than 7 days.
• Soaking through a pad or tampon every hour for several hours in a row.
• Needing to double up on protection to get through it.
• Having to get up in the night to change a pad or tampon.
• Passing clots the size of a coin or larger.
• Bleeding between periods, or a sudden clear change from your usual pattern.

None of these means something is seriously wrong. They do mean it is worth asking, both because heavy periods have treatable causes and because steady blood loss on that scale is a common route to iron deficiency, which is its own drag on how you feel. Feeling wiped out, breathless or dizzy around your period is a reason to go sooner rather than later.

Questions people ask

Do I count the first day if it starts in the evening?

Yes, if it is proper flow rather than a bit of spotting. Day 1 is the first day of real bleeding, whatever time it starts. Light spotting the day before does not count, which trips people up more often than you would think and can quietly shift every date you calculate.

My period is late but the tool said today. Should I worry?

Usually not. A few days either side is ordinary, and stress, illness, travel, poor sleep and changes in exercise or weight all push ovulation around, which pushes your period with it. If pregnancy is possible, a test is the fastest way to settle the question. If your periods go missing for three months or more, or the timing changes sharply and stays changed, that is worth a doctor's opinion.

Can I delay my period for a holiday?

Sometimes, but not with a calculator. It is a medical question, and depending on what you already take, there are options a doctor or pharmacist can talk you through. Worth asking well ahead of the trip rather than the week before.

References

Where the figures come from. The definition of normal versus prolonged menstrual duration, the 8-day boundary used above, follows the FIGO criteria set out in the StatPearls physiology reference. The signs of heavy menstrual bleeding listed above are those published by the American College of Obstetricians and Gynecologists.

1. Physiology, Menstrual Cycle. In: StatPearls. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK500020/
2. American College of Obstetricians and Gynecologists. Heavy Menstrual Bleeding (patient FAQ). https://www.acog.org/womens-health/faqs/heavy-menstrual-bleeding