Period calculator
Enter the first day of your last period and your usual cycle length to see when the next six are expected. This projects dates only — it does not estimate ovulation or a fertile window.
Enter your details
Result
These are estimates from population-level equations, not a measurement of you and not medical advice. Individual results vary widely. Talk to a doctor or a registered dietitian before making significant changes to how you eat or train.
How this is calculated
Cycle projectiondefault
next period = first day of last period + cycle length
nth period = first day of last period + (cycle length × n)
- Source
- Arithmetic rather than a published equation. Cycle length ranges are described in Mihm M., Gangooly S., Muttukrishna S., "The normal menstrual cycle in women", Animal Reproduction Science 124(3–4), 2011, pp. 229–236.
- Assumes
- That every future cycle is exactly as long as the one you entered. That is the only assumption, and it is a strong one: cycle length varies from one cycle to the next in most people.
- Accuracy
- Each projection is as accurate as the cycle length you gave, and no more. Cycle-to-cycle variation of a few days is normal, and because the arithmetic multiplies the interval, that error multiplies too — the sixth date is roughly six times as uncertain as the first. Projecting from an average of several recent cycles is better than projecting from one.
Days from the start of one period to a later one, by cycle length
Read across from your cycle length. The figures are simply the cycle length multiplied out, which is the whole of the arithmetic — and also its limitation. Cycle length varies from one cycle to the next, so the error in the twelfth column is roughly twelve times the error in the first.
| Cycle length | 1 cycle | 2 cycles | 3 cycles | 6 cycles | 12 cycles |
|---|---|---|---|---|---|
| 21 days | 21 days | 42 days | 63 days | 126 days | 252 days |
| 24 days | 24 days | 48 days | 72 days | 144 days | 288 days |
| 26 days | 26 days | 52 days | 78 days | 156 days | 312 days |
| 28 days | 28 days | 56 days | 84 days | 168 days | 336 days |
| 30 days | 30 days | 60 days | 90 days | 180 days | 360 days |
| 32 days | 32 days | 64 days | 96 days | 192 days | 384 days |
| 35 days | 35 days | 70 days | 105 days | 210 days | 420 days |
Questions
- How accurate is a projected period date?
- As accurate as the cycle length you entered, and it degrades the further ahead it looks. Cycle length varies by a few days from one cycle to the next in most people, and this arithmetic assumes it does not, so the error compounds across cycles. The first date is a reasonable estimate; the sixth is a rough one.
- What counts as the first day of a period?
- The first day of full bleeding. Spotting the day before is generally not counted. Using the same rule every time matters more than which rule you pick, because cycle length is measured from one first day to the next.
- What is a normal cycle length?
- Roughly 21 to 35 days in adults, with 28 as the commonly quoted midpoint rather than a target. Cycles are often longer and more variable in the years after menstruation begins and in the years approaching menopause. A cycle consistently outside that range, or a sudden change in it, is worth raising with a doctor.
- Does this calculator show my fertile window or ovulation date?
- No, and that is deliberate. This site does not publish ovulation, conception or due date calculators — they are the highest-scrutiny category in health and are properly served by established medical publishers. This page projects period start dates and nothing else.
- Can I use this to avoid pregnancy?
- No. This is not a contraceptive method and gives no information about fertility. Calendar-based methods of any kind are among the least reliable forms of contraception, and this page does not implement one. Talk to a doctor or a sexual health service about contraception.
- Why did my period not arrive on the projected date?
- Because the projection assumed a fixed cycle length and cycles vary. Illness, stress, travel, weight change, exercise load, hormonal contraception and perimenopause all shift timing. A single late or early period is common. A pattern of missed or very irregular periods is worth a conversation with a doctor.