Period Calculator
Predict your upcoming periods from the first day of your last period, your average cycle length and how long bleeding lasts. The calculator gives the start and end of your next period, your estimated ovulation day and fertile window, and projects several cycles ahead in a calendar table. You can also solve your cycle length from two real period dates and estimate what period products cost you each year.
Formula
Worked example
Last period started 10 May with a 28-day cycle and a 5-day bleed. The next period runs 7 Jun to 11 Jun. With a 14-day luteal phase, ovulation lands about 24 May, and the fertile window spans 19 May to 24 May. Later periods follow on 5 Jul and 2 Aug.
How period dates are predicted
A menstrual cycle is counted from the first day of one period to the first day of the next, so cycle length is the gap between consecutive period start dates. To predict your next period, the calculator adds your average cycle length to the first day of your last period. The cycle after that is two cycle lengths later, and so on. The end of each period is the start plus your usual bleeding length minus one day, since the first day counts. This calendar method assumes your cycles stay close to your average, so the more regular your cycles, the more reliable the prediction.
Ovulation and the fertile window
Ovulation usually happens a fairly fixed number of days before the next period rather than a fixed number after the last one. That gap is the luteal phase, commonly about 14 days, so the calculator places ovulation one luteal phase before each predicted period start. The fertile window is the six days ending on ovulation day, because sperm can survive up to about five days and the egg lives for roughly one day. Turn on advanced options to set your own luteal phase if you have tracked it. Remember that these are estimates and the fertile window can shift, so calendar timing should not be relied on to prevent or achieve pregnancy on its own.
Project ahead, solve your cycle length, and cost it out
Set how many cycles to project and the calendar table lists each upcoming period with its bleeding days, estimated ovulation and fertile window, which is handy for planning around events or travel. If you are not sure of your average cycle length, switch on the date solver and enter the start dates of two consecutive periods; the gap between them is your measured cycle length, which you can copy back into the main field. The optional cost estimate multiplies what you spend on products each period by the number of periods in a year (about 365 divided by your cycle length) to show a yearly figure, useful when comparing tampons, pads, cups or period underwear.
What counts as a normal cycle
Most adult menstrual cycles last between 21 and 35 days, with 28 days often quoted as the average, and a period itself usually lasts two to seven days. Cycles in the first few years after periods begin, and in the years approaching menopause, tend to be longer and more variable. Stress, illness, intense exercise, significant weight change, travel across time zones and hormonal contraception can all move ovulation and period timing. If your cycle regularly falls outside 21 to 35 days, or changes suddenly, it is worth discussing with a healthcare professional. A calendar estimate cannot be used as a reliable method of contraception.
Where your next periods fall (28-day cycle, 5-day bleed, from 10 May)
| Cycle | Period | Ovulation (est.) | Fertile window |
|---|---|---|---|
| Next period | 7 Jun to 11 Jun | 24 May | 19 May to 24 May |
| Following period | 5 Jul to 9 Jul | 21 Jun | 16 Jun to 21 Jun |
| Third period | 2 Aug to 6 Aug | 19 Jul | 14 Jul to 19 Jul |
Each predicted period starts one cycle length after the previous one. Ovulation sits one luteal phase (14 days here) before each start, and the fertile window is the six days ending on ovulation.
Frequently asked questions
How do I find my average cycle length?
Note the first day of bleeding for several periods in a row, then count the number of days from the start of one period to the start of the next and average those gaps. You can use the built-in date solver: enter two consecutive period start dates and it returns the cycle length between them. Tracking three or more cycles gives a more dependable figure than a single month.
How does the calculator estimate ovulation and the fertile window?
Ovulation is placed one luteal phase before each predicted period, about 14 days by default, because the luteal phase is fairly fixed per person while the first half of the cycle varies more. The fertile window is the six days ending on ovulation day, covering the roughly five days sperm can survive plus the day the egg is released. You can set your own luteal phase under advanced options.
Why is my period not arriving on the predicted day?
Cycle length naturally varies by a few days, and factors like stress, illness, travel, weight change and contraception can shift it further. A predicted date is a best estimate based on your average; an actual period a few days early or late is usually normal.
Can I rely on this to avoid or achieve pregnancy?
No. Calendar-based prediction is not a reliable method of contraception, and the fertile window it shows is an estimate, not a precise signal of ovulation. Ovulation and period timing can shift unpredictably. Speak to a healthcare provider about effective contraception or about timing intercourse if you are trying to conceive.