Period calculator: predict your next periods and fertile days
This period calculator predicts when your next periods will start and where the fertile window falls in each cycle, from the first day of your last period and your usual cycle length.
How this is calculated
Period n = first day of last period + (cycle length × n)
Ovulation in each cycle = first day of that cycle + (cycle length − luteal phase length)
Fertile window = the 5 days before ovulation, and the day of ovulation
The luteal phase, the days from ovulation to the next period, lasts 14 days on average and 11 to 17 days in a normal cycle. Sperm survive up to 5 days in the reproductive tract and an egg about 12 to 24 hours after ovulation, which is why the fertile window ends on the day of ovulation.
A calendar cannot detect ovulation in the current cycle. In a study of 221 women, the fertile window fell entirely within days 10 to 17 of the cycle for only about 30 in 100, and its timing was highly unpredictable even in regular cycles.
Sources: ACOG, on the fertile window; Wilcox, Weinberg and Baird, Timing of sexual intercourse in relation to ovulation (New England Journal of Medicine, 1995; 221 women, 625 cycles); ASRM Practice Committee, Diagnosis and treatment of luteal phase deficiency: a committee opinion (Fertility and Sterility, 2021); Wilcox, Dunson and Baird, The timing of the fertile window in the menstrual cycle (BMJ, 2000; 221 women, 696 cycles).
A menstrual cycle runs from the first day of one period to the first day of the next
The first day of a period is day 1 of the cycle, and the cycle length is the number of days from that day to the day before the next period starts. A menstrual calculator adds that length to the date your last period started to predict the next one, then repeats the step for each cycle after it.
Your next period is due one cycle length after your last period started
Each prediction assumes that every cycle lasts exactly the length you enter. If your cycles vary, the difference accumulates from one month to the next, so the next period is the most reliable prediction and the later ones are progressively less certain. Even in women whose cycles are usually regular, the timing of the fertile window can be highly unpredictable (Wilcox, Dunson and Baird, BMJ, 2000).
No day of the cycle is reliably safe from pregnancy by the calendar alone
In a study of 221 women, on every day from day 6 to day 21 of the cycle at least 10 in 100 women were in their fertile window, and some were still potentially fertile in the fifth week of the cycle (Wilcox, Dunson and Baird, BMJ, 2000; 221 women, 696 cycles). A period calculator is not a method of contraception.
A pregnancy test is most reliable from the first day of a missed period
If your period has not started by the predicted date, a home pregnancy test gives its most reliable result from the first day of the missed period (NHS, Doing a pregnancy test). The NHS adds that a negative result is less reliable than a positive one, particularly if the test is taken too early.
Sources
- Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321(7271):1259–1262. doi:10.1136/bmj.321.7271.1259. PMID 11082086.
- NHS. Doing a pregnancy test. nhs.uk. Accessed 21 September 2026.