Cycle day & next period
Period calculator
Next period
- Period
- Fertile days (estimated)
- Ovulation (estimated)
- Period (estimated)
- Today
This is a calendar-based estimate of a possible fertile phase and expressly makes no statement about actual fertility or about when ovulation actually takes place.
Your entries stay in your browser. The calculator stores nothing and sends nothing.
How the calculator works
- Day 1 is the day you enter. Today is the day your device shows.
- It estimates the start of your next period with a fixed rule: day 1 plus your usual cycle length. Cycles vary, so the actual start may differ.
- It estimates fertile days and ovulation with a fixed calendar-based rule that is applied in the same way for everyone: it counts back 14 days from the estimated start of your next period, marks that day as ovulation and the 5 days before, the day itself and the day after as fertile days. Physical signs or measurements are not evaluated.
Sources
- The 5 days before the calculated ovulation date and ovulation day itselfWilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby. N Engl J Med. 1995;333(23):1517–1521.
- The 14 days between ovulation and the next periodLenton EA, Landgren BM, Sexton L. Normal variation in the length of the luteal phase of the menstrual cycle: identification of the short luteal phase. Br J Obstet Gynaecol. 1984;91(7):685–689.
- Why the display is an estimate and includes the following day as a bufferWilcox 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.
This period calculator displays estimated cycle phases, including a fertile window. It is not a medical device. Its results are estimates only and do not constitute medical advice or a diagnosis. Do not use it for contraception or pregnancy planning. Full disclaimer