About

Timing determines conception success more than any other factor. The biological window for fertilization is narrow. Sperm survives inside the reproductive tract for up to five days while the egg remains viable for only 12 to 24 hours after release. This calculator aligns these two biological timelines to project the most probable dates for conception.

Standard fertility algorithms rely on the Luteal Phase which is the time between ovulation and the next period. Medical consensus sets this duration at a constant 14 days for most regular cycles. By subtracting this constant from your total cycle length you identify the estimated day of ovulation. Accuracy depends on cycle regularity. This tool projects these dates six months into the future to assist with long-term family planning.

Formulas

The calculation relies on the retrospective subtraction of the Luteal Phase from the expected start of the next cycle.

{

D_next = D_LMP + L_cycleD_ovulation = D_next − 14 (days)W_fertile = [D_ovulation − 5, D_ovulation + 1]

Where D_LMP is the date of the Last Menstrual Period and L_cycle is the average length of the menstrual cycle in days.

Reference Data

Cycle Length	Ovulation Day (Day of Cycle)	Fertile Window (Cycle Days)	Next Period Starts (Day of Cycle)
21 Days	Day 7	Day 2 to Day 8	Day 22
22 Days	Day 8	Day 3 to Day 9	Day 23
23 Days	Day 9	Day 4 to Day 10	Day 24
24 Days	Day 10	Day 5 to Day 11	Day 25
25 Days	Day 11	Day 6 to Day 12	Day 26
26 Days	Day 12	Day 7 to Day 13	Day 27
27 Days	Day 13	Day 8 to Day 14	Day 28
28 Days (Avg)	Day 14	Day 9 to Day 15	Day 29
29 Days	Day 15	Day 10 to Day 16	Day 30
30 Days	Day 16	Day 11 to Day 17	Day 31
31 Days	Day 17	Day 12 to Day 18	Day 32
32 Days	Day 18	Day 13 to Day 19	Day 33
33 Days	Day 19	Day 14 to Day 20	Day 34
34 Days	Day 20	Day 15 to Day 21	Day 35
35 Days	Day 21	Day 16 to Day 22	Day 36

Frequently Asked Questions

The Luteal Phase is the second half of the menstrual cycle. While the follicular phase (first half) varies significantly among women and causes differences in cycle length, the post-ovulatory phase remains relatively constant at 12 to 14 days for the vast majority of healthy reproductive systems. This calculator uses 14 days as the clinical standard for estimation.

Conception outside the fertile window is highly unlikely. The egg survives for a maximum of 24 hours. Once the egg disintegrates, fertilization cannot occur until the next cycle. Sperm can survive up to 5 days, so intercourse prior to ovulation (within the window) allows sperm to wait for the egg. Intercourse 2 days after ovulation has a near-zero probability of conception.

Standard calculators lose accuracy with irregular cycles. If your cycle varies by more than a few days month-to-month, the calculation of the ovulation date (Next Period minus 14) becomes a moving target. In such cases, combining this calculation with physical tracking methods like Basal Body Temperature (BBT) or ovulation predictor kits (OPK) is recommended.

Yes. Hormonal contraceptives typically prevent ovulation entirely. If you are on the pill, the "bleeding" you experience is a withdrawal bleed rather than a true period, and there is usually no egg release to track. This tool is designed for natural cycles.