Ovulation Calculator
Tell us the first day of your last period and how long your cycle usually runs. We predict your ovulation day, your full six-day fertile window, your next period, and the earliest day a pregnancy test can be meaningful.
Your details
The highest-conception days are the two days before ovulation and ovulation day itself. If your cycles are irregular, use the shortest and longest values to get a window range instead of a single day.
The six days ending on ovulation day are the only days in a cycle when sex can lead to pregnancy: up to five days before ovulation, sperm survive in the reproductive tract waiting for the egg, and the egg itself lives roughly 12–24 hours after release. Everything else in the cycle is biologically off-limits, which is why a two- or three-day error in predicting ovulation can cancel out an entire month of well-intentioned effort.
Nearly every ovulation calculator online runs the same hidden math: last period plus 14 days. That is only correct for a 28-day cycle. If your cycle is 33 days, you ovulate around day 19, not day 14 — a five-day difference that puts the "fertile window" label on entirely the wrong week. This calculator takes your actual cycle length as a first-class input and shows the window for the cycle you actually have.
If your cycles vary, run the calculator twice — once with your shortest typical cycle and once with your longest — and treat the overlapping window as your reliable zone. For cycles longer than 35 days or shorter than 21 days, or if tracking keeps disagreeing with predictions, a clinician can check for ovulation with blood tests or ultrasound.
How the calculation works
We estimate ovulation as: first day of last period, plus cycle length, minus 14 days. The 14 is the approximate luteal phase — the fixed-ish stretch between ovulation and the next period — which holds reasonably steady even when the first half of the cycle varies.
The fertile window runs from five days before the predicted ovulation (sperm survival) through the day after (egg survival), six days total. Your next period is predicted as last period plus one full cycle length, and the first day a test can pick up a pregnancy is roughly 14 days after ovulation — the day your period would be due.
Frequently asked questions
How do I count my cycle length correctly?
Start counting on the first day of proper menstrual bleeding (day 1) and stop on the first day of the next period. The count includes both bleeding days. Do not start from the last day of bleeding — that is one of the most common counting errors and it shifts every prediction by several days.
My cycle is irregular. Can I still use this calculator?
Yes, but treat the output as a range, not a pinpoint. Run it with your shortest cycle in the last six months and again with your longest; your realistic fertile window is where the two outputs overlap. If cycles swing by more than about 9 days month to month, ovulation predictor kits (LH strips) or basal body temperature give you a much sharper signal than any calendar method.
Which days are best for conceiving?
Probability is highest on the two days before ovulation and on ovulation day itself — intercourse on those three days captures most of the realistic chance in a cycle. The two days before ovulation beat ovulation day itself because sperm need several hours to reach the egg, and being early costs nothing while being late costs everything.
When can I take a pregnancy test after ovulation?
The earliest sensitive blood tests can detect pregnancy is about 8–10 days after ovulation, and home urine tests become reliable from the day your period is due — roughly 14 days after ovulation. Testing earlier produces frequent false negatives that reflect timing, not pregnancy status. If you are not pregnant, a period is predicted about 14 days after ovulation.
Can stress, travel, or illness delay ovulation?
Yes. The first half of the cycle (the follicular phase) is the variable part, and it stretches readily under stress, travel across time zones, illness, or significant weight change. The luteal phase is much more stable. This is why a late period often means late ovulation rather than a problem with the pregnancy itself.
References & sources
- Wilcox AJ, et al. Timing of sexual intercourse in relation to ovulation. N Engl J Med. 1995.
- ACOG Committee Opinion 762: Prepregnancy Counseling (2019).
- Lenton EA, et al. Normal variation in the length of the follicular phase of the menstrual cycle. Br J Obstet Gynaecol. 1984.
- Stanford JB, et al. Timing intercourse to achieve pregnancy: current evidence. Fertil Steril. 2002.