Ovulation Calculator - Predict Your Fertile Window

How to use this tool

1. Enter your LMP. Select the start date of your last menstrual period.
2. Set cycle length. Enter the average number of days between periods (default 28).
3. Set luteal phase. Enter the typical length of your luteal phase (default 14 days).
4. Choose cycles to show. Select how many upcoming cycles to project (1–6).
5. Review results. Review the six-day core fertile window, estimated peak days, possible day after ovulation, and next period dates. These are estimates, not contraception.

How the ovulation calculator works

Ovulation is estimated using the standard formula: LMP + Cycle Length − Luteal Phase. For example, with an LMP of January 1, a 28-day cycle, and a 14-day luteal phase, the estimate is January 15: cycle day 15 when the LMP is counted as day 1. That is about 14 days before the next period, estimated for January 29. The dates are calendar estimates based on the cycle and luteal-phase lengths you enter.

The core fertile window shown here is the six calendar days from five days before estimated ovulation through ovulation day. Sperm can survive for several days, while an egg may remain viable for about 12–24 hours after ovulation. Because timing estimates and descriptions of the fertile window vary, the tool separately notes the possible day after ovulation; it does not promise that conception is or is not possible on any date. The calendar is not contraception and does not identify safe days.

Understanding the luteal phase

The luteal phase is the second half of the cycle, from ovulation until the next period. It often lasts around 12–16 days, though individual cycles vary. This calculator uses the luteal length you select to estimate ovulation by counting back from the next expected period. The default of 14 days is only a starting estimate, not a measurement of your own cycle.

Improving accuracy

Calendar estimates cannot confirm the day ovulation occurs. Basal body temperature (BBT) charting can help identify a temperature shift after ovulation, while over-the-counter LH (luteinizing hormone) test strips can detect a hormone surge that often precedes ovulation. These signs can add context, but they also have limits; ask a clinician about interpreting them or choosing fertility or birth-control methods.

Cycle phase overview

• Menstrual phase: bleeding begins on cycle day 1, the first day of the period. Bleeding length varies from person to person.
• Follicular phase: begins on cycle day 1 and continues up to ovulation. Its length can vary, which is one reason calendar estimates may shift from cycle to cycle.
• Ovulatory phase: in the calculator's 28-day-cycle, 14-day-luteal example, ovulation is estimated around cycle day 15 (about 14 days before the next period). Actual timing can differ.
• Luteal phase: follows ovulation and lasts until the next period. The calculator uses your selected luteal-phase length as an estimate.

BBT charting guide

Basal body temperature (BBT) is your temperature at complete rest, taken immediately upon waking before any movement. After ovulation, progesterone causes a sustained rise of approximately 0.2°C (0.4°F) that persists until menstruation - this is called the biphasic pattern.

To chart BBT effectively:

• Take your temperature at the same time each morning (within 30 minutes is acceptable).
• Use a dedicated BBT thermometer (reads to two decimal places: 36.65°C, not just 36.7°C).
• Measure before sitting up, eating, or drinking.
• Note disturbances (alcohol, illness, travel, poor sleep) as they can raise BBT abnormally.

BBT charting confirms that ovulation has already occurred - it cannot predict it in advance. Combine with LH testing for both prediction and confirmation.

LH test strips (OPK) guide

Ovulation predictor kits (OPKs) detect the luteinizing hormone (LH) surge in urine. The LH surge typically begins 24–36 hours before ovulation.

• When to start testing: for a 28-day cycle, begin on day 10–11. For shorter or longer cycles, start testing about 4–5 days before your estimated ovulation day.
• Reading the result: a positive OPK is when the test line (T) is as dark as or darker than the control line (C). A faint test line is negative - LH is always present at low levels.
• Timing intercourse: a positive OPK detects an LH surge that often precedes ovulation, but it does not confirm the exact ovulation date. Follow the test's instructions and ask a clinician for personal fertility or birth-control guidance.