When is my fertile window?

People type this into search as an ovulation calculator.

The fertile window calculator uses the calendar method in the ASRM committee opinion on natural fertility. The luteal phase is taken as about 14 days. The fertile window is the 6-day interval that ends on the estimated ovulation day. ASRM notes that the real window moves, including in cycles that look regular, and that calendar apps often miss the ovulation day. Use the result as a planning sketch. A contraception plan belongs in a conversation with a clinician. This calendar is a conception sketch.

Cycle length minus 14

ovulation cycle day = cycle length − 14

ovulation date = first day of the last period + (cycle day − 1)

The minus 1 is there because the first day of bleeding is cycle day 1. Day 14 is 13 days after that date, not 14.

window start = ovulation date − 5 days

That start, plus the ovulation day, is six days. The next period on this cycle is the last period plus the cycle length.

The field allows 20 through 45 days, the same range as the due date calculator. The due date post adds 280 days to the same last period. This page stops at the next period and marks the six days at the end of the follicular phase.

A 28-day cycle puts ovulation on day 14. A longer cycle keeps the 14-day luteal phase and puts the extra days before ovulation. A shorter cycle pulls ovulation forward. The window is still six days. Only its place on the calendar moves.

Example 1: 15 March 2026, 28-day cycle

This is the calculator default.

• last period: 15 March 2026, cycle day 1
• cycle: 28 days

Ovulation cycle day: 28 − 14 = 14

Ovulation date: 15 March + 13 days = Mar 28, 2026

Window: 28 March minus 5 days = 23 March. The card prints Mar 23, 2026–Mar 28, 2026

Next period: 15 March + 28 days = Apr 12, 2026

The six cycle days are 9, 10, 11, 12, 13, and 14. Day 9 is 15 March + 8 days = 23 March. Count them: 23, 24, 25, 26, 27, 28. That is six dates.

Six days out of a 28-day cycle is 6 ÷ 28 = 21.4 percent. The other 22 days are 78.6 percent. The window is a short slice of the cycle even when the dates are right, and ASRM’s own caution is that the dates move.

Example 2: a 30-day cycle, same last period

Cycle length becomes 30.

Ovulation cycle day: 30 − 14 = 16

Ovulation date: 15 March + 15 days = Mar 30, 2026

Window: Mar 25, 2026–Mar 30, 2026

Next period: 15 March + 30 = Apr 14, 2026

Compared with Example 1, ovulation moved from 28 March to 30 March. That is two days, and the cycle is two days longer than 28. The window moved with it. It did not grow. It is still six days: 25 through 30 March.

Example 3: the mistake of always using day 14

Day 14 is correct when the cycle is 28 days. On any other length, day 14 is a different date from cycle length minus 14.

Last period 15 March 2026. “Always day 14” is always 28 March, because 15 + 13 = 28.

The gap equals the cycle’s distance from 28. A 32-day cycle is 4 days over 28, and day 14 lands 4 days before the formula’s ovulation. The window is wrong by those same 4 days. The formula window for 32 days is Mar 27, 2026–Apr 1, 2026. The day-14 window would be 23 March through 28 March, which is Example 1’s window pasted onto a longer cycle.

32 − 14 = 18. 15 March + 17 days = 1 April. Minus 5 days is 27 March. Next period: 15 March + 32 = Apr 16, 2026.

Example 4: a 21-day cycle and a 35-day cycle

Still 15 March 2026.

21 days. Ovulation day: 21 − 14 = 7. Date: 15 March + 6 = Mar 21, 2026. Window: Mar 16, 2026–Mar 21, 2026. Next period: Apr 5, 2026.

The window opens the day after the period starts. Day 2 through day 7. Six days, crowded against the start of bleeding.

35 days. Ovulation day: 35 − 14 = 21. Date: 15 March + 20 = Apr 4, 2026. Window: Mar 30, 2026–Apr 4, 2026. Next period: Apr 19, 2026.

From the 21-day ovulation to the 35-day ovulation is 14 days on the calendar, 21 March to 4 April. The cycles themselves differ by 14 days. The luteal phase stayed 14 days in both, so the whole difference shows up before ovulation.

Six days is 6 ÷ 21 = 28.6 percent of the short cycle, and 6 ÷ 35 = 17.1 percent of the long one. The window did not change length. The cycle around it did.

Example 5: the short end and the long end of the field

20-day cycle, the minimum. Ovulation day: 20 − 14 = 6. Date: Mar 20, 2026. Window: Mar 15, 2026–Mar 20, 2026.

The window opens on the first day of the period. Day 6 minus 5 days is day 1. There is no earlier day in this cycle for the calendar method to use. Next period: Apr 4, 2026.

45-day cycle, the maximum. Ovulation day: 45 − 14 = 31. Date: 15 March + 30 days = Apr 14, 2026. Window: Apr 9, 2026–Apr 14, 2026. Next period: Apr 29, 2026.

A 45-day cycle in this formula is a long follicular phase plus a 14-day luteal phase. The six fertile days sit in April. The period that started the count was 15 March. If your cycles really run that long, the average you type matters a lot, because each extra day pushes ovulation by one day. ASRM’s note that the real day moves is doing a lot of work at this end of the field.

Day 22 on a 36-day cycle is 36 − 14. Day 30 on a 44-day cycle is 44 − 14. The line rises one day per day of cycle length. That is the whole formula.

Example 6: what if the last period moves by a week

Same 28-day cycle. The last period moves from 15 March to 22 March 2026.

Ovulation is still cycle day 14. The date is 22 March + 13 days = Apr 4, 2026.

Window: Mar 30, 2026–Apr 4, 2026

Next period: 22 March + 28 = Apr 19, 2026

Every date moved by 7 days. Example 1’s ovulation was 28 March. This one is 4 April. 28 March plus 7 days is 4 April. The window’s length stayed six days. A wrong start day shifts the whole sketch by the same number of days, and it does not show up as a wider window or a different cycle day.

The due date for a 28-day cycle would shift by the same week. The due date post has 15 March landing on 20 December. 22 March plus 280 days is 27 December 2026. Same 7-day shift. The two calculators share the last-period date. They answer different questions.

Example 7: other months, including a leap day

1 January 2026, 28 days. Window Jan 9, 2026–Jan 14, 2026. Ovulation Jan 14, 2026, day 14. Next period Jan 29, 2026.

Day 14 of a period that starts on the 1st is the 14th. The window opens on day 9, which is the 9th. Easy to check by eye.

31 January 2026, 28 days. Window Feb 8, 2026–Feb 13, 2026. Ovulation day 14 is Feb 13, 2026. Next period Feb 28, 2026. 2026 is not a leap year. The next period lands on the last day of February, 28 days after 31 January: January contributes nothing after the 31st, and February’s 28 days finish the count.

29 February 2024, 28 days. 2024 was a leap year, so this start date exists. Window Mar 8, 2024–Mar 13, 2024. Ovulation Mar 13, 2024. Next period Mar 28, 2024. The six days all fall in March. February has no days left after the 29th, so day 2 is 1 March.

1 December 2025, 30-day cycle. Ovulation day 16 is Dec 16, 2025. Window Dec 11, 2025–Dec 16, 2025. Next period Dec 31, 2025. A 30-day cycle from 1 December ends on the 31st, which December has. A 31-day month can hold a 30-day cycle without spilling. January’s 31-day start in the row above spilled into February because the start was the 31st, not the 1st.

Example 8: a date that fails, and a cycle the field refuses

31 February 2026. The result is Need a valid LMP date. The day has to exist. February 2026 has 28 days.

15 March 2026, 19-day cycle. The result is Need a usual cycle length. The range is 20 through 45. The formula does not estimate ovulation on a 19-day cycle, even though 19 − 14 = 5 would be a tidy day number. Below 20, you get the message instead of a window.

The same message appears at 46 days. Example 5 already ran the longest accepted cycle, 45 days, with ovulation on 14 April.

One more limit sits inside the formula and outside the field. If the cycle day of ovulation were under 1, the result would say the cycle is too short for a 14-day luteal phase. That needs a cycle under 15 days. The field stops at 20, so a normal visit will see “Need a usual cycle length” first. Either way, you do not get a window pasted onto a cycle the method cannot fit.

Cycles that change length every month have the same practical limit as a single average. The card uses the one number you type. ASRM says the real ovulation day can move even when cycles look regular. A recent average is the input the method asks for. The shortest month and the longest month are different sketches, and Example 3 shows how far apart those sketches get.

The examples next to each other

Open the ovulation calculator with the first day of the last period and a usual cycle length. The due date calculator will take those same fields and add 280 days. Confirm timing with a clinician when it matters. The six dates are a calendar sketch, and ASRM is plain that the real ovulation day often lands somewhere else.