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A 28-day cycle starting 15 March 2026
Enter year 2026, March, day 15, and 28 days. ASRM sets ovulation on cycle day 14, which is 28 March. The 6-day window is 23 March through 28 March.
Day of last period: 25% lower
Change day of last period to 11 while keeping the other example values the same.
Day of last period: 25% higher
Change day of last period to 19 while keeping the other example values the same.
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Quick answer
What does the Ovulation fertile window calculate?
When is the 6-day fertile window if ovulation is 14 days before your next period? This calculator uses year of last period, month of last period, day of last period, and usual cycle length to estimate calendar fertile window immediately in your browser.
With the values currently entered, the result is Mar 23, 2026–Mar 28, 2026 — ASRM 6-day fertile window ending on ovulation. It also shows estimated ovulation, ovulation cycle day, next period on this cycle, and last period used.
How to use the Ovulation fertile window
- Replace the example values with your own numbers.
- Review the result and supporting figures as they update automatically.
- Check the formula and assumptions before using the estimate for a decision.
Inputs used
- Year of last period
- Month of last period
- Day of last period
- Usual cycle length — entered in days
Ovulation fertile window formula
Ovulation cycle day = cycle length − 14. Fertile window = that day and the 5 days before it. Next period = LMP + cycle length.
Assumptions
- ASRM’s calendar method treats the luteal phase as about 14 days. Ovulation is cycle length minus 14, counting the first day of the last period as day 1.
- ASRM defines the fertile window as the 6-day interval ending on the day of ovulation. The real window can move, even in regular cycles. This is not contraception advice.
Verify the inputs
Authoritative sources
These sources explain the definitions, factors, or rules behind this tool. Their geographic scope is shown because an official source for one country is not automatically valid somewhere else.
Sources do not endorse Calculum. Check the source date, scope, and your own documents before making a financial, tax, insurance, or reporting decision.
Practical guide
Ovulation fertile window example and edge cases
ASRM’s calendar method puts ovulation 14 days before the next period. The fertile window is that day and the 5 days before it.
Example: A 28-day cycle starting 15 March 2026
Enter year 2026, March, day 15, and 28 days. ASRM sets ovulation on cycle day 14, which is 28 March. The 6-day window is 23 March through 28 March.
- Year of last period
- 2,026
- Month of last period
- March
- Day of last period
- 15
- Usual cycle length
- 28 days
Calculated resultMar 23, 2026–Mar 28, 2026ASRM 6-day fertile window ending on ovulation
A 30-day cycle moves ovulation to day 16. If you want the 280-day due date from the same last period, open the pregnancy due date calculator.
Example results use the default display profile. The calculator above follows your selected country and units.
How to read the result
- The highlighted range ends on the estimated ovulation day. ASRM calls that the 6-day fertile window.
- The next-period line is last period plus the cycle length you typed. It is the date the 14-day luteal count is measured from.
Edge cases worth checking
The cycle length changes each month
Use a recent average, not the shortest month and not the longest. ASRM says the real window can move even when cycles look regular.
You need contraception timing
Stop here. A calendar window is a conception sketch, not a birth-control method. ASRM is explicit that apps miss ovulation often.
What changes the result most
Year of last period
Keep year of last period on the same time basis as the other inputs. Monthly and annual values are easy to mix up.
Month of last period
Keep month of last period on the same time basis as the other inputs. Monthly and annual values are easy to mix up.
Day of last period
Keep day of last period on the same time basis as the other inputs. Monthly and annual values are easy to mix up.
Try a different scenario
Small changes show whether the answer is stable or sensitive.
Day of last period: 25% lower
11Mar 19, 2026–Mar 24, 2026ASRM 6-day fertile window ending on ovulation
Day of last period: 25% higher
19Mar 27, 2026–Apr 1, 2026ASRM 6-day fertile window ending on ovulation
Year of last period: 25% higher
2,100Mar 23, 2100–Mar 28, 2100ASRM 6-day fertile window ending on ovulation
Common mistakes
Check year of last period
ASRM’s calendar method treats the luteal phase as about 14 days. Ovulation is cycle length minus 14, counting the first day of the last period as day 1. Make sure this matches the number you enter.
Keep usual cycle length consistent
ASRM defines the fertile window as the 6-day interval ending on the day of ovulation. The real window can move, even in regular cycles. This is not contraception advice. Use the same units and time period throughout the calculation.
Do not rely on one ovulation fertile window scenario
Run a cautious case and an optimistic case. The range is often more useful than one exact-looking number.
Use this result well
When is the 6-day fertile window if ovulation is 14 days before your next period?
Use it to set a starting budget, then confirm real quotes and each person’s needs.