Example 1
10-15 Aug 2026
With 1 August, a 28-day cycle, age 30, no smoking, and a blank luteal phase, the chance is 20%.
Fertile days calculator
You enter the first day of the last period, the cycle length, and age. With 1 August 2026, a 28-day cycle, and a blank luteal phase, the window runs from 10 to 15 August and ovulation falls on 15 August. At age 30, with no smoking and BMI 22, the cycle chance is 20%. This is not an ovulation test, and it is not a contraceptive method.
Result
Enter the first day of the last period, the cycle length in days, and age in years. The window and calendar will show here.
This calculator does not replace a visit to a doctor, and it is not a contraceptive method. The ovulation calculator shows only the window, without the chance based on age. You can calculate a due date in the due date calculator.
How it works
LMP1 Aug 2026
Ovulation28 − 14 = day 14
Window10–15 Aug
Chance · age 3020%
Ovulation day is the cycle length minus the luteal phase, counted from the first day of the period. A blank luteal phase counts as 14 days. The window covers that day and the five days before it. With 1 August 2026, a 28-day cycle, and a blank phase, ovulation falls on 15 August and the window runs from 10 to 15 August. The next period falls on 29 August. With a 32-day cycle from 1 July, also with a blank phase, ovulation falls on 19 July and the window runs from 14 to 19 July.
The chance depends on age: 25% up to age 25, 20% up to 30, 15% up to 35, 12% up to 37, 8% up to 40, 5% up to 42, and 2% after that. Smoking multiplies that number by 0.7. BMI under 18.5 or from 30 multiplies it by 0.85. The result does not go above 40%. The calculator shows the chance in whole percents. At age 40, with smoking and BMI 31, starting from the age-based value of 8%, the result is 4.76, and the calculator rounds that to 5%.
The calculator accepts age as a whole number from 18 to 50 years and cycle length as a whole number from 21 to 45 days. For smoking, choose “I do not smoke” or “I smoke”. A blank BMI field counts as 22. The unit and currency switches do not change the window dates or the chance percent.
The calculator shows a window, a calendar for this cycle, the next two windows, and a chance estimate. The ovulation calculator shows only the window, without age. The due-date calculator uses the same start date and a different rule. You can calculate BMI on its own page.
The chance is not a lab result. This calculator does not recommend supplements, and it is not meant for contraception.
LMP stands for last menstrual period, the first day of the last period. From that date, the calculator subtracts the luteal phase from the cycle length. A blank phase field counts as 14 days. In the BMI field you can type a decimal, for example 18.5. Read the date from a calendar rather than assuming the period started around mid-month.
How to use
- Enter 1 August 2026, a cycle of 28 days, and age 30 years. Leave “I do not smoke”. Enter BMI 22 or leave it blank. Leave the luteal phase blank.
- Click Calculate. The window runs from 10 to 15 August, ovulation falls on 15 August, the next period falls on 29 August, and the chance is 20%.
- At age 40, with smoking and BMI 31, on the same 28-day cycle and a blank luteal phase, the chance is 5%. The window still runs from 10 to 15 August.
- With 1 July and a 32-day cycle, and a blank luteal phase, the window runs from 14 to 19 July. With a longer cycle, ovulation falls later.
- Enter a luteal phase of 12 days with the same 1 August and a cycle of 28 days. Ovulation falls on 17 August, and the window runs from 12 to 17 August. The next period is still 29 August, because the cycle length did not change.
Formula
Ovulation day is the cycle length minus the luteal phase, counted from the first day of the period. A blank phase field counts as 14 days. The window starts five days before ovulation and ends on ovulation day. The next period falls after a full cycle length. The chance depends on age. Smoking multiplies it by 0.7, and BMI under 18.5 or from 30 multiplies it by 0.85. The result stays between 0% and 40%.
Six days from a 1995 study
In 1995 Wilcox, Weinberg, and Baird described 625 cycles with a known ovulation day in the New England Journal of Medicine. In those cycles, pregnancy began only when intercourse fell on one of the six days ending on that day. The chance from a single day was 10% five days before ovulation and 33% on ovulation day.
Separately, the same model estimated the chance for a whole cycle: 37% with intercourse every day, 33% every other day, and 15% about once a week, with no day chosen. That second 33% means something different from the 33% on ovulation day. One figure is for a single day. The other is for a whole cycle. The day of intercourse did not change the chance of a boy or a girl in that study. Of 192 pregnancies, 129 ended in a live birth.
The calculator shows that window for the date you enter. With 1 August 2026, a 28-day cycle, and a blank luteal phase, ovulation falls on 15 August and the window runs from 10 to 15 August. In that study, the chance on 10 August is 10%, and on 15 August it is 33%. 16 August is already outside the window. The 20% at age 30, with no smoking and BMI 22, is a separate estimate, based on age, of the chance for a whole cycle. The calculator does not multiply it by the 10% or the 33% from the study.
Window, age and chance
With 1 August 2026, a 28-day cycle, and a blank luteal phase, the window runs from 10 to 15 August. At age 30, with no smoking and BMI 22, the chance is 20%.
- Window
- The window covers the five days before ovulation and ovulation day itself. With 1 August 2026, a 28-day cycle, and a blank luteal phase, it runs from 10 to 15 August.
- Age
- The chance depends on age. At 30, it is 20%. At 40, it is 8%, before the calculator accounts for smoking and BMI.
- Chance
- An estimate for the whole cycle, not a blood test. Smoking multiplies it by 0.7. BMI under 18.5 or from 30 multiplies it by 0.85.
- LMP
- Last menstrual period: the first day of the last period. From that date, the calculator subtracts the luteal phase from the cycle length. A blank phase field counts as 14 days.
- Luteal phase
- The days from ovulation to the next period. A blank field counts as 14 days. A number from 10 to 16 changes the ovulation day, but not the cycle length.
- 10% and 33%
- The chance from intercourse on one day in a study of 625 cycles. The 10% falls five days before ovulation, and the 33% falls on ovulation day. That is different from the 20% the calculator estimates from age.
Worked examples
Example 2
5%
With 1 August, a 28-day cycle, age 40, smoking, and BMI 31, the window still runs from 10 to 15 August, and the calculator shows 5%.
Example 3
14-19 Jul 2026
With 1 July, a 32-day cycle, age 25, and a blank luteal phase, ovulation falls on 19 July, the next period falls on 2 August, and the chance is 25%.
Suggested uses
- You can check the window after 1 August on a 28-day cycle instead of assuming ovulation always falls on day 14.
- At age 30, with no smoking and BMI 22, the estimate is 20%. At age 40, with smoking and BMI 31, it drops to 5%.
- With 1 August, a 28-day cycle, and a blank luteal phase, the study reports a 10% chance on 10 August and 33% on 15 August. The calculator does not multiply those percentages by the 20% from age.
Frequently asked questions
When does the fertile window fall after 1 August 2026 on a 28-day cycle?
The window runs from 10 to 15 August when the luteal-phase field is blank. A blank field counts as 14 days, so the calculator subtracts 14 from the 28-day cycle and ovulation falls on 15 August, 14 days after 1 August. The chance at age 30, with no smoking and BMI 22, is 20%.
What does cycle chance mean?
Cycle chance is an estimate based on age. Smoking multiplies it by 0.7, and BMI under 18.5 or from 30 multiplies it by 0.85. It is not a lab result.
What is the chance at age 40 with smoking and BMI 31?
The chance is 5%. Starting from the age-based value of 8%, after multiplying by 0.7 and then by 0.85, the result is 4.76. The calculator shows whole percents, so it rounds 4.76 to 5%. The window still runs from 10 to 15 August, because the cycle did not change.
Is 20% a pregnancy guarantee?
No. It is only an estimate based on age. The formula does not include a partner or a lab result.
How is this different from the ovulation calculator?
This calculator includes age, smoking, BMI, a calendar, and the next two windows. The ovulation calculator shows only the window estimated from the date and the cycle length.
Does a blank BMI change the result?
No. A blank field counts as 22, so the calculator does not multiply the chance by 0.85.
Is this contraception?
No. This is not a contraceptive method.
Which supplements do you recommend?
We do not recommend any supplements.
What happens if you leave the luteal phase blank?
A blank field counts as 14 days. Ovulation day is then the cycle length minus 14, counted from the first day of the period. A number from 10 to 16 changes the ovulation day. The next period follows the cycle length, not this phase.
Where do 10% and 33% come from?
From Wilcox, Weinberg, and Baird, New England Journal of Medicine (1995), across 625 cycles. 10% is the chance from intercourse on one day, five days before ovulation. 33% is the chance on ovulation day. The age-based cycle chance, for example 20% at age 30, is a different figure and is not multiplied by those percentages.
Do the unit or currency switches change the result?
No. The unit and currency switches do not change the window dates or the chance percent. The numbers in the fields stay the same.
Knowledge sources
The calculator estimates the window from the date, the cycle length, and the luteal phase. The 10% and 33% come from a study of 625 cycles. This is not a contraceptive method.
Page updated in 2026.