Example 1
10-15 Aug 2026
1 August, cycle 28, age 30, no smoke. Chance 20%.
Fertile days calculator
You enter last period, cycle and age. After 1 August 2026 and 28 days the window is 10-15 August, ovulation on the 15th. At age 30 and BMI 22 the cycle chance is 20%. Not an LH test and not contraception.
This is not a clinic and not a contraceptive method. Window only, no chance: ovulation calculator. Due date: due date.
Result
Period date, cycle and age. The result will show here.
How it works
Ovulation = first day + (cycle − 14). The window is that day and the five days before. 1 August 2026 and 28 days give ovulation on 15 August and a window of 10-15. At 32 days from 1 July that is 19 July and 14-19.
Chance starts from age: to 25 it is 25%, to 30 20%, to 35 15%, to 37 12%, to 40 8%, to 42 5%, then 2%. Smoking multiplies by 0.7. BMI under 18.5 or from 30 multiplies by 0.85. The cap is 40%.
Age 18-50. Cycle 21-45. For smoking pick “I do not smoke” or “I smoke”. Blank BMI counts as 22.
Here you get a window and a chance sketch. The ovulation calculator is the window only, with no age. Due date uses the same start date with another rule. Human BMI lives on its own card.
Chance is not a lab result. We do not recommend supplements. This is not contraception.
Window is those six days. LMP is the first day of the last period. From there you count cycle minus 14. A comma in 18.5 works. Take the date from a calendar, not from “around mid-month”.
How to use
- Type 1 August 2026, cycle 28 and age 30. Leave “I do not smoke”. Type BMI 22 or leave it blank.
- Click Calculate. Window 10-15 August, ovulation on the 15th, chance 20%.
- Age 40, smoking and BMI 31 on the same cycle leave about 5%.
- 1 July and cycle 32 give a window of 14-19 July. A longer cycle moves the day.
- We will not run a date without age. The calendar without chance is on ovulation.
Formula
ovulation = LMP + (cycle − 14). window = ovulation−5 … ovulation. chance = age_band × 0.7 if smoking × 0.85 if BMI < 18.5 or ≥ 30. 0-40%.
Window, age and chance
1 August 2026 and 28 days: window 10-15 Aug. Age 30: 20%.
- Window
- Five days before plus ovulation day. 1 Aug 2026 and 28 days is 10-15 August.
- Age
- The chance band. Age 30 is 20%. At 40, it is 8% before habits.
- Chance
- A cycle sketch, not AMH. Smoking ×0.7. BMI outside 18.5-30 ×0.85.
- LMP
- First day of last period. From there you count cycle − 14.
Worked examples
Example 2
about 5%
1 August, 28 days, age 40, smoking and BMI 31. Same window, lower chance.
Example 3
14-19 Jul 2026
1 July and cycle 32, age 25. Ovulation 19 July, chance 25%.
Suggested uses
- You check the window after 1 August on a 28-day cycle, before you treat “day 14” as a rule.
- At 30 the sketch is about 20%. At 40, with smoking and BMI 31, it drops to about 5%.
Frequently asked questions
When is the window after 1 August 2026 on a 28-day cycle?
10-15 August. Ovulation is 15 August, because 28 − 14 = 14 days from 1 August. Chance at age 30 and BMI 22 is 20%.
What does cycle chance mean?
An age-band sketch, then ×0.7 for smoking and ×0.85 for BMI outside 18.5-30. It is not a lab result.
What about age 40, smoking and BMI 31?
About 5%. Start 8%, times 0.7 and 0.85. The window stays with the cycle; chance drops with habits.
Is 20% a pregnancy guarantee?
No. It is an age band. The couple and the lab stay outside the formula.
How is this different from the ovulation calculator?
Here you also use age, smoking and BMI. There you only get a window from the date and cycle.
Does a blank BMI break the sum?
No. A blank field counts as 22 and does not multiply by 0.85.
Is this contraception?
No. The window is a model with a 14-day luteal phase. You do not plan protection on it.
Which supplements do you recommend?
None. This card does not pick pills.
Knowledge sources
The window uses LMP and cycle length. This is not a contraceptive method.
Page updated in 2026.