Example 1
- LMP: January 1, 2026
- Cycle: 28 days
Ovulation: January 14
Window: January 9–15
Typical 28-day cycle: ovulation roughly mid-cycle under a fixed 14-day luteal model.
Ovulation Calculator
Enter the first day of your last period and cycle length. From January 1, 2026 and 28 days: ovulation January 14, window 9–15. At 32 days it is January 18. A calendar model, not an LH test.
Result
Enter the first day of your last period.
Ovulation ≈ LMP + (cycle − 14). Window: ovulation−5 … ovulation+1. Not contraception. Pregnancy gain: weight gain.
How the results are calculated
LMPJan 1
Cycle28
Luteal14
OvulationJan 14
Ovulation: LMP + (cycle length − 14 days).
Fertile window: from 5 days before ovulation through 1 day after (ovulation−5 … ovulation+1).
Next period: LMP + cycle length.
January 1, 2026 and a 28-day cycle: ovulation January 14, window 9–15, next period January 29. At 32 days ovulation is January 18, window 13–19. At 24 days: January 10, window 5–11. The luteal phase stays fixed at 14 days; the difference sits before ovulation.
Enter the first bleeding day. Leave cycle length at 28, tap a 24/28/32 chip, or enter a 21–45 average. The visualization bar shows the window and the next period.
This is a calendar, not contraception and not a diagnosis. Stress, illness, or routine change can shift the day by several days. An LH test does not enter the ovulation date.
Article hook: the same LMP on Jan 1, 2026 with a 28-day cycle gives ovulation Jan 14 and window 9–15, but at 32 days it is already Jan 18 and 13–19 — the bar shows the luteal phase stays 14 days. A calendar, not an LH test.
How to use
- Enter the first day of your last period, for example January 1, 2026.
- Enter average cycle length, for example 28. If you do not know, leave 28; a blank cycle field also uses 28.
- Click Calculate. On 28 days from January 1, ovulation lands on January 14, window 9–15.
- At 32 days the same start date gives January 18 and window 13–19. At 24 days: January 10, window 5–11.
- Once you are pregnant, gain lives on the pregnancy-gain page, not here.
Formula
ovulation day = cycle − luteal (at least 1)
ovulation date = LMP + (ovulation day − 1)
window = ovulation − 5 … ovulation + 1; next period = LMP + cycle
Model: calendar luteal phase of 14 days (textbook simplification). Not an LH test and not contraception.
Examples
Example 2
- LMP: January 1, 2026
- Cycle: 32 days
Ovulation: January 18
Window: January 13–19
A longer cycle moves ovulation later — the luteal phase stays 14 days.
Example 3
- LMP: January 1, 2026
- Cycle: 24 days
Ovulation: January 10
Window: January 5–11
A shorter cycle means earlier ovulation in the same model.
LMP, ovulation, and luteal phase
Start is the first bleed day plus cycle length minus 14. From January 1, 2026 on 28 days, ovulation is January 14, window 9–15.
- LMP
- Last menstrual period — first day of the last period; the formula start.
- ovulation
- Estimated day LMP + (cycle − 14). At 32 days from the same date it is January 18.
- LH
- Luteinizing hormone from a strip test. This calculator does not read it.
Related calculators
FAQ
When am I ovulating according to this calculator?
Ovulation ≈ first day of the last period + (cycle length − luteal phase). From 1 January 2026 with a 28-day cycle and a blank luteal phase (14) that is 15 January. A calendar estimate, not an ovulation test.
What is the fertile window on this page?
The fertile window is the five days before ovulation plus ovulation day. From 1 January 2026 with a 28-day cycle and blank luteal phase it runs 10–15 January. Chance-by-age lives on the fertile-days calculator, not here.
Is this an ovulation calendar?
Yes in the calendar sense: you enter LMP and cycle length, and the calculator returns ovulation day, the fertile window, and the next period. It does not read LH strips or basal temperature.
How does the calculator estimate ovulation day?
Ovulation ≈ LMP + (cycle length − luteal phase). From 1 January 2026 with a 28-day cycle and 14-day luteal phase that is 14 January. With luteal 12 from the same LMP it is 16 January.
How is the fertile window counted?
From 5 days before estimated ovulation through 1 day after (ovulation−5 … ovulation+1). With ovulation on 14 January the window is 9–15 January. The separate fertile days calculator adds a chance estimate with age and BMI.
Where does the next period date come from?
Next period ≈ LMP + cycle length. From 1 January 2026 and 28 days that is 29 January. With 32 days it is 2 February.
What is the luteal-phase field for?
Default is 14 days. You can set 10–16 if you know your average. It shortens or lengthens the pre-ovulation phase at the same cycle length.
What do the 24 / 28 / 32 buttons do?
They set the cycle-length field. The timeline and dates recompute immediately. An empty field still defaults to 28.
What does marking an irregular cycle do?
It does not change the formula. It shows a warning that a calendar estimate is less reliable when cycles vary a lot.
What is follicular length in the result?
It is the cycle day of ovulation: cycle length minus luteal phase. With 28 and 14 it is 14. With 28 and 12 it is 16.
Is this contraception or an LH test?
No. It is a calendar estimate. Stress and illness can shift the day by several days. An LH strip is not part of the date.
Why is cycle length limited to 21–45 days?
Outside that range a teaching calendar model loses sense. Consider specialist assessment instead of LMP alone.
Is an empty cycle or luteal field OK?
Yes. Empty cycle = 28 days. Empty luteal = 14 days. You can change either before Calculate.
Does the result work in pregnancy?
No. After a positive pregnancy test, due date and pregnancy weight gain belong on their own calculators (for example due date, pregnancy weight gain), not here.
What are the three last-cycle fields for?
When you enter at least one length, the calculator uses their average instead of the single mean-cycle field. At 28, 32, and 30 the mean is 30 days.
What does fertile-window length change?
The window still ends the day after ovulation. A shorter window moves the start closer to ovulation; a longer one opens earlier. The range is 3–10 days.
What do the next-cycle projections show?
The calculator adds the same cycle length to the ovulation and next-period dates. It is a calendar repeat, not an LH measurement.
Is the luteal phase always 14 days?
By default yes, but you can set 10–16. A shorter luteal phase with the same cycle means earlier ovulation.
Does the result replace an ovulation test?
No. It is a calendar estimate. An LH test or clinical exam measures something else.
Sources
Calendar model with a 14-day luteal phase. Not contraception.
- Wikipedia — ovulation
- ACOG — fertility awareness methods (context; this page is not an FAB method)
Page updated October 29, 2026.