Explain it like I'm 12
The ovulation calculator estimates the next period from cycle length, then subtracts luteal phase length to estimate ovulation and a fertile-window range.
Health & Lifecycle
Ovulation / Fertility Calculator
Estimate ovulation date, fertile window and next period from last menstrual period, cycle length and luteal phase.
Calculator
Working calculator
Live result2026-06-15Estimated fertile window 2026-06-10 to 2026-06-15. Next period estimate 2026-06-29 from 28-day cycle and 14-day luteal phase.
Formula used
Next period = LMP + cycle length. Estimated ovulation = next period − luteal phase. Fertile window = ovulation − 5 days through ovulation day.
This is the method behind the answer, so the result can be checked rather than simply trusted.Visual grid
This number is one point on a larger pattern
Ovulation / Fertility is not just a final answer. It is a step on a line: before and after, input and output, assumption and result.
Micro-timehours, minutes, shiftsHuman scaledays, weeks, projectsMacro-timemonths, years, calendars
InputFormulaResult
2026-06-15CalculationTime keeps the path visible: the input, the method and the final number belong together.
CalculationTime
Ovulation / Fertility Calculation Report
2026-06-15Estimated fertile window 2026-06-10 to 2026-06-15. Next period estimate 2026-06-29 from 28-day cycle and 14-day luteal phase.
Inputs
- LMP year
- 2,026
- LMP month
- 6
- LMP day
- 1
- Cycle length
- 28 days
- Luteal phase
- 14 days
Method
Next period = LMP + cycle length. Estimated ovulation = next period − luteal phase. Fertile window = ovulation − 5 days through ovulation day.
- For LMP 2026-06-01, 28-day cycle and 14-day luteal phase, estimated ovulation is 2026-06-15 and the fertile window starts around 2026-06-10.
Assumptions
- Cycle tracking is an estimate and does not guarantee pregnancy or contraception outcomes.
- Irregular cycles, postpartum, medication, health conditions and age can change timing.
- Use clinical advice for fertility treatment or contraception decisions.
Notes
Use this space on the printed report for client, supplier, classroom, job-location, measurement, quote or approval notes.
Source references
- Office on Women’s Health - Ovulation and the menstrual cycle — cycle education reference
Formula
Next period = LMP + cycle length. Estimated ovulation = next period − luteal phase. Fertile window = ovulation − 5 days through ovulation day.
Worked example
For LMP 2026-06-01, 28-day cycle and 14-day luteal phase, estimated ovulation is 2026-06-15 and the fertile window starts around 2026-06-10.
Professional note
Master’s Tip: record real cycle dates over several months before trusting a single calculated window.
Regional and unit assumptions
General menstrual-cycle estimate. Not contraception or medical advice.
Assumptions and limitations
- Cycle tracking is an estimate and does not guarantee pregnancy or contraception outcomes.
- Irregular cycles, postpartum, medication, health conditions and age can change timing.
- Use clinical advice for fertility treatment or contraception decisions.
Methodology & Accuracy
How this calculator is checked
CalculationTime pages are built around visible arithmetic: the formula, assumptions, worked example and practical limitations are shown so the result can be checked rather than simply trusted.
Formula used
Next period = LMP + cycle length. Estimated ovulation = next period − luteal phase. Fertile window = ovulation − 5 days through ovulation day.
Standard or basis
General menstrual-cycle estimate. Not contraception or medical advice.
Where a calculator follows a named legal, trade or industry standard, that standard is cited visibly. Otherwise the page uses transparent general arithmetic and states its limits.Master's Tip
Master’s Tip: record real cycle dates over several months before trusting a single calculated window.
Related calculators
Questions
Is ovulation always 14 days after a period starts?
No. This calculator estimates ovulation by subtracting luteal phase from expected next period. Cycle timing varies.
Can this be used as contraception?
No. A simple calculator is not reliable contraception.