Ovulation & Fertile Window Calculator
Calculate your ovulation date and 7-day fertile window from your last period. Estimate peak fertility days and next expected period instantly.
See step-by-step calculation
When to use this calculator
- Identifying the best days to try to conceive
- Tracking cycles for natural family planning
- Predicting next period date for scheduling
- Understanding how cycle length affects ovulation timing
- Logging fertility data alongside BBT or OPK tests
- Preparing questions for a gynecologist or fertility specialist
Fertile Window by Cycle Length (Luteal Phase = 14 days)
| Cycle Length (days) | Estimated Ovulation Day | Fertile Window Start | Fertile Window End | Peak Fertility Days | Next Period (from LMP) |
|---|---|---|---|---|---|
| 24 | Day 10 | Day 5 | Day 11 | Days 8–10 | Day 24 |
| 25 | Day 11 | Day 6 | Day 12 | Days 9–11 | Day 25 |
| 26 | Day 12 | Day 7 | Day 13 | Days 10–12 | Day 26 |
| 27 | Day 13 | Day 8 | Day 14 | Days 11–13 | Day 27 |
| 28 | Day 14 | Day 9 | Day 15 | Days 12–14 | Day 28 |
| 29 | Day 15 | Day 10 | Day 16 | Days 13–15 | Day 29 |
| 30 | Day 16 | Day 11 | Day 17 | Days 14–16 | Day 30 |
| 31 | Day 17 | Day 12 | Day 18 | Days 15–17 | Day 31 |
| 32 | Day 18 | Day 13 | Day 19 | Days 16–18 | Day 32 |
| 33 | Day 19 | Day 14 | Day 20 | Days 17–19 | Day 33 |
| 34 | Day 20 | Day 15 | Day 21 | Days 18–20 | Day 34 |
| 35 | Day 21 | Day 16 | Day 22 | Days 19–21 | Day 35 |
Fuente: Ogino-Knaus calendar method as described in WHO Family Planning Guidance (2018) and NEJM (1995). Formula: Ovulation = Cycle Length − 14; Fertile Window = Ovulation − 5 to Ovulation + 1; Peak = Ovulation − 2 to Ovulation. All days counted from Day 1 (LMP). Assumes a fixed luteal phase of 14 days; actual timing varies by individual.
How it works
What is the fertile window?
The fertile window is the 5-6 day period when pregnancy is most likely to occur, centered around ovulation. For a 28-day cycle, it typically spans days 9-15 of your cycle, with peak fertility on days 12-14. This window shifts based on cycle length variations and individual ovulation timing differences.
How It Works
This calculator uses the Ogino-Knaus calendar method, the same framework underlying standard ovulation prediction. The core formula is:
Ovulation Day = LMP + (Cycle Length − Luteal Phase Length)
Fertile Window Start = Ovulation Day − 5
Fertile Window End = Ovulation Day + 1
Peak Fertility = Ovulation Day − 2 to Ovulation Day
Next Period = LMP + Cycle LengthThe luteal phase (time from ovulation to next period) is relatively fixed at 12–16 days for most people, with 14 days as the standard default. The follicular phase (LMP to ovulation) is what varies between individuals and cycles.
Why 5 Days Before + 1 Day After?
Sperm can survive in the female reproductive tract for up to 5 days under favorable cervical mucus conditions. The egg itself is viable for only 12–24 hours after release. This creates a biological window where intercourse on any of the 6 days leading up to and including ovulation can result in fertilization. The day after ovulation (+1) is included as a conservative buffer.
Worked Example
| Input | Value |
|---|---|
| LMP Date | June 1, 2026 |
| Cycle Length | 30 days |
| Luteal Phase | 14 days |
| Output | Calculation | Result |
|---|---|---|
| Ovulation | June 1 + (30 − 14) | June 17 |
| Fertile Window Start | June 17 − 5 | June 12 |
| Fertile Window End | June 17 + 1 | June 18 |
| Peak Fertility | June 15 – June 17 | June 15–17 |
| Next Period | June 1 + 30 | July 1 |
Limitations & When NOT to Rely on This Alone
Frequently asked questions
What is the fertile window?
How accurate is the calendar method for predicting ovulation?
What if my cycle is shorter than 25 days or longer than 35 days?
What is the luteal phase and should I change the default of 14 days?
Can I use this calculator to avoid pregnancy?
What are peak fertility days?
My cycle varies each month. How should I use this?
Does stress or illness affect ovulation timing?
What does 'cycle day today' mean in the results?
Should I use this calculator if I am trying to conceive after age 35?
Sources & references
- Fertility Awareness-Based Methods — WHO Family Planning Guidance — World Health Organization (2018)
- Timing of Sexual Intercourse in Relation to Ovulation — NEJM — New England Journal of Medicine (1995)
- Menstruation in Girls and Adolescents — ACOG Committee Opinion — American College of Obstetricians and Gynecologists (2015)
- Female Fertility: Why Lifestyle Choices Count — Mayo Clinic — Mayo Clinic (2023)
- Reproductive Health — CDC — Centers for Disease Control and Prevention (2024)
Methodology & trust
Calculadora de familia revisada por el equipo editorial de Hacé Cuentas, contrastada con Fertility Awareness-Based Methods — WHO Family Planning Guidance, según nuestra política editorial y metodología.
Última revisión: June 20, 2026. Los parámetros se verifican periódicamente con las fuentes citadas.
Calculations run 100% in your browser. We do not store or transmit your data.
Indicative results. For critical decisions, consult a professional.
Rodríguez, M. (2026). Ovulation & Fertile Window Calculator. Hacé Cuentas. https://hacecuentas.com/ovulation-fertile-window-calculator
Contenido bajo licencia CC-BY 4.0 — reutilizable citando la fuente con enlace a Hacé Cuentas.