How do I calculate my fertile window after my period?

Enter the first day of your last period and your cycle length. The tool's fertile window runs from 5 days before your predicted ovulation day through ovulation day itself — e.g. Oct 10 – Oct 15, 2026 for a 28-day cycle starting Oct 1. The 5-day lead accounts for sperm survival; the egg itself is viable roughly 12–24 hours.

What are my most fertile days to conceive — calculator?

The daily conception chances list shows it: per the engine's table, the single best day is the day BEFORE ovulation (~33% estimated chance), followed by 2 days before (~27%) and ovulation day itself (~20%). For a 28-day cycle starting Oct 1, 2026, that peak day is Oct 14, 2026.

How do I calculate ovulation day after my period?

Ovulation day after your period = first day of your last period + (cycle length − 14). A 28-day cycle means ovulation around day 14; a 32-day cycle means around day 18. The tool does this math for you — just enter your last-period date and cycle length.

Can I use an ovulation calculator with irregular periods?

Only as a rough guide. The calendar method assumes ovulation happens 14 days before your next period, which breaks down with irregular cycles — stress, illness, PCOS, and perimenopause all shift it. If your cycles vary by more than a few days, ovulation test strips or basal body temperature tracking will be more reliable than any ovulation calculator with irregular periods.

How accurate is an ovulation calculator?

For regular cycles it is a reasonable estimate, not a guarantee: it predicts based on averages and cannot know your actual hormone surge. It cannot be used as contraception, and it cannot detect cycles where ovulation does not happen. Confirm the fertile window with ovulation predictor kits or your clinician if timing matters.

Ovulation Calculator

Q: When do I ovulate — calculator?

This "when do I ovulate" calculator answers it from your last period: ovulation usually happens 14 days before your next period, so the tool computes last period + (cycle length − 14). With a 28-day cycle starting October 1, 2026, that is October 15, 2026. Enter your own last-period date and cycle length above for your prediction.

What is Ovulation calculator?

Ovulation Calculator — Free ovulation calculator: enter your last period and cycle length to predict ovulation day, fertile window, conception chances, and next period.

The engine is the open-source ovulation-calculator (DarnOsint, MIT), using the standard calendar method: ovulation day = first day of last period + (cycle length − 14); the fertile window runs from 5 days before ovulation through ovulation day itself (sperm can survive several days, and an egg lives roughly 12–24 hours); the next period estimate = last period + cycle length. The per-day conception chances come from the engine's published table (5 days before: 10%, 4: 12%, 3: 16%, 2: 27%, 1: 33% — the peak — ovulation day: 20%, day after: ~0%). Limitations: this is an estimate, not a medical diagnosis — it assumes you ovulate 14 days before your next period, which holds for regular cycles but shifts with stress, illness, or irregular cycles; it cannot detect anovulatory cycles and is not a contraceptive method. For pregnancy planning, confirm with ovulation test strips or your clinician.

Why use this ovulation calculator?

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Built for ovulation calculator

Free ovulation calculator: enter your last period and cycle length to predict ovulation day, fertile window, conception chances, and next period.

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Transparent & honest

Informational estimate — not a medical diagnosis and not a contraceptive method.

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Private by design

Everything runs in your browser. Your inputs never leave your device.

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Works everywhere

Fully responsive — perfect on desktop, tablet, and mobile. No app needed.

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Regularly updated

Regularly maintained and reviewed to ensure accuracy.

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Free forever

No accounts, no limits, no watermarks. Use Ovulation Calculator as much as you need.

How to use Ovulation Calculator

1Enter the first day of your most recent period in the date field.
2Enter your average cycle length in days (first day of one period to the first day of the next) — 28 is the default.
3Optionally enter your average period length for context.
4Run the tool to see your predicted ovulation day, fertile window, and next period estimate.
5Check the daily conception chances list — the day before ovulation carries the highest estimated chance.

What can you do with Ovulation Calculator?

When do i ovulate calculator — Free ovulation calculator: enter your last period and cycle length to predict ovulation day, fertile window, conception chances, and next period.
Fertile window calculator — Free ovulation calculator: enter your last period and cycle length to predict ovulation day, fertile window, conception chances, and next period.
Best days to conceive calculator — Free ovulation calculator: enter your last period and cycle length to predict ovulation day, fertile window, conception chances, and next period.
Ovulation day after period calculator — Free ovulation calculator: enter your last period and cycle length to predict ovulation day, fertile window, conception chances, and next period.
Irregular period ovulation calculator — Free ovulation calculator: enter your last period and cycle length to predict ovulation day, fertile window, conception chances, and next period.

Good to know

Informational estimate — not a medical diagnosis and not a contraceptive method.
Assumes ovulation 14 days before the next period (standard for regular cycles); irregular cycles reduce accuracy.
Conception chances are population averages from the engine's table, not your personal probability.
Cycle lengths outside 21–35 days and period lengths outside 1–10 days are rejected by the tool.

How it works

The engine is the open-source ovulation-calculator (DarnOsint, MIT), using the standard calendar method: ovulation day = first day of last period + (cycle length − 14); the fertile window runs from 5 days before ovulation through ovulation day itself (sperm can survive several days, and an egg lives roughly 12–24 hours); the next period estimate = last period + cycle length. The per-day conception chances come from the engine's published table (5 days before: 10%, 4: 12%, 3: 16%, 2: 27%, 1: 33% — the peak — ovulation day: 20%, day after: ~0%). Limitations: this is an estimate, not a medical diagnosis — it assumes you ovulate 14 days before your next period, which holds for regular cycles but shifts with stress, illness, or irregular cycles; it cannot detect anovulatory cycles and is not a contraceptive method. For pregnancy planning, confirm with ovulation test strips or your clinician.

Examples

28-day cycle starting October 1, 2026

Predicted ovulation: Oct 15, 2026 — fertile window Oct 10 – Oct 15, next period Oct 29, 2026.

32-day cycle starting October 1, 2026

Predicted ovulation: Oct 19, 2026 — fertile window Oct 14 – Oct 19, next period Nov 2, 2026.

Peak conception day example

Daily chances: the highest estimated chance (33%) falls on Oct 14, 2026 — one day before ovulation.

People also search for these — this tool handles them all:

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