# Ovulation Calculator: Fertile Window and Next Period Dates

Estimate your fertile window, ovulation day and next period for the next three cycles from the first day of your last period and your usual cycle length.

Interactive tool: https://lookup.sg/tools/ovulation-calculator

## How this works
Each period is predicted by adding your average cycle length to the first day of the last one. Ovulation is estimated as 14 days before the next period, because the second half of the cycle (the luteal phase) is usually fairly constant, while the first half varies more. For a 28-day cycle that is about 14 days after your period starts.

The fertile window is the 5 days before ovulation plus ovulation day itself, 6 days in all, because sperm can survive for several days while an egg lasts about a day. The chance of conceiving is highest in the 3 days ending on ovulation day.

This is the calendar method, an estimate only. Cycles of 21 to 35 days are normal, but ovulation can shift from month to month, especially with irregular cycles, after stopping hormonal contraception or while breastfeeding. Do not use it as contraception. Ovulation test kits, temperature charting or a GP or gynaecologist can give a more accurate picture.

## FAQs
### When do I ovulate on a 28-day cycle?
Around 14 days after your period starts, which is 14 days before the next one. If your last period started on 1 October, ovulation is around 15 October and your fertile window is about 10 to 15 October.

### How long is the fertile window?
About 6 days: the 5 days before ovulation and ovulation day itself. Sperm can survive in the body for up to 7 days, so sex a few days before ovulation can still lead to pregnancy.

### When do I ovulate on a 32-day cycle?
About 18 days after your period starts (32 minus 14). If your last period began on 1 October, ovulation is around 19 October, the fertile window about 14 to 19 October and your next period is due on 2 November.

### When should I take a pregnancy test?
Most home pregnancy tests can be used from the first day of a missed period. On a 28-day cycle starting 1 October, that is 29 October.

## Sources
- [NHS: Periods and fertility in the menstrual cycle](https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/)
- [Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995](https://pubmed.ncbi.nlm.nih.gov/7477165/)
- [NHS: Doing a pregnancy test](https://www.nhs.uk/pregnancy/trying-for-a-baby/doing-a-pregnancy-test/)

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Source: Lookup (https://lookup.sg) · Data as of October 2026 · Listings compiled from Singapore government open data (data.gov.sg), OneMap and Overture Maps places. Free listings never show contact details; contact details appear only on Lookup Verified listings.