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Ovulation Calculator

Find your most fertile days. Enter the first day of your last period and your usual cycle length to see your estimated ovulation date, fertile window and next period for the next three cycles.

By BMI Calculator NHS Editorial TeamLast reviewed: Checked against NHS & NICE sources
Quick answer

Ovulation usually happens about 12 to 16 days before your next period – around day 14 of a 28-day cycle. Your fertile window is the 5 days before ovulation plus ovulation day, because sperm can survive for up to 7 days. The NHS advises having sex every 2 to 3 days throughout your cycle for the best chance of pregnancy.

Ovulation Calculator

Fertile window for 3 cycles
days
From the first day of one period to the day before the next
days
Usually 12–16 days; 14 if unsure

How ovulation works

Each cycle starts on the first day of your period. Hormones cause an egg to mature, and it is released from the ovary at ovulation. The egg survives for about 12 to 24 hours, but sperm can survive for several days, which is why the days just before ovulation are the most fertile.

Signs of ovulation

  • Cervical mucus becomes clear, slippery and stretchy, like raw egg white.
  • A slight rise in body temperature after ovulation.
  • Mild pain on one side of the lower tummy for some people.
  • Ovulation predictor kits detect the hormone surge 24–36 hours before ovulation.

Tips if you are trying to get pregnant

  • Have sex every 2 to 3 days rather than timing it exactly.
  • Start taking 400 micrograms of folic acid daily.
  • Stop smoking and cut out alcohol.
  • Aim for a healthy weight – a BMI in the healthy range can improve fertility. Check your BMI.

See your GP if you have not conceived after 1 year of trying (or 6 months if you are 36 or over), or sooner if you have irregular periods or a known condition.

Already pregnant? Use the due date calculator.

Frequently asked questions

When am I most fertile?

In the 5 days before ovulation and on ovulation day. For a 28-day cycle that is usually around days 10 to 15.

How accurate is an ovulation calculator?

It is an estimate based on an average cycle. Cycles vary, so ovulation tests or tracking signs can help.

Can I use this as contraception?

No. Calendar methods alone are not a reliable form of contraception.

My periods are irregular. Can I still use it?

It will be less accurate. Ovulation tests may help, and it is worth speaking to your GP about irregular periods.

Due Date CalculatorEstimated due date (EDD).Open tool →BMI CalculatorCheck your BMI, category and healthy weight range.Open tool →Ideal Weight by Height & AgeHow much should I weigh for my height and age?Read guide →Waist-to-Height RatioNICE waist check: keep waist under half your height.Open tool →Calorie Deficit CalculatorDaily calories to lose weight safely.Open tool →

About this page

Written and reviewed by the BMI Calculator NHS Editorial Team

Our team writes in plain English and checks every figure, threshold and formula against the official NHS, NICE and peer-reviewed sources listed below before publishing. Read our editorial policy or report a correction.

Last reviewed27 October 2026
Next review due1 April 2027
Applies toEngland, Scotland, Wales & Northern Ireland

How this tool works

Ovulation day = first day of last period + cycle length − luteal phase (14 days unless you enter another value). Fertile window = 5 days before ovulation to 1 day after. Dates repeat for three cycles; real cycles vary, so these are estimates.

Sources

  1. NHS – Trying for a baby
  2. NHS – Pregnancy due date calculator

Medical disclaimer: this tool gives an estimate for adults and is not a diagnosis or medical advice. bmicalculatornhs.net is independent and not affiliated with the NHS. Speak to your GP, pharmacist or another qualified health professional about your own health. Full disclaimer.