Conception calculator

Use your last period or your due date to see the most likely conception date and the days around it.

Your details

Work it out from
First day of your last period
Usual cycle length
Due date (if you chose due date)
Crown rump length from your dating scan (optional)
Date of that scan
Shortest recent cycle (optional)
Longest recent cycle (optional)

Conception most likely around

Sat 5 September 2026

within a few days either side

  • Likely range2 Sept to 8 Sept
  • Due date used29 May

The calendar assumes ovulation about 2 weeks before the next period. Sperm can live up to 5 days, so sex a few days before this date can also lead to pregnancy. A dating scan at 11 to 14 weeks is more accurate than any calendar, and when the two differ your midwife will use the scan.

From a due date

Working back from your due date

Choose due date and the calculator counts back 266 days (38 weeks) to the likely conception date, with the few days either side when it most probably happened. A dating scan due date is more accurate than one from your last period.

The working

Two weeks after your period

Conception usually follows ovulation, about 2 weeks after the first day of your last period in a 28 day cycle. A period starting 1 October 2026 points to conception around 15 October. From a due date, count back 266 days (38 weeks).

From your scan

The crown rump length

At the dating scan the sonographer measures the baby from crown to rump. Robinson's formula, which NICE uses for lengths of 45 to 84 mm, turns that into weeks: 8.052 × √length in mm + 23.73 days. A 55 mm baby is 83 days, 11 weeks 6 days, so conception was about 69 days before the scan. If that differs from the calendar, the scan date is the one your midwife uses.

Why it is a range

Bodies are not calendars

  • Ovulation can come a few days early or late
  • Sperm can wait up to 5 days for an egg
  • Due dates from a scan are accurate to about a week
  • So the true date may be several days either side

Scan dates

The most accurate guide

The dating scan at 11 to 14 weeks measures your baby and gives the due date the NHS uses. If it differs from your period dates, go by the scan.

If your cycles vary

A wider range

The calendar assumes ovulation 14 days before the next period. If your cycles vary, add the shortest and longest of recent ones: ovulation could fall anywhere from the shortest minus 14 to the longest minus 14 days after your last period. With cycles of 26 to 35 days and a period starting 22 August, ovulation could be from 3 to 12 September, so conception from 29 August to 13 September, counting the 5 days sperm can live. With a week or more of variation, the dating scan is the best guide.

Read with care

What this cannot tell you

  • CertaintyIt cannot pin down a single day.
  • PaternityDates cannot settle who the father is; talk to your midwife or GP in confidence.
  • IVFWith IVF, use your clinic's transfer date.

Questions

Common questions

How is conception date calculated?

About 14 days after the first day of your last period, or 266 days before the due date.

How accurate is a conception date?

Usually within a few days either side.

Is conception the same day as sex?

Not always: sperm can live up to 5 days.

Sources

Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.

Support when you need it

Contact your midwife or GP any time you are worried, and call NHS 111 if you cannot reach them. If a date or a result brings up difficult feelings, Tommy's and the Miscarriage Association offer kind, free support.

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