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Pregnancy Conception Calculator

Estimate conception, ovulation, fertile-window and due dates from an LMP or due date.

By Nitin Raturi · Last reviewed

Your dates

How to use this Pregnancy Conception Calculator

Use one date and your usual cycle length to see calendar-based estimates, not a confirmed conception date.

  1. Choose whether you know your last menstrual period or an estimated due date.
  2. Enter that date and your usual cycle length in days.
  3. Review the estimated ovulation date, fertile window and conception date together.
  4. If your cycles are irregular or clinical dating differs, discuss the dates with a healthcare professional.

Good to know

  • The ⓘ beside each input explains what to enter.
  • The fertile window shown is the six calendar days ending on estimated ovulation.
  • This tool is not suitable for contraception or confirming pregnancy timing.

Starting point

Choose the date you know: the first day of your last menstrual period or an estimated due date.

How to fill it in

  • Choose one starting point. The other date field will appear below.
  • The same cycle length is used to estimate the other dates.

Example

Starting from the default LMP, the estimated due date is 8 Jul 2027.

Keep in mind: A due date entered here is treated as an estimate based on the same cycle-length rule.

First day of last menstrual period

Enter the calendar date bleeding began for your most recent menstrual period.

How to fill it in

  • Use the first day, not the last day, of bleeding.
  • Enter a date from 1 Jan 1900 to 31 Dec 2100.

Example

With the default 28-day cycle and an LMP of 1 Oct 2026, estimated ovulation is 15 Oct 2026.

Keep in mind: If the date is uncertain or cycles are irregular, these calendar estimates may be less useful.

Usual cycle length

This is the number of days from the first day of one period to the first day of the next.

How to fill it in

  • Enter a whole number from 21 to 35 days.
  • Use your usual cycle length, not the number of days you bleed.
  • If it varies, choose a typical length and treat the result as a broad estimate.

Example

A 32-day cycle moves estimated ovulation to 19 Oct 2026.

Keep in mind: The calculation assumes cycle length is known and ovulation occurs about 14 days before the next period.

Estimated dates

Based on a 28-day cycle and an estimated LMP of 1 Oct 2026.

Estimated conception date

15 Oct 2026

Assumed to be the estimated ovulation date.

Estimated ovulation

15 Oct 2026

About 14 days before the next period in the selected cycle.

Estimated fertile window

10 Oct 2026 – 15 Oct 2026

Six calendar days ending on estimated ovulation.

Estimated due date

8 Jul 2027

Based on 280 days from LMP, adjusted for cycle length.

Estimates only. This is not medical advice and must not be used as contraception.

A due date is an estimate; about 5% of babies are born on that exact date. Ultrasound dating and clinical advice may differ.

ChangelogHow we test this calculatorEmbed this calculator

How this calculator works

These are calendar estimates based on a chosen starting date and cycle length, not confirmed biological dates.

  1. Step 1Choose a date1 Oct 2026Start with an LMP or estimated due date.
  2. Step 2Adjust for cycle28 daysA longer or shorter cycle shifts the estimate.
  3. Step 3Estimate ovulation15 Oct 2026About 14 days before the next period.
  4. Step 4Show the window10 Oct 2026–15 Oct 2026Six days ending on estimated ovulation.

The due-date convention starts with 280 days after the first day of the last menstrual period (LMP). For cycles other than 28 days, this calculator adds the difference from 28 days. This is an assumption, not an assessment of your pregnancy.

What this estimate leaves out

  • Individual timingIt does not know when ovulation actually happened or whether a pregnancy occurred.
  • Clinical datingUltrasound findings and clinician-provided dates can differ from a calendar calculation.
  • Irregular cyclesA single usual cycle length cannot represent cycle-to-cycle variation.

Worked example

Example: LMP 1 Oct 2026, cycle 28 days
Estimated ovulation
LMP + cycle length − 14 days
Estimated due date
LMP + 280 days + (cycle length − 28 days)
  1. LMP1 Oct 2026
  2. Estimated ovulation and conceptionLMP + 28 − 14 days15 Oct 2026
  3. Estimated fertile windowfive days before through ovulation10 Oct 2026–15 Oct 2026
  4. Estimated due dateLMP + 280 days8 Jul 2027

Reading your result

  • Estimated conception15 Oct 2026Set to the estimated ovulation date for this simple calendar method.
  • Estimated fertile window10 Oct 2026–15 Oct 2026A six-day calendar window ending on estimated ovulation.

The date shown is not proof of when conception happened. Only about 5% of babies are born on their estimated due date; actual delivery can occur before or after it. A clinician may use other information to establish pregnancy dates.

Once a pregnancy is confirmed, your care team will date it. These tools help with the questions that follow.

The pregnancy weight gain calculator shows the IOM ranges for your pre-pregnancy BMI and week, and the BMI calculator finds that category. To count days between two dates yourself, use the date difference calculator.

Common mistakes

  • Mistake: Entering the last day of bleeding as the LMP.

    Instead: Use the first day bleeding began.

  • Mistake: Entering the number of days bleeding lasts as cycle length.

    Instead: Count from the first day of one period to the first day of the next.

  • Mistake: Treating the fertile window as a reliable way to prevent pregnancy.

    Instead: Do not use this calculator for contraception.

Sources and methodology

The standard due-date arithmetic follows the 280-day LMP convention described by the American College of Obstetricians and Gynecologists (ACOG).

The 14-day pre-period ovulation timing and fertile-window framing are estimates informed by reproductive medicine guidance. See the linked source list below for the ACOG and American Society for Reproductive Medicine (ASRM) documents.

This calculator uses pure Gregorian whole-day arithmetic. Dates are estimates, and the cycle-length adjustment assumes the luteal phase is approximately 14 days. A clinician’s assessment may differ.

A separate script re-derives every test date two ways, with a calendar library and with Julian day numbers. Read how we test our calculators, or browse more health calculators.

Frequently asked questions

How does the calculator estimate conception?

It estimates ovulation from the LMP and usual cycle length, then uses that same date as an approximate conception date. It cannot confirm when conception occurred.

How is ovulation estimated from my last period?

The tool adds the cycle length minus 14 days to the first day of the LMP, using the assumption that ovulation is about 14 days before the next period.

How is the fertile window defined here?

It displays the six calendar days from five days before estimated ovulation through the ovulation date. Individual timing can differ.

Can I use this to avoid pregnancy?

No. Calendar estimates are not reliable contraception. Talk with a qualified healthcare professional about contraception options.

What if my cycles are irregular?

A single cycle length may not reflect irregular or changing cycles, so the estimated dates may be less useful. A healthcare professional can discuss your circumstances.

How accurate is an estimated due date?

It is an estimate rather than a prediction. Only about 5% of babies arrive on the estimated due date; clinical dating may use additional information.

What if my due date came from an ultrasound?

Enter the date you were given to see a calendar-based reverse estimate, but use your clinician’s dating for care because it may differ.

Sources and methodology

The formulas are standard time-value-of-money results. We check every figure against golden values worked out independently, by closed form and by month-by-month simulation; see the methodology.

Written and maintained by Nitin Raturi, a software engineer (about the author). Last reviewed 22 Oct 2026; next review due by . Found an error? Tell us.

Changelog

  • : First release with LMP/due-date modes, cycle-length adjustment, and explicit date-estimate caveats.

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