Reverse Due Date Calculator · Due, Conception & Intercourse Dates

Reverse Due Date Calculator

Enter any of the three dates below, the other two update instantly, along with total pregnancy length and every key milestone

Your dates — edit any field
How the dates relate: Due date = LMP + 280 days. Conception = due date − (294 − cycle length). Intercourse ≈ conception − 2 days. Pregnancy length is 280 days (40 weeks 0 days) from LMP, or 294 − cycle length days from conception.
Your reverse due date results
Pregnancy length
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Due — Conception — Intercourse —
📅—
Last menstrual period
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Conception
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Intercourse
—
Countdown
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Where you are on the 0–42 week scale
1st trimester
0 – 14 wks 2nd trimester
14 – 28 wks 3rd trimester
28 – 42 wks
Pregnancy milestone timeline
MilestoneGestational ageDate
Reverse calculation reference
What you're findingFormulaValue
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Faiq Ur Rahman

Founder & CEO, Toolraxy

Faiq Ur Rahman is a web designer, digital product developer, and founder of Toolraxy, a growing platform of web-based calculators and utility tools. He specializes in building structured, user-friendly tools focused on health, finance, productivity, and everyday problem-solving.

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Introduction

Most pregnancy date calculators go one way. You enter an LMP or a scan measurement and get a due date. This one runs the math in reverse too. Enter the due date your provider gave you, and it back-calculates a conception date, an intercourse window, and an LMP. Enter a conception date, and the due date resolves from that. Type in an intercourse date, and the tool fills in the rest.

That flexibility matters when you already know one piece of the puzzle. A clinic hands you a due date at your first appointment. You remember the weekend you conceived. You’re working out whether a specific encounter could have led to the pregnancy. Any of those starting points produces a complete set of dates in one step.

The calculator also shows pregnancy length from two anchors, tracks milestones across all 40 weeks, and counts down the days remaining. Nothing you enter leaves your browser.

 

How to Use the Reverse Due Date Calculator

  1. Enter any one of three dates: due date, conception date, or intercourse date. The tool updates the others.

  2. Set your average cycle length. The range is 20 to 45 days, default 28.

  3. Adjust the “calculate as of” date if you want results for a future or past day. Today is the default.

  4. Read the hero panel. Pregnancy length appears from both LMP and conception.

  5. Check the context row for the resolved due date, conception date, and intercourse date.

  6. Scan the four tiles for LMP, conception, intercourse, and the countdown.

  7. Follow the gauge to see where you sit across the 42-week timeline.

  8. Scroll the milestone table for dated events from LMP through post-term.

 

How the Reverse Due Date Calculator Formula Works

Every date in this tool derives from the same fixed anchor: the 280-day interval from LMP to due date.

Formula: LMP = due date − 280 days

That’s the only fixed constant. Everything else adjusts based on your cycle length.

Formula: Conception = due date − (294 − cycle length)

For a 28-day cycle, that’s 266 days before the due date. For a 35-day cycle, it’s 259 days before. The 294 figure exists because the standard 40-week count starts 14 days before conception, and cycle length shifts where that 14-day offset lands.

Formula: Intercourse date ≈ conception − 2 days

The two-day offset reflects the typical relationship between intercourse and fertilization, since sperm survive in the reproductive tract for up to five days but conception usually happens on the day of ovulation or within 24 hours.

Formula: Pregnancy length (from conception) = 294 − cycle length

A 28-day cycle gives 266 days. A 35-day cycle gives 259. That’s why conception-to-birth length varies between women while LMP-to-birth stays fixed at 280 days.

Cycle length is clamped between 20 and 45 days. The tool flags any input outside that range rather than producing a nonsensical result.

 

Worked Example

Say your provider gave you a due date of November 20, 2026, and your cycles average 30 days. You want to know when you conceived and roughly which intercourse window led to the pregnancy.

  • Due date: November 20, 2026

  • Cycle length: 30 days

  • Conception offset = 294 − 30 = 264 days

  • Conception date = November 20 − 264 days = March 1, 2026

  • Intercourse date ≈ March 1 − 2 days = February 27, 2026

  • LMP = November 20 − 280 days = February 13, 2026

  • Pregnancy length from LMP: 280 days (40 weeks 0 days)

  • Pregnancy length from conception: 264 days (37 weeks 5 days)

The calculator would show “40 weeks 0 days” in the hero, with the subtext confirming 280 days from LMP and 264 from conception. The tiles would show February 13, 2026 as your LMP, March 1 as conception, and February 27 as the estimated intercourse date. The countdown would update based on today’s date.

Try the same due date with a 35-day cycle. Conception shifts to February 24, and the intercourse window to February 22. That’s a full week earlier than with the 30-day cycle, which is why entering your real average matters.

Frequently Asked Questions

How do you calculate conception from a due date?

Subtract (294 − cycle length) days from the due date. For a 28-day cycle, that’s 266 days. For a 35-day cycle, it’s 259 days. The result is an estimate, since ovulation timing varies.

 

Can you reverse-calculate conception from a due date?

Yes, but only as an estimate. The calculation assumes ovulation on the standard cycle day for your cycle length. Tracking ovulation with an OPK or BBT chart gives a more accurate conception date.

 

How accurate is the intercourse date?

It’s the least precise of the three dates. It assumes intercourse happened two days before conception, which matches the population average. Intercourse up to five days before ovulation can also lead to pregnancy.

 

What is the difference between conception and intercourse date?

Conception is the moment of fertilization. Intercourse is when sex happened. Sperm can survive up to five days, so intercourse often occurs several days before conception.

 

How does cycle length affect the reverse calculation?

A longer cycle pushes conception later relative to the due date, while the 40-week LMP count stays fixed. A 35-day cycle puts conception about a week later than a 28-day cycle.

 

Why is pregnancy 280 days from LMP but 266 from conception?

The LMP-to-due-date count includes about two weeks before ovulation. Clinical dating starts from the LMP because it’s observable, while ovulation happens silently. Conception-to-birth length is 266 days in a 28-day cycle.

 

What if I don’t know my LMP?

You don’t need it. The calculator derives the LMP from whichever date you enter. The equivalent LMP is what you’d report to a clinic that asks for it, and it appears in the tile row.

 

Can a due date and conception date disagree?

Yes, often by a few days. Ovulation timing varies between cycles, and the intercourse-to-conception window spans several days. An early ultrasound is more reliable than a reverse calculation for dating a pregnancy.

 

Does the calculator work for IVF pregnancies?

Not accurately. IVF uses the embryo’s age at transfer rather than a standard ovulation offset. Use an IVF-specific due date calculator or ask your clinic for the equivalent LMP they’re using.

 

How is the “as of” date used?

It sets the reference point for gestational age and countdown. Today is the default. Change it to see how far along you were on a past date or will be on a future one.

 

What does the pregnancy length number mean?

It’s the total expected duration from either LMP or conception to the estimated due date. The LMP figure is always 280 days. The conception figure is 294 − cycle length.

 

When should I use this calculator instead of a standard due date calculator?

Use it when you already have a due date, conception date, or intercourse date and want the others. A standard calculator works forward from an LMP or scan.

Disclaimer

This reverse due date calculator is for educational purposes only and is not medical advice. It applies standard dating formulas to the dates you provide, including an approximate intercourse date that assumes a two-day gap before conception. Real conception timing varies based on ovulation, sperm survival, and factors the calculator cannot see. Always rely on your healthcare provider’s dating for clinical decisions about screening windows, induction timing, or any other clinical matter.

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