Conception Calculator
Last updated: June 2026 · Free · No sign-up required
Enter values above and click Calculate to see your result instantly.
Quick reference
Ways to estimate conception:
| You know | Estimate |
|---|---|
| Last menstrual period | ~14 days after LMP start |
| Due date | Due date minus 266 days |
| Ultrasound (gestational age) | Back-calculate from weeks |
monitor_weight A starting point, not a diagnosis
This calculator gives you the same number a clinician would compute by hand from your inputs. It is informational, never a substitute for a doctor. If a result surprises you, write it down and discuss it at your next appointment — that conversation is the goal, not a verdict from a webpage.
science Built from peer-reviewed formulas
Every health calculator on CalcaTools uses an equation from the WHO, NIH, ACOG, CDC, or a published peer-reviewed paper. The exact citation is in the "Formula & methodology" section. We re-check sources quarterly and update the page when guidance changes — the "Last reviewed" date is at the top.
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Health calculations only help if you can actually use them. We use plain-English labels (no jargon), accept both metric and US units, and show what a "normal" range looks like alongside your result — so you understand what the number means, not just what it is.
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Your weight, height, age, pregnancy details, and any other health input never leave your browser. Calculations run client-side; nothing is logged or stored on our side. We don't want your medical data — we just want the formula to work for you.
Interpretation guide
Pregnancy timing terms:
| Term | Meaning |
|---|---|
| Gestational age | Counted from LMP, not conception |
| Conception | Usually around ovulation |
| Full term | ~40 weeks from LMP |
| Conception to due date | About 266 days |
Formula & methodology
Formula: Conception date approx = LMP first day + 14 days = due date - 266 days
Conception usually occurs around ovulation, about 14 days after the first day of your last menstrual period for a typical cycle. The calculator estimates the conception date from your LMP, your due date (subtracting 266 days), or an ultrasound's gestational age. Note that pregnancy 'weeks' are counted from the LMP, roughly two weeks before conception, which is why gestational age exceeds the time since conception.