Conception Calculator
Use this conception calculator to estimate when conception most likely happened from your due date, last period, or an ultrasound scan.
Updated October 22, 2026
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Your Conception Date Is Calculated
Conception, also called fertilization, is when a sperm joins an egg. It happens on or very close to the day of ovulation, because the egg appears to survive only a short time after release². Doctors count pregnancy from the first day of the last period, about two weeks before conception, so this calculator works backward from those standard dates¹.
What Your Conception Date Estimate Measures
The result is the most likely date of fertilization, with a range of days around it. It also shows the days of sex that could have led to conception on that date, because sperm can survive in the body for up to five days². It cannot confirm a pregnancy or show which day sex led to it.
The Formula
Conception date = due date minus 266 days
Conception date = first day of last period + (cycle length minus 14) days
Conception date = scan date minus (gestational age at scan minus 14 days)
Possible days of sex = 5 days before the conception date through the conception date
By convention, the due date is 280 days after the first day of the last period, which assumes a 28-day cycle with ovulation 14 days after the period starts¹. Taking away those 14 days leaves 266 days (38 weeks) from conception to the due date, the same total built into ACOG's IVF dating rule¹.
How This Calculator Computes Your Result
Choose one option. Enter your due date; or the first day of your last period and your average cycle length (21 to 45 days, default 28); or a scan date and the gestational age on the report (6 weeks 0 days to 41 weeks 6 days). Cycle length is only used with the last period option, because scan ages already follow the standard 28-day scale. This option follows the due date convention of 14 days from period start to conception, so it lands one day after the ovulation day (cycle day = cycle length minus 14) shown by our Ovulation Calculator. Results are whole calendar days. To work forward from a known conception date, use the Due Date Calculator.
What Your Result Means
Result | What it shows |
Most likely conception date | The single best estimate of the day of fertilization |
Likely range | The days either side where conception most likely fell |
Days of sex for that date | The six days ending on the conception date |
All possible days of sex | From 5 days before the range starts to the day it ends |
Equivalent last period | 14 days before conception, where pregnancy weeks start |
Due date | Conception date plus 266 days |
Pregnancy week today | Weeks since the equivalent last period, and since conception |
Pregnancy weeks run about two weeks ahead of the time since conception, so 8 weeks pregnant means about 6 weeks since conception¹. If you conceived through IVF, your clinic's dates are more exact than any of these options¹.
How Wide the Range Is and Why
No dating method can pin conception to one day. ACOG puts the accuracy of a first-trimester scan at about 5 to 7 days, and a third-trimester scan at only 21 to 30 days¹. For scans, this calculator uses ACOG's redating limits, the gap at which doctors change a due date¹.
How the date was set | Range shown |
Scan up to 8 weeks 6 days | 5 days either side |
Scan at 9 weeks 0 days to 15 weeks 6 days | 7 days either side |
Scan at 16 weeks 0 days to 21 weeks 6 days | 10 days either side |
Scan at 22 weeks 0 days to 27 weeks 6 days | 14 days either side |
Scan at 28 weeks or later | 21 days either side |
Last period, regular cycles | 7 days either side |
Due date | 7 days either side |
The 7-day range for a last period date reflects real cycles: in 28-day cycles, ovulation fell anywhere from cycle day 10 to day 22³. The due date option assumes your date came from an early scan or a reliable last period. If a later scan set it, enter that scan instead. The range never runs past today.
Which Days of Sex Could Have Led to Conception
In a 1995 study, 221 women collected daily urine samples so researchers could estimate the day of ovulation from hormone levels. Every pregnancy followed sex in the six days ending on ovulation day². The chances below are for sex on that one day only, and this calculator treats ovulation day as conception day.
Day of sex | Estimated chance of conception |
5 days before ovulation | About 10% |
4 days before | About 16% |
3 days before | About 14% |
2 days before | About 27% |
1 day before | About 31% |
Ovulation day | About 33% |
Day after ovulation | No pregnancies seen |
Why Conception Is Not Always Two Weeks After Your Period
The two-week rule is an average, and ACOG notes it does not account for irregular cycles or variable ovulation timing¹. In one study, ovulation came 14 days before the next period in only 10% of 28-day cycles, and across all cycles it ranged from day 8 to day 60³. In over 600,000 cycles, 28-day cycles averaged 15.4 days from period start to ovulation, and the authors concluded that ovulation does not reliably happen on day 14⁴.
Chance of Conception by Day of Sex
The estimated chance that sex on one day led to conception, counted from the day of ovulation.
Show data table
| Day of sex | Chance of conception (%) |
|---|---|
| 5 days before ovulation | 10 |
| 4 days before | 16 |
| 3 days before | 14 |
| 2 days before | 27 |
| 1 day before | 31 |
| Ovulation day | 33 |
Day after ovulation: no pregnancies seen.
How Wide the Conception Date Range Is
Days either side of the most likely conception date that the calculator shows, by how your date was set.
- Likely range
- Most likely date
Show data table
| How the date was set | Likely range | Most likely date |
|---|---|---|
| Scan by 8 weeks | Scan up to 8 weeks 6 days: up to 5 days before, Scan up to 8 weeks 6 days: up to 5 days after | Most likely conception date |
| Scan at 9 to 15 weeks | Scan at 9 weeks 0 days to 15 weeks 6 days: up to 7 days before, Scan at 9 weeks 0 days to 15 weeks 6 days: up to 7 days after | Most likely conception date |
| Scan at 16 to 21 weeks | Scan at 16 weeks 0 days to 21 weeks 6 days: up to 10 days before, Scan at 16 weeks 0 days to 21 weeks 6 days: up to 10 days after | Most likely conception date |
| Scan at 22 to 27 weeks | Scan at 22 weeks 0 days to 27 weeks 6 days: up to 14 days before, Scan at 22 weeks 0 days to 27 weeks 6 days: up to 14 days after | Most likely conception date |
| Scan at 28+ weeks | Scan at 28 weeks or later: up to 21 days before, Scan at 28 weeks or later: up to 21 days after | Most likely conception date |
| Last period | Last period, regular cycles: up to 7 days before, Last period, regular cycles: up to 7 days after | Most likely conception date |
| Due date | Due date: up to 7 days before, Due date: up to 7 days after | Most likely conception date |
A Note on Accuracy
This calculator gives a likely range, not a proven date. It uses standard dating rules and population averages, so it cannot show when sex happened or who the biological father is. Only about half of women recall their last period date accurately, and irregular cycles widen the true range¹. ACOG calls a first-trimester scan the most accurate way to date a pregnancy, so if you have more than one scan, the earliest one is the best input¹. The due date in your prenatal records takes priority over this estimate.
Frequently Asked Questions
How do I calculate my conception date from my due date?
Subtract 266 days (38 weeks) from your due date. For example, a due date of May 20, 2027 points to conception around August 27, 2026, give or take about a week¹.
How many days after your period do you conceive?
In a 28-day cycle, conception usually happens about 14 days after your period starts, and later in longer cycles¹. Real cycles vary, so ovulation can come several days earlier or later³.
How accurate is a conception calculator?
It gives a range, not an exact day. A first-trimester scan is accurate to about 5 to 7 days either side and later scans are less precise¹, while a last period date with regular cycles gives about a week either side³.
Can a conception calculator tell who the father is?
No. Sperm can survive up to five days and every estimate carries a range, so sex on different days can fit the same pregnancy². Only a DNA paternity test can establish paternity, and it can be done during pregnancy or after birth⁵.
Sources
Methods for Estimating the Due Date, ACOG, 2017
Timing of Sexual Intercourse in Relation to Ovulation, Wilcox et al., 1995
The Timing of the Fertile Window in the Menstrual Cycle, Wilcox et al., 2000
Real-World Menstrual Cycle Characteristics of More Than 600,000 Menstrual Cycles, Bull et al., 2019
DNA Paternity Test, Cleveland Clinic, 2024