Due Date Calculator
Estimate a pregnancy due date from last menstrual period, from a conception date, or from an ultrasound that reports gestational age. LMP adds 280 days; conception adds 266 days; ultrasound subtracts the reported age from the scan date and then adds 280.
Due Date Calculator tool
About the Due Date Calculator
Clinicians often keep the ultrasound date when it disagrees with LMP by more than a few days in the first trimester. This page lets you run all three methods; it does not choose among them.
Results are calendar estimates, not a scheduled delivery and not medical advice. Ultrasound weeks must be 0–45 and extra days 0–6. The as-of date is only used to show current gestational age; it does not change the due date. Twins and IVF transfers use clinical protocols this page does not implement.
How the Due Date Calculator works
Ultrasound dating reconstructs an equivalent LMP by walking backward from the scan, then applies Naegele’s 280-day rule.
From LMP: due = LMP + 280
From conception: due = conception + 266
From ultrasound: LMP = scan − (weeks × 7 + days); due = LMP + 280
Example
A scan on 16 April 2026 at 15 weeks 0 days implies LMP 1 January 2026 and due date 8 October 2026.
How to Use the Due Date Calculator
- Choose LMP, conception, or ultrasound.
- Enter the corresponding date.
- For ultrasound, enter whole weeks 0–45 and extra days 0–6.
- Set the as-of date to see current gestational age.
- Read the due date, equivalent LMP, and conception estimate.
Due Date Calculator Examples
LMP
LMP 1 January 2026 → due 8 October 2026.
Conception
Conception 15 January 2026 → due 8 October 2026.
Ultrasound
15 weeks 0 days on 16 April 2026 → the same 8 October 2026 due date.
How to Calculate a Due Date
Pick the data you have. LMP + 280 is the default classroom method. Conception + 266 matches that if conception was LMP + 14. Ultrasound walks back to an LMP equivalent.
The same pregnancy, three entries
All three rows describe one timeline.
| Method | Input | Due date |
|---|---|---|
| LMP | 2026-01-01 | 2026-10-08 |
| Conception | 2026-01-15 | 2026-10-08 |
| Ultrasound | 2026-04-16 at 15w0d | 2026-10-08 |
Naegele on a calendar
A paper shortcut is: add one year to the LMP, subtract three months, add seven days. On 1 January 2026 that is 8 October 2026 — the same as +280 days in a non-leap span that includes 28 February 2026. The three modes on this page can disagree when ovulation was not day 14 or when the ultrasound age does not match LMP. The tool will not reconcile them; it only applies the rule you picked. Twins and IVF transfers use clinical protocols that are not implemented here. Results remain calendar estimates, not a scheduled delivery.
Frequently Asked Questions
Which method should I trust?
Follow the clinician who is dating the pregnancy. First-trimester ultrasound often overrides LMP when they disagree.
What extra days mean
Gestational age is weeks + days, such as 8 weeks 3 days. Days must be 0–6.
Is 40 weeks exact?
280 days is 40 weeks. Many births occur from 37 to 42 weeks.
Twins or IVF?
Those pregnancies are dated with clinical protocols this tool does not implement.
Medical advice?
No.