Most due date tools only work in one direction: you give them a last period date and they hand you a due date. This reverse due date calculator flips that around. Enter any one of three dates, your due date, your conception date, or the date you had intercourse, and it works backward (or forward) to fill in the other two. It is built for anyone who already knows one date for certain and wants to reconstruct the rest of the timeline.

How to use this reverse due date calculator

1. Look at the three date fields: Due Date, Conception Date, and Intercourse Date. You only need to fill in one of them, not all three.
2. Type or select the one date you actually know. If it’s your due date (from a doctor or an earlier due date calculator), enter it in the Due Date field. If you know roughly when conception happened, use the Conception Date field. If you’re working from a specific intercourse date, use that field instead.
3. Check the Pregnancy Length setting. It defaults to 40 weeks (280 days), the standard full-term pregnancy length, but you can change it if you want to model a shorter or longer cycle.
4. Pick the unit for the pregnancy length from the dropdown: days, weeks, months, years, or a combined weeks/days format. Adjust the number to match whatever length you want to calculate against.
5. Click Calculate Other Dates.
6. Review the three results: Due Date, Conception Date, and an Intercourse Window (shown as a date range rather than a single day, since ovulation timing and sperm viability naturally vary).
7. Use Save as Image or Download PDF if you want to keep a copy of the results, or Clear All Fields to start over with a new date.

What this reverse due date calculator does

The calculator takes whichever single date you provide and applies a fixed pregnancy length (280 days by default) to work out the other two dates on the timeline. Because a due date, a conception date, and an intercourse date are all separated by known, fairly consistent intervals, knowing just one of them is enough to estimate the rest. This is useful if you’re trying to figure out roughly when conception happened based on a due date you were already given, or if you want to check what due date a specific intercourse date would point to.

The dating convention it uses

The “conception date” this reverse due date calculator produces follows the common due-date convention: it’s calculated as the pregnancy length minus a set number of days back from the due date, similar to how LMP-based (last menstrual period) dating works in standard due date calculators. It is not the same as a strict, medically confirmed fertilization date, which usually can’t be pinned to a single day without clinical monitoring.

Why intercourse is shown as a window, not a single date

The calculator deliberately gives you an intercourse window rather than one exact day. Ovulation timing shifts from cycle to cycle, and sperm can remain viable inside the body for several days before fertilization actually occurs. Because of that variability, pinning conception to a single 24-hour period would be misleading. A range is a more honest reflection of what’s actually knowable from date math alone.

Adjusting pregnancy length

Not every pregnancy runs exactly 280 days, and this reverse due date calculator lets you account for that. If you’re working from a different assumed length, whether for a preterm scenario, a post-term estimate, or just to compare against a doctor’s figure, change the number and unit in the Pregnancy Length field and recalculate. The table below shows how the same due date shifts under different assumed lengths.

These outputs are date estimates built on a fixed-length model, not a clinical assessment. Actual conception timing, gestational age, and due dates can only be confirmed by a doctor or midwife using ultrasound dating and other clinical measurements. Use this reverse due date calculator as a planning reference, not as medical advice.

Frequently asked questions