Pregnancy Due Date Calculator

How to use this calculator

1. Enter first day of last period — The calculator loads with a worked example already filled in, so you can see what a realistic set of figures looks like before replacing them with your own.
2. Fill in the remaining fields — Every field has a sensible default. Change the ones that apply to you and leave the rest.
3. Read the result — The answer updates as you type — there is nothing to submit. The rows beneath the headline show how it breaks down.
4. Expand the schedule — Where a year-by-year table is offered, open it. The breakdown over time usually shows something the single headline figure does not.
5. Share or print — Use the share button to copy a link with your figures already filled in, or print the result — the print stylesheet strips the navigation and adverts.

What this calculator works out

To use the Pregnancy Due Date Calculator, enter your first day of last period and average cycle length. The result — estimated due date — updates as you type, with no button to press and nothing to submit. The calculation runs entirely in your browser, so the figures you enter are never sent anywhere.

How this is calculated

The pregnancy due date calculator applies its formula to the values you enter and returns the result immediately.

A worked example

Take the values this page loads with:

• First day of last period: 2026-07-26
• Average cycle length: 28

On those figures the estimated due date is Sunday, 2 May 2027. Broken down:

• Gestational age today: 8 weeks, 0 days
• Trimester: First trimester
• Estimated conception: 9 August 2026
• Days until due date: 223
• End of first trimester: 25 October 2026

Change any field above and every figure here recalculates — these numbers are produced by the same formula the calculator runs, so they cannot drift apart from it.

What the figures mean

The headline figure answers the immediate question. These are the ones worth paying attention to:

• Gestational age today — the figure worth watching when you change your inputs.

Where a schedule is shown, expand it: the year-by-year breakdown usually reveals something the single headline number hides.

What this calculator assumes

Uses Naegele's rule — 280 days from the first day of the last period, adjusted for cycle length. Fewer than 5% of babies arrive on the estimated date; a dating scan is more accurate.

Why it runs in your browser

Every calculation on this page is arithmetic, and arithmetic does not need a server. Running it locally means the result is instant, it keeps working if your connection drops, and — the part that matters — your salary, your debts and your medical measurements are never transmitted to us. There is no figure on our side to store, log or lose.

Where this calculator stops

Every figure here is an estimate derived from population averages. None of these equations measures an individual, and several of them were built for a purpose other than the one they are now used for — the ideal-weight formulae, for instance, were developed for drug dosing rather than for setting personal goals.

Use them to notice a trend over time, not to make a decision about your health. If a result concerns you, or if you have a condition that changes how these figures should be read, speak to a qualified healthcare professional rather than acting on a web page.

Keeping the figures current

The formula behind this page does not change, but the assumptions you should bring to it do — interest rates, inflation and market returns all move. Revisit the inputs rather than trusting a figure you calculated a year ago.