Due Date Calculator

Based on Naegele's rule: 280 days from the first day of your last menstrual period.

First day of your last period

Result

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estimated due date

About this calculator

Based on Naegele's rule: 280 days from the first day of your last menstrual period. To use it, fill in first day of your last period. The figure in the result box changes the instant you edit any of those values, so you can try several variations in a few seconds without reloading anything.

Due Date Calculator sits in our pregnancy collection, and it exists because this particular question is genuinely awkward to answer by hand. Pregnancy is tracked in weeks, but life is organised in dates. Underneath the headline number you also get a breakdown that restates the same answer in other units, so you can copy whichever version fits the form, message or spreadsheet you are filling in.

A worked example

To show exactly what the calculator does with your numbers, here is the same tool run against a fixed reference date of Monday, March 16, 2026, using the values listed below. Open the calculator above with today’s date and the method is identical — only the starting moment differs.

First day of your last period2026-03-16

ResultMonday, December 21, 2026 estimated due date

Days until due date280

Second trimester startsMonday, June 22, 2026

Third trimester startsMonday, September 28, 2026

Weeks remaining40

You're around 0 weeks and 0 days pregnant. Change any input and every figure in the table above is recalculated from scratch rather than scaled from the previous answer.

When people use this

Most visitors arrive with a practical reason rather than idle curiosity. These are the situations this page comes up in most often:

Booking scans, appointments and antenatal classes around the right week.
Timing the start of parental leave with an employer.
Keeping family updated with the current week and trimester rather than a vague estimate.

What the due date calculator tool does

Based on Naegele's rule: 280 days from the first day of your last menstrual period. The answer appears the moment the page loads, and it recalculates as soon as you change an input — there is no submit button, no sign-up and no waiting. Everything runs locally in your browser, which is why the result is instant and why none of the values you type are ever transmitted.

Pregnancy is tracked in weeks, but life is organised in dates. Converting between the two — due date, current week, trimester boundaries, days remaining — is something expectant parents do constantly when booking appointments, planning leave and preparing at home.

How the calculation works

Standard dating counts forty weeks from the first day of the last menstrual period, which is the method used here. From that anchor the calculator derives the estimated due date, the current gestational week and the time remaining.

An estimated due date is exactly that: only a small minority of babies arrive on it, and a dating scan may shift the estimate. This tool is for information and planning and is not a substitute for advice from a midwife or doctor.

Getting the most out of it

A few practical notes make this pregnancy tool more useful in everyday situations:

If a scan has given you a revised due date, work back forty weeks and use that date as your input.
Trimester boundaries are conventionally set at weeks 13 and 27.
The related pregnancy calculators below cover due dates, weeks and countdowns from the same information.

Common questions people bring to this page

Visitors usually arrive here with a very specific question — the exact figure shown in the result box. Once they have it, the follow-up is almost always about context: what the date falls on, how the number looks in a different unit, or how it compares with another period. The breakdown under the result and the related calculators at the bottom of the page are there for precisely those follow-ups, so you rarely need to open a second tool.

If a figure looks surprising, it is usually down to one of three things: which endpoint is included in the count, whether weekends and holidays are excluded, or the time zone your device is set to. Each of those is explained above, and adjusting the input accordingly will reconcile the number with whatever you are comparing it against.

Accuracy, privacy and cost

TimeQuick calculators are free, unlimited and require no account. Because the arithmetic happens in your browser rather than on a server, results are available offline once the page has loaded, and the dates you enter — including birth dates and pregnancy dates — stay on your device.

Every page is mobile-first: large type, tappable inputs and a result box that stays readable on a small screen. If you spot a figure you believe is wrong, check the assumptions listed above first, since most differences come from a different counting convention rather than a calculation error.

Frequently asked questions

How accurate is a due date?+

It's an estimate — only about 4% of babies arrive on the exact due date, but most arrive within two weeks either side.

What if my cycle isn't 28 days?+

Naegele's rule assumes a 28-day cycle. A longer or shorter cycle shifts the estimate by the difference.

Should I use this instead of a scan?+

No. An early ultrasound is the most accurate dating method — treat this as a quick reference.

Is the due date calculator free to use?+

Yes. Every TimeQuick calculator is completely free, has no usage limit and does not require an account or email address.

Does this work on a phone?+

Yes. The page is built mobile-first, so inputs are large enough to tap and the result stays readable on any screen size. It also works offline once loaded.

Is my data stored anywhere?+

No. The calculation runs entirely in your browser using your device clock, so nothing you enter is sent to or stored on a server.