How to use the Due Date Calculator
- Choose what you know: the first day of your last period, the conception date or an IVF transfer.
- Enter the date (and your usual cycle length for the period method).
- See your due date, how many weeks along you are and the key dates.
How the due date is estimated
Pregnancy is counted from the first day of the last menstrual period (LMP), about two weeks before conception. The standard estimate (Naegele’s rule) is LMP + 280 days (40 weeks), adjusted if your cycle is longer or shorter than 28 days. From a known conception date it’s + 266 days; for IVF, the transfer date + 261 days (day 5 embryo) or + 263 days (day 3).
Trimesters
- First trimester: weeks 1–13
- Second trimester: weeks 14–27
- Third trimester: week 28 until birth
Only about 4% of babies are born on the due date; most arrive within two weeks either side. Count days between other dates with the Date Difference Calculator.
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Tools that pair well with the Due Date Calculator
- Age Calculator — Enter a date of birth to get the exact age in years, months and days — plus total days and weeks lived, the weekday you were born on, a countdown to the next birthday and your star sign.
- Calorie Calculator — Estimate how many calories you burn per day — your BMR at rest and TDEE with activity — and the daily calories to lose, maintain or gain weight.
- BMI Calculator — Calculate your body mass index in metric or imperial units, see the WHO category on a scale and the healthy weight range for your height.
- Countdown Timer — Set a countdown in seconds — type hours, minutes and seconds or tap a preset — and get an alarm sound and an optional notification when it reaches zero.
Frequently asked questions
How is my due date calculated?
From the first day of your last period plus 280 days (40 weeks), adjusted for your cycle length — or from conception plus 266 days, or from an IVF transfer date.
How many weeks pregnant am I?
Weeks are counted from the first day of your last period. The calculator shows weeks and days, for example 12 weeks 3 days.
Is the due date exact?
No. It’s an estimate; most babies are born between 37 and 42 weeks. An early ultrasound can give a more precise date.
Can I install Due Date Calculator as a web app and use it offline?
Yes. Due Date Calculator is an installable web app (PWA) — installed from your browser, not downloaded from an app store. Click Install app at the top of this page in Chrome or Edge (Windows, macOS, Linux, ChromeOS) or on Android, use Share → Add to Home Screen on iPhone and iPad, or File → Add to Dock in Safari 17+ on a Mac. It then opens in its own window with its own icon, and because it runs entirely in your browser it keeps working offline. Firefox on desktop can’t install web apps. How installing works.