How to use the OB wheel calculator: every mode explained

The OB wheel calculator has six modes, and picking the right one takes about two seconds once you know what each is for. This guide walks through every mode: when to use it, exactly what to enter, and how to read what comes back. If you only remember one thing: you enter one date you know, and the calculator works out all the others.

The two shared settings (used by every mode)

Before the mode-specific fields, two inputs sit at the top and apply throughout:

• “As of” date. This is the day you want the gestational age calculated for. It defaults to today, which is what you want almost always. Change it only if you’re checking how far along a pregnancy was (or will be) on some other date.
• Cycle length. The average length of your menstrual cycle, defaulting to 28 days. This matters for last-period dating because it sets when you ovulate — see menstrual cycle length and dating. If you don’t know it, leave it at 28.

Mode 1 — Last period (the default)

Use it when: you know the first day of your last menstrual period (LMP). This is the most common starting point and the clinical default.

What to enter: the first day of your last period. That’s the first day of bleeding, not the last.

What you get: your estimated due date (LMP + 280 days, adjusted for cycle length), how far along you are today, your trimester, and every milestone date. The method behind it is Naegele’s rule.

Example: last period on 1 January, 28-day cycle → due date 8 October. Change the cycle to 32 days and the due date shifts to 12 October.

Mode 2 — Known due date

Use it when: a clinician has already given you a due date (say, from an early scan) and you want everything else — how far along you are today, your trimester, upcoming milestones — without re-deriving the date.

What to enter: the estimated due date you were given.

What you get: the calculator works backwards to an equivalent last period (due date − 280 days) and then fills in gestational age and milestones. It’s the fastest way to turn “your due date is X” into a full picture.

Mode 3 — Ultrasound

Use it when: you have a dating scan that reported a gestational age on a specific date — for example, “9 weeks 2 days” on the scan date. Ultrasound is the most accurate dating method, especially in the first trimester; see crown-rump length dating.

What to enter: the date of the scan, and the gestational age the scan reported in weeks and days.

What you get: the calculator back-calculates your dating from the scan and carries it forward to today, giving an updated due date and current gestational age.

Mode 4 — Conception / ovulation

Use it when: you know the day you conceived or ovulated — for instance, if you tracked ovulation, or had timed insemination (this is also the right mode for IUI dating).

What to enter: the conception or ovulation date.

What you get: a due date of conception + 266 days, plus gestational age counted from an equivalent last period 14 days earlier. More on the reasoning in conception date and due date.

Mode 5 — IVF

Use it when: you conceived through IVF and know your transfer or retrieval date. This is the most precise dating of all, because the timing is known exactly.

What to enter: whether it was an embryo transfer or egg retrieval, the date, and — for a transfer — the embryo’s age (day-3 or day-5).

What you get: a due date matched to your exact IVF timing (for example, day-5 transfer + 261 days). The full breakdown is in IVF due date calculation.

Mode 6 — Reverse look-up (“when will I be X weeks?”)

Use it when: you already have a pregnancy dated and want the calendar date for a specific milestone — say, when you’ll reach 36 weeks, or the start of the third trimester.

What to enter: after running any mode above, type a target gestational age (weeks and days) into the look-up.

What you get: the exact date the pregnancy reaches that point. Handy for planning appointments, leave, or travel around key weeks.

Reading your results

Whatever mode you use, the results panel shows the same things: the estimated due date, your gestational age today in weeks and days, your trimester, days pregnant and days remaining, and a milestone table with the calendar date of each key point (second trimester, viability, the term windows). If two methods disagree — say your last-period date and a scan — the scan usually wins; that’s covered in how accurate is your due date.

Sharing and re-checking

The copy link button builds a URL containing your inputs so you can save or share a result — nothing is sent to us; the values simply travel in the link. Copy result puts the text on your clipboard, and print gives a clean printout. To start over, use reset. And remember: every calculation runs in your browser, so none of your dates leave your device.

Which mode should I pick? (quick answer)

Use ultrasound if you’ve had a dating scan — it’s the most accurate. Otherwise use last period if you know your LMP, IVF or conception if you know those exact dates, and known due date if a clinician already handed you a date. Then use the reverse look-up any time you want the date of a particular week. When in doubt, last-period mode with an accurate cycle length is a solid default.