Pregnancy Calculators
Ten free tools for working out dates, timings and measurements — built by a diagnostic ultrasound clinic, with honest limits on what a calculator can tell you.
Every calculator here does arithmetic on dates and measurements. That is genuinely useful for planning — when you are due, when a scan is worth booking, whether a measurement fits your dates. What none of them does is examine you, and the difference matters, so it is set out plainly further down.
Which calculator do you need?
| What you want to know | Calculator |
|---|---|
| When is my baby due? | Due date calculator |
| How many weeks pregnant am I today? | Gestational age calculator |
| When did I conceive? | Conception date calculator |
| When am I likely to ovulate? | Ovulation calculator |
| Which trimester am I in? | Trimester calculator |
| My report gives a CRL in mm — how many weeks is that? | CRL to gestational age |
| When should I book which scan? | Scan timing calculator |
| When would I be eligible for NIPT? | NIPT timing calculator |
| Are my hCG levels rising as expected? | hCG doubling calculator |
| What weight gain is expected for my BMI? | BMI and weight gain |
What a calculator can and cannot do
What they do well. Arithmetic you would otherwise do badly on the back of an envelope — counting weeks, projecting a due date, working out which scan window you are in, translating a millimetre measurement into a gestational age.
What they cannot do. Every date calculator starts from your last menstrual period and assumes you ovulated on day 14 of a 28-day cycle. Plenty of people do not. A longer cycle, an irregular one, or simply later ovulation shifts everything — which is why a calculated date and a scanned date so often disagree, and why the measurement wins. Dating by ultrasound in the first trimester is accurate to within a few days; dating by calendar is an assumption.
They also cannot tell you whether a pregnancy is developing normally, where it is located, or how many there are. Those are examination findings, not calculations. The hCG calculator in particular gives a pattern, not a diagnosis — hCG that rises slowly or plateaus is interpreted alongside a scan and your symptoms, never on its own.
If you have bleeding, severe or one-sided pain, shoulder-tip pain, or feel faint, contact NHS 111, an Early Pregnancy Assessment Unit or A&E — do not work through a calculator first. Later in pregnancy, reduced fetal movements need your maternity unit straight away.
The point at which a calculator stops helping
A calculator is the right tool when you are planning. A scan is the right tool when you need to know something about this pregnancy. In practice that means booking rather than calculating if you are unsure of your dates and it matters, if you want viability confirmed, if you need to know the pregnancy is in the right place, or if a previous loss makes waiting difficult.
Our early pregnancy scan answers viability, location and dates from 6 weeks; the dating scan gives the most accurate due date of the whole pregnancy between 8 and 14 weeks.
Your questions answered
How accurate are pregnancy due date calculators?
Why does my scan date differ from my calculated date?
Can a calculator tell me if my pregnancy is healthy?
My hCG is not doubling — should I be worried?
What is CRL and why does it matter?
Are these calculators free?
Where a calculator points you next
Confirm your dates with a scan
Measured dating by HCPC-registered sonographers, explained on screen as it happens, with a written report usually within two hours for your GP or midwife. No referral needed.
Kensington, London W8 4ED
3 mins from Notting Hill Gate
These calculators are planning tools, not medical advice, and cannot assess an individual pregnancy. Dating by ultrasound in the first trimester is more accurate than dating from a last menstrual period, and is what NHS maternity care uses. Bleeding, severe pain or reduced fetal movements need urgent assessment — contact NHS 111, an Early Pregnancy Assessment Unit, your maternity unit or A&E.
Written and clinically reviewed by the HCPC-registered sonographers who perform this scan at IUS London — a CQC-registered diagnostic ultrasound clinic (Provider ID 1-2775844974). Your own findings are explained to you at the scan and set out in your report.
Author: Yianni Kiromitis, Senior Sonographer, HCPC RA38415
Medically reviewed: 4 August 2026