Average three gestational sac diameters and estimate gestational age from the mean sac diameter.
Result
How to use this tool
Use the three orthogonal sac measurements in mm from the ultrasound.
The three values are averaged.
Add 30 to the mean in mm to get days of gestation.
Once an embryo is visible, crown-rump length dating replaces this estimate.
Formula & worked example
Diameters of 20, 18 and 16 mm average to 18 mm, giving an estimated gestational age of 18 + 30 = 48 days, or 6 weeks 6 days.
Limitations — read this before you rely on it
Early-pregnancy approximation only. It does not date a pregnancy definitively and never replaces ultrasound assessment. This tool is for education only and is not medical advice. Talk to a clinician about any result that worries you, and never change treatment because of a calculator result.
Frequently asked questions
The average of the three orthogonal diameters of the gestational sac on early ultrasound.
By the rule that gestational age in days is roughly the mean sac diameter in mm plus 30.
In very early pregnancy, before a crown-rump length can be measured.
It is an approximation with several days of variation; later measurements are more accurate.
Management follows clinical guidance on sac size and follow-up scans, not this estimate alone.