How this calculator works
Mean sac diameter is the arithmetic mean of three internal, perpendicular ultrasound dimensions. Do not include the surrounding decidual tissue in those dimensions. Reverse mode is a consistency check of that arithmetic only. ACOG advises against using mean sac diameter to estimate the due date, so this page does not turn millimetres into gestational age. Sac size alone also cannot establish pregnancy viability. The result is an estimate for information only and is not medical advice.
Formula
MSD = (internal length + internal width + internal height) / 3. Missing length = 3 × MSD - width - height.
Worked example
Dimensions 15, 12 and 9 mm give a mean of 12 mm. Dimensions 20, 25 and 30 mm give 25 mm.
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Frequently asked questions
Why is no gestational age shown?
ACOG does not recommend MSD for due-date assignment. A precise-looking age from a simple shortcut would be misleading.
Can I measure this from a screenshot?
Measurements should come from a properly performed ultrasound examination and report.
Does a sac measurement prove viability?
No. Clinicians use additional findings, timing and sometimes repeat scans.