# Mean Sac Diameter Calculator (MSD)

URL: https://speedycalculator.com/calculators/health/mean-sac-diameter/
Last updated: 2026-10-30

> The mean sac diameter calculator averages the three diameters of the gestational sac on early ultrasound, estimates the gestational age (MSD + 30 days) and flags findings that suggest early pregnancy loss.

## Inputs

- Length (mm)
- Width (mm)
- Height (mm)
- Embryo seen?

## Results

- Mean sac diameter
- Estimated gestational age
- Gestational age – weeks
- …plus days
- Interpretation (SRU criteria)

## Formula

```
MSD (mm) = (length + width + height) ÷ 3
Gestational age (days) ≈ MSD + 30
No embryo with MSD ≥ 25 mm is diagnostic of pregnancy failure (SRU 2013)
```

## Explanation

The gestational sac is the first structure seen on ultrasound, from about 4.5–5 weeks. It grows about 1 mm a day in early pregnancy. Diagnosis of pregnancy loss is never made from a single number without careful follow-up.

### How to use the Mean Sac Diameter Calculator

- **Length**
- **Width**
- **Height**
- **Embryo seen?**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter the three perpendicular sac diameters measured from inner edge to inner edge, and whether an embryo is seen.

### Inputs explained

FieldWhat to enterLengthIn mmWidthIn mmHeightIn mmEmbryo seen?Choose one: Yes, No

### What each result means

- **Mean sac diameter** = ([Length] + [Width] + [Height])÷3
- **Estimated gestational age** = [Mean sac diameter] + 30
- **Gestational age – weeks** = floor([Estimated gestational age]÷7)
- **…plus days** = mod(round([Estimated gestational age]), 7)

### Example values

How the main result changes when **Length** changes and everything else stays at the example values above:

Length (mm)Gestational age – weeks (weeks)106156206257307407
*Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.*

## Worked example

Inputs — Length: 20 mm; Width: 16 mm; Height: 18 mm; Embryo seen?: Yes.

Results — Mean sac diameter = 18 mm; Estimated gestational age = 48 days; **Gestational age – weeks = 6 weeks**; …plus days = 6 days; Interpretation (SRU criteria) = No concerning finding.

## Key terms

- **MSD**: Mean sac diameter.

## FAQ

**When is the gestational sac seen?**
From about 4.5–5 weeks with a transvaginal scan.

**How fast does the sac grow?**
About 1 mm per day.

**Does an empty sac always mean miscarriage?**
Not if it is small; a repeat scan is needed before any diagnosis.


## Sources

- [Doubilet PM et al. Diagnostic criteria for nonviable pregnancy early in the first trimester. N Engl J Med, 2013](https://pubmed.ncbi.nlm.nih.gov/24106939/)