Mean Sac Diameter Calculator

Created by Alexa Ruiz and Dominika Śmiałek, MD, PhD candidate
Reviewed by Dominik Czernia, PhD and Jack Bowater
Based on research by
Bottomley, C.; Bourne, T. Dating and growth in the first trimester Best Practice and Research in Clinical Obstetrics and Gynaecology (August 2009)
Last updated: Jan 04, 2023

Welcome to the mean sac diameter calculator. It'll help you calculate the size of the gestational sac. The meaning of a gestational sac will be explained shortly, but just to give you an idea: it's an important structure observed in the first trimester of pregnancy.

By calculating the size of the gestational sac, an estimated gestational age can be determined from the mean sac diameter chart. This can be used to find how far along a pregnancy is. In the following article, we'll also give you some additional information on how to calculate mean sac diameter and how to interpret mean sac diameter values. Keep reading!

What is a gestational sac in pregnancy?

The gestational sac is what encloses the baby during pregnancy and is filled with amniotic fluid. During the first ultrasound a new mom-to-be receives, the doctor or nurse will likely comment on the presence or absence of a gestational sac. Whether or not the gestational sac is there is the earliest sonographic finding that can be made. For that estimation, you can also use the gestational age calculator of the baby. It will appear on the sonogram as a white rim around a clear center.

The gestational sac is usually visible at about 5 weeks after the last menstrual period (around 3 weeks after
conception - check out our conception calculator), but it's sometimes possible to see it between 4.3 to 5 weeks on an ultrasound. The size of the gestational sac can be determined in one of two possible ways:

  • Measuring the largest diameter; and
  • Taking the mean of three diameters (known as the mean sac diameter).

What is the gestational sac in pregnancy used for? Well, you can find out what the estimated gestational age of the baby is using the mean sac diameter chart.

How to measure gestational sac?

There are usually 3 steps to confirming a pregnancy:

  1. Home pregnancy test - Depending on the test, two vertical lines mean pregnant, a "plus" sign means pregnant, "pregnant" means pregnant (this one is kind of obvious), and so on. Read the instructions that come with your test if you are unsure. Also, you may use our pregnancy test calculator to see when it's the best time to use it.
  2. Blood test - This test measures the amount of human chorionic gonadotropin (hCG) present in the blood. This is a hormone that is produced by the placenta during pregnancy.
  3. Ultrasound - If the blood test confirms pregnancy, now you need an ultrasound. It is also a way of measuring the gestational sac. There are two types of ultrasound: transvaginal ultrasound and transabdominal ultrasound, each one having its advantages:
  • The transabdominal ultrasound is performed when the maternal bladder is full. It provides a wider field of view and the greatest depth of view; and
  • The transvaginal ultrasound is performed when the bladder is empty. It gives a better resolution with finer details. This type is usually preferred to the transabdominal ultrasound and is considered superior.

How to calculate mean sac diameter?

The formula for calculating the mean sac diameter is as follows:

mean sac diameter = (length + height + width) / 3

where length, width, and height are the dimensions of the gestational sac, all in the same units.

The value for the mean sac diameter can then be used to estimate gestational age from the mean sac diameter gestational age chart or with the following equation:

Days of pregnancy = mean sac diameter (in mm) + 30

The mean sac diameter increases by 1 mm per day, but there can be variations in the rate of growth. The accuracy of the mean sac diameter measurement is about +/- 5 days.

Mean sac diameter gestational age chart

Below you can find the mean sac diameter chart we used to build this tool.

Mean sac diameter (mm)

Gestation (weeks/days)


4w 4d


4w 5d


4w 6d




5w 1d


5w 2d


5w 3d


5w 4d


5w 5d


5w 6d




6w 1d


6w 2d


6w 3d


6w 4d


6w 5d


6w 6d




7w 1d


7w 2d


7w 3d


7w 4d


7w 5d


7w 6d




8w 1d


8w 2d


8w 3d


8w 4d

How to use the mean sac diameter calculator?

To use this mean sac diameter calculator:

  1. Determine the length, height, and width of the gestational sac located on the ultrasound and enter these values in the calculator.
  2. The mean of these summed values will be calculated automatically.
  3. Based on this value, we will find the estimated gestational age in days using the built-in mean sac diameter chart.
  4. If you would like to know the estimated gestational age in weeks, you can switch between the units of time, and the calculator will do the conversion from days to weeks or months for you.


Is mean sac diameter the only way to date a pregnancy?

No, mean sac diameter is not the only way to date a pregnancy.

The mean sac diameter measurement is used to determine gestational age before a crown rump length (CRL) can be clearly measured. The crown rump length is the length of the fetus from the top of its head to the bottom of the torso and is the most accurate way to determine gestational age.

Which other structures are found in the gestational sac?

The yolk sac and the amniotic sac are also found in the gestational sac. Of course, there is also the embryo itself.

Where in the uterus is the gestational sac exactly located?

The gestational sac is located in the part of the uterus called the fundus, which is the top part of the uterus that is across from the cervix (the opening of the uterus).

What does an empty gestational sac indicate?

Depending on the size of the gestational sac, an empty gestational sac could mean an embryo has failed to develop and a miscarriage has occurred.

It may happen early in the first trimester, and a person might not even realize that they are pregnant.

Alexa Ruiz and Dominika Śmiałek, MD, PhD candidate
Gestational sac dimenstions
Mean sac diameter
Gestational age
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