&lt;p&gt; I encountered a strange behavior of mongo and I would like to clarify it a bit...&lt;br&gt;
My request is simple as that: I would like to get a size of single document in collection.
I found two possible solutions:&lt;br&gt; &lt;/p&gt;
+ Object.bsonsize - some javascript method that should return a size in bytes&lt;/li&gt;
&lt;li&gt;db.collection.stats() - where there is a line &#39;avgObjSize&#39; that produce some &quot;aggregated&quot;(average) size view on the data. It simply represents average size of single document.&lt;/li&gt;
&lt;br&gt;
When I create test collection with only one document, both functions returns different values. How is it possible? &lt;br&gt;
Does it exist some other method to get a size of a mongo document?


Here, I provide some code I perform testing on:

1. I created new database &#39;test&#39; and input simple document with only one attribute: type:&quot;auto&quot;

        db.test.insert({type:&quot;auto&quot;})

2. output from stats() function call: *db.test.stats()*:

        { 
          &quot;ns&quot; : &quot;test.test&quot;,
          &quot;count&quot; : 1,
          &quot;size&quot; : 40,
          &quot;avgObjSize&quot; : 40,
          &quot;storageSize&quot; : 4096,
          &quot;numExtents&quot; : 1,
          &quot;nindexes&quot; : 1,
          &quot;lastExtentSize&quot; : 4096,
          &quot;paddingFactor&quot; : 1,
          &quot;systemFlags&quot; : 1,
          &quot;userFlags&quot; : 0,
          &quot;totalIndexSize&quot; : 8176,
          &quot;indexSizes&quot; : {
                &quot;_id_&quot; : 8176
        },
        &quot;ok&quot; : 1
       }

3. output from bsonsize function call: *Object.bsonsize(db.test.find({test:&quot;auto&quot;}))*

        481



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How to get the size of single document in Mongodb?