BSON Document Size Calculator

More information about BSON document size

MongoDB stores documents as BSON (Binary JSON), a binary encoding of your JSON-like data. BSON is not the same size as the JSON text: each field carries a type byte and a length prefix, and values are stored in fixed binary widths. This tool estimates that encoded size.

The 16 MB limit

A single BSON document cannot exceed 16 MB. Hit that ceiling and writes fail, so it's worth watching for documents with large arrays or embedded blobs. If you're approaching the limit, reference large sub-documents in a separate collection or store binary payloads in GridFS.

How BSON size is calculated

• A document is a 4-byte length, its elements, and a 1-byte terminator.
• Each element is a type byte, the field name as a null-terminated string, and the value.
• Strings are a 4-byte length plus their UTF-8 bytes plus a null byte; an int32 is 4 bytes, a double or date is 8, an ObjectId is 12, a Decimal128 is 16.

Frequently asked questions