MongoDB Index Size Estimator

Estimate MongoDB / WiredTiger index footprint from document count, key width, multikey cardinality, and compression.

Estimate WiredTiger index footprint for single-field, compound, multikey, and TTL indexes.

Documents Index type Key field size bytes / doc Compound extra keys bytes Avg array cardinality multikey Record locator / _id bytes Per-entry overhead bytes WiredTiger compression Internal / freelist factor % Also estimate _id index

Heuristic for capacity planning. Real WiredTiger sizes depend on key prefix compression, fragmentation, and cache. Confirm with collStats / db.collection.totalIndexSize().

What Drives Index Size

Key width — ObjectIds, UUIDs, and long strings add up fast
Multikey — array indexes create one entry per element
Compound keys — sum of field sizes in the key pattern
Compression — WiredTiger prefix + block compression often ~0.35–0.7× raw

Always validate with db.collection.stats() / totalIndexSize() on a realistic dataset.