# Atlas index build tier defaults > Source-anchored research for Atlas index build tier defaults. Run status: COMPLETED. Parent: [Index build resource limits (maxNumActiveUserIndexBuilds, disk space)](https://llms-explorer.com/tree/index-build-resource-limits-maxnumactiveuserindexbuilds-disk-space/) · 6 facets · 10 facts · page: https://llms-explorer.com/tree/atlas-index-build-tier-defaults/ ## Structure and components - A diagnostic gist created on 2023-08-15 contains Atlas-named hosts and maxIndexBuildMemoryUsageMegabytes=250. It demonstrates a historical configured value, but does not identify an Atlas tier or establish a current service default. (low confidence) (verified as of 2023-08-15) — [source](https://gist.github.com/Salil999/ff0ca3f9a08021c038bcd7aef2a03dc1) ## How it works - The index-build memory allowance is shared across indexes created by one createIndexes command, separately from WiredTiger cache memory; exceeding the allowance causes temporary files to be used. (high confidence) — [source](https://www.mongodb.com/docs/v8.3/reference/parameters/) - A createIndexes request consumes one concurrency slot even when it builds multiple indexes; the request shares its memory budget among those indexes. (medium confidence) — [source](https://www.mongodb.com/docs/manual/reference/parameters.md) ## Measurements and reference values - Atlas maxIndexBuildMemoryUsageMegabytes defaults are M10–M20: 100 MB; M30: 250 MB; M40: 400 MB; M50–M60: 512 MB; M80–M140: 1024 MB; M200+: 2048 MB. The documentation labels tiers as comparable to instance-RAM bands. (high confidence) (verified as of 2026-10-23) — [source](https://www.mongodb.com/docs/manual/reference/parameters.md) - The generic 200 MB per-createIndexes default is explicitly scoped to self-hosted deployments; it should not replace the Atlas tier table in capacity estimates. (high confidence) (verified as of 2026-10-23) — [source](https://www.mongodb.com/docs/v8.3/reference/parameters/) - Percona’s August 5, 2019 article describes a 500 MB default and configurable memory, so its figures should not substitute for the current Atlas tier table. (medium confidence) — [source](https://www.percona.com/blog/mongodb-impact-free-index-builds-using-detached-replicaset-nodes/) ## Problems, failure modes and limitations - MongoDB documents Atlas index-build memory values as cluster-tier defaults that users cannot change. (high confidence) (verified as of 2026-10-23) — [source](https://www.mongodb.com/docs/v8.3/reference/parameters/) ## Comparisons and alternatives - The generic 200 MB memory and three-concurrent-build defaults describe self-managed deployments; Atlas uses tier-specific values. Memory exhaustion during sorting can spill to temporary files rather than immediately aborting the build. (medium confidence) (verified as of 2026-10-19) — [source](https://www.mongodb.com/docs/manual/reference/parameters.md) ## Facts and statements - Atlas maxNumActiveUserIndexBuilds defaults are 1 for M10–M40, 2 for M50–M60, and 3 for M80 and above including NVMe variants; corresponding low-CPU R tiers use the same defaults. (medium confidence) (verified as of 2026-10-23) — [source](https://www.mongodb.com/docs/manual/reference/parameters.md) - A diagnostic gist created August 15, 2023 records maxIndexBuildMemoryUsageMegabytes=250 and maxNumActiveUserIndexBuilds=3. It is a deployment snapshot, not a current tier-default specification. (low confidence) — [source](https://gist.github.com/Salil999/ff0ca3f9a08021c038bcd7aef2a03dc1)