CloudWatch Metric Guide
BucketSizeBytesAmazon S3 CloudWatch metric
BucketSizeBytes reports the total size, in bytes, of all objects stored in a bucket for a given storage class — including current and noncurrent object versions, object metadata, and the parts of any incomplete multipart uploads. It is a free, always-on storage metric published once per day.
What it measures
About BucketSizeBytes
BucketSizeBytes reports the total size, in bytes, of all objects stored in a bucket for a given storage class — including current and noncurrent object versions, object metadata, and the parts of any incomplete multipart uploads. It is a free, always-on storage metric published once per day.
| Namespace | AWS/S3 |
| Metric name | BucketSizeBytes |
| Unit | Bytes |
| AWS docs | Official Amazon S3 metrics reference |
Why this metric matters
BucketSizeBytes is a storage metric, not a request metric — AWS publishes it automatically for every bucket at no extra cost, with no setup required. The tradeoff is granularity: it updates roughly once every 24 hours, so it is useless for real-time incident detection but genuinely useful for catching slow-moving cost and capacity problems — runaway storage growth, a lifecycle policy that silently stopped working, or an application bug writing far more data than intended.
Because BucketSizeBytes is reported per StorageType (StandardStorage, GlacierStorage, and so on, per AWS documentation), it also surfaces storage-class drift — for example, objects that should have transitioned to Glacier under a lifecycle rule but didn't, quietly inflating your Standard storage bill.
Recommended alarm threshold for BucketSizeBytes
Recommended threshold
Alarm on a >20% day-over-day increase (or use CloudWatch Anomaly Detection) rather than an absolute byte value
AWS does not publish a universal 'good' byte threshold for BucketSizeBytes because normal size is entirely workload-dependent — a fixed number breaks the first time your bucket legitimately grows. A percentage-based day-over-day threshold (Nuberio recommendation) catches runaway writes, a broken lifecycle policy, or an abandoned multipart-upload cleanup job without needing to be manually revised as the bucket grows.
Is your BucketSizeBytes alarm already set up correctly?
The free Nuberio Audit scans your CloudWatch setup and flags missing or misconfigured alarms — including BucketSizeBytes — in 5 minutes.
Common failures that show up in BucketSizeBytes
When BucketSizeBytes reaches an alarm threshold, these are the most common root causes — in order of how often Nuberio sees them across customer AWS accounts.
Lifecycle policy missing or misconfigured, so noncurrent object versions and expired objects are never deleted
Versioning enabled without a noncurrent-version expiration rule, so every overwrite permanently adds a new copy
Incomplete multipart uploads that were never aborted — AbortIncompleteMultipartUpload not set, so failed large-file uploads leave billed orphaned parts
Application bug or retry storm writing duplicate or unintended objects at high volume (e.g., a logging loop)
Backup, replication, or migration job misconfigured to write far more data than expected
How the Nuberio Audit checks BucketSizeBytes
The free Audit scans every Amazon S3 resource in your account and checks whether a BucketSizeBytes alarm exists, whether its threshold and evaluation period are configured sensibly, and flags it if it's missing entirely. Every finding comes with the exact CLI command to fix it — read-only access, no credit card, results in about 5 minutes.
Nuberio Audit
Scans your CloudWatch setup for missing or misconfigured BucketSizeBytes alarms. Free, 5-minute read-only scan.
Related Amazon S3 metrics
BucketSizeBytes rarely fails in isolation. These metrics tend to correlate — monitor them together for complete Amazon S3 coverage.
FAQ
Frequently asked questions about BucketSizeBytes
Common questions about setting up CloudWatch alarms for BucketSizeBytes in Amazon S3.
What is the recommended CloudWatch alarm threshold for BucketSizeBytes?+
Alarm on a >20% day-over-day increase (or use CloudWatch Anomaly Detection) rather than an absolute byte value. AWS does not publish a universal 'good' byte threshold for BucketSizeBytes because normal size is entirely workload-dependent — a fixed number breaks the first time your bucket legitimately grows. A percentage-based day-over-day threshold (Nuberio recommendation) catches runaway writes, a broken lifecycle policy, or an abandoned multipart-upload cleanup job without needing to be manually revised as the bucket grows.
Which CloudWatch namespace does BucketSizeBytes belong to?+
BucketSizeBytes is published in the AWS/S3 namespace with a unit of Bytes. You can find it in the CloudWatch console under "Metrics" → "AWS/S3". See the Amazon S3 CloudWatch metrics reference in the AWS documentation.
Does Nuberio automatically create CloudWatch alarms for BucketSizeBytes?+
No — the free Nuberio Audit does not create alarms for you. It scans your CloudWatch setup and tells you which Amazon S3 resources are missing a BucketSizeBytes alarm, then gives you the exact CLI command to create it yourself.
Can I use Nuberio without already having a BucketSizeBytes alarm set up?+
Yes. The free Audit works whether or not you have any BucketSizeBytes alarm configured — it reads the resource directly from CloudWatch and tells you which resources need one, plus the copy-paste AWS CLI command to create it.
This page is part of the CloudWatch metric guide — thresholds and debugging guidance for every metric across RDS, Lambda, ECS, ALB, EC2, and DynamoDB. To find which Amazon S3 alarms your account is missing — including BucketSizeBytes — run the free CloudWatch alarm audit. The scan takes under 5 minutes and requires no account.