How to estimate replication GB/month from write volume

When people search for S3 replication cost, the hardest input is usually not the price. It's the volume: how many GB per month actually replicate? The key idea is simple: replication is driven by writes and changes, not by your total stored dataset size.

Start with the right question

Instead of asking "how big is my bucket?", ask:

How many GB/day are newly written or changed?
How often do objects get overwritten (churn)?
Do you replicate everything or only a prefix/tags subset?

Method 1: From GB/day of writes

If you can estimate or measure writes in GB/day, monthly replication volume is straightforward:

Replicated GB/month ≈ write GB/day x 30.4 x replication coverage

Replication coverage is the fraction of writes that are replicated (for example, 0.6 if only 60% of objects match the replication rules).

Method 2: From requests and average object size

If you do not have GB/day but you know how many objects are written, approximate GB/day from request volume:

Write GB/day ≈ PUTs/day x avg object size (MB) / 1024

This works well when your object sizes are reasonably consistent. If sizes vary a lot, break the workload into a few buckets (small/medium/large) and sum.

Method 3: From change rate on an existing dataset

Some datasets are mostly static but have a measurable churn rate (percentage of data replaced per day or per week). If your total dataset is S GB and churn is c% per month:

Replicated GB/month ≈ S x (c / 100)

Example: 100,000 GB stored with 5% churn per month implies about 5,000 GB/month replicated (plus any new data).

Common pitfalls

Using total stored GB instead of changed GB: replication volume is usually far smaller.
Forgetting destination storage: replication often doubles storage for replicated objects.
Ignoring extra requests: replication workflows can increase PUT/GET/LIST operations.

Turn replicated GB/month into dollars

Once you have replicated GB/month, estimate the replication line item with Storage replication cost calculator. If replication is billed as cross-region transfer in your setup, price it with cross-region transfer.

For the full picture (extra storage + requests + transfer), use the checklist in copy storage pricing and add storage pricing with storage pricing calculator.

A practical checklist to estimate cloud cost without missing major line items: requests, compute, storage, logs/metrics, and network transfer. Includes a worksheet template, validation steps, and the most common double-counting traps.

Database costs explained: compute, storage growth, backups, and network

A practical framework to estimate managed database bills: baseline compute, storage GB-month growth, backups/snapshots, and the network patterns that cause surprises.

Estimate Secrets Manager API calls per month (GetSecretValue volume)

A practical workflow to estimate Secrets Manager API request volume (especially GetSecretValue): measure and scale when possible, model from runtime churn when not, and validate with CloudTrail so your budget survives peaks.

Security costs explained: WAF, keys/secrets, and request-driven spikes

A practical security cost model: WAF request-based pricing, key management (KMS/Key Vault) operations, secrets access patterns, and how bot traffic or high-frequency crypto can surprise budgets.

Estimate CloudTrail Events per Month (cost planning)

Methods to estimate CloudTrail event volume: Lake queries, S3 logs, and workload-driven approximations.

S3 replication pricing: estimate replicated GB/month and total impact

A practical guide to S3 replication pricing: replication volume (changed data), extra destination storage, transfer-like charges, and when CRR vs SRR changes the bill.

FAQ

Should I use total bucket size to estimate replication?

Usually no. Ongoing replication volume is typically driven by changed data (writes and churn), not the entire dataset size, unless you're doing a one-time backfill.

What is replication coverage?

The fraction of writes that match replication rules (for example, only a prefix or a tag set). If only 60% of writes replicate, coverage is 0.6.

What about retries and overhead?

Treat this model as a baseline. Add a small overhead factor if your workload has frequent retries, reprocessing, or multi-destination replication.

Last updated: 2026-01-21