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SQL Server Transaction Log Sizing Calculator

A full transaction log stops every posting in your ERP. This calculator gives a defensible log size from your peak log rate, backup interval, longest transaction and the biggest index rebuild you run, and shows how a shorter log backup interval shrinks the requirement.

Your numbers

Peak log generation

MB/min

Measure with sys.dm_db_log_stats or by sampling the log_bytes_used counter during month end or MRP.

Log backup interval

15 min

How often the transaction log backup job runs.

Longest open transaction

min

The log cannot be truncated past the oldest active transaction. Think of nightly integrations and posting runs.

Largest index rebuilt online

Size of the biggest index you rebuild in one operation.

Recovery model during rebuild

Full recovery logs the whole rebuild. Bulk-logged can minimally log offline rebuilds.

Safety headroom

Your results

Required log size (GB)

14.1973

Suggested fixed growth increment

1,792

A fixed growth step in MB, rounded to 256 MB and capped at 8 GB, so the log grows in a few large steps rather than many small ones.

Log per backup file

600

Roughly how large each log backup is at peak.

Log held by backup cycle and long transaction

1,800

Log is held until the oldest transaction commits and the next log backup completes.

Log from largest index rebuild

9,830.4

Required log size

14,538

Conservative planning figure in MB.

A planning model, not a guarantee. Replication, availability groups, long-running reports under snapshot isolation and uncommitted ETL loads can hold the log longer. Monitor log_reuse_wait_desc in sys.databases.

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Save results:

Required log size (GB) versus log backup interval

Sweeps Log backup interval from 5 to 120, holding your other inputs fixed.

Database health review

Optional: we can review log, backup and growth settings on your ERP database read-only and send a short findings list.

Optional. The tool and every export work without this. No spam.

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How to use it

Measure peak log generation during your busiest window such as period close, MRP or a large integration batch. Enter your current log backup interval and the longest transaction you see, often a nightly interface that opens a transaction and keeps it for half an hour. Then enter the largest index you rebuild and your recovery model during the rebuild. The headroom percentage covers surprises. Read the required size and the suggested fixed growth increment.

Measure the rate at peak, not on average
Include interface jobs that hold a transaction open
Set a fixed growth value, never percent growth

The logic

In the full recovery model, log space is only reused after a log backup and after every active transaction older than that point has committed. So the log must hold the peak rate for the backup interval plus the age of the longest open transaction. An index rebuild is fully logged in full recovery and writes about as much log as the index size, which is why it is added on top. The sum is multiplied by the headroom factor.

Worked example

At 40 MB per minute, a 15 minute backup cycle and a 30 minute long transaction hold 1,800 MB. Rebuilding an 8 GB index in full recovery adds about 9.8 GB. With 25 percent headroom you need about 14.5 GB of log. Cutting the backup interval to 5 minutes lowers the requirement only modestly, because the rebuild dominates. Rebuilding in smaller pieces or using bulk-logged during the rebuild is the bigger lever.

Common mistakes

Teams often set a huge log once and never revisit it, or leave the log on 10 percent growth, which creates thousands of virtual log files and slow recovery. Another mistake is assuming a log backup frees space while a transaction is still open. Check log_reuse_wait_desc to see what is actually holding the log.

Frequently Asked Questions

Should I use the simple recovery model for an ERP?

Usually not. Simple recovery removes point-in-time restore, which most ERP owners need for audit and recovery. Use full recovery with frequent log backups unless the business accepts losing everything since the last full or differential backup.

Why is percent autogrowth a problem?

Percent growth creates ever larger steps and, when the file is small, many small steps. Both cause many virtual log files, which slows startup, restore and log backups. Use a fixed increment such as the suggested value and size the file correctly up front.

Does a shorter backup interval always help?

It reduces how long log is retained and limits data loss, but it cannot free log held by an open transaction or a running index rebuild. Past a point, the chart flattens and the rebuild or long transaction dominates.

How do I measure peak log rate?

Sample the Log Bytes Flushed per sec performance counter or query sys.dm_db_log_stats on a schedule during your busiest job. Multiply MB per second by 60 for MB per minute. Use the highest sustained value, not a one-second spike.

If log growth keeps stopping your ERP, we can wire a read-only monitor on your own servers that alerts on log_reuse_wait before users notice.

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