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SQL Server Index Size Estimator Calculator

Before you add an index to a big ERP table, you need to know how much disk, backup and restore time it will cost. This calculator estimates the size of a nonclustered index from row count, key width, included columns and fill factor, and charts how fill factor changes the answer.

Your numbers

Rows in the table

rows

Use the row count from sys.dm_db_partition_stats or your ERP transaction history table.

Index key width

bytes

Sum of the key column widths, including the clustering key that SQL Server appends to every nonclustered row.

Included columns width

bytes

Sum of the INCLUDE column widths. Average length for variable columns.

Fill factor

90 %

100 means pages are packed full. Lower values leave room for inserts but increase size.

Number of similar indexes

indexes

Use this to size a family of indexes of similar width across the same table.

Your results

Size of one index

1,059.31

Total size of one index in MB.

Size of all indexes

4.1379

Combined storage in GB for the number of similar indexes you entered.

Leaf row size

Key plus included columns plus an approximate 7 byte row header.

Usable bytes per page

7,286

An 8 KB page holds about 8096 bytes of row data before the fill factor is applied.

Rows per leaf page

148

Includes the 2 byte slot array entry per row.

Leaf pages

135,136

Non-leaf fan-out

298

Index rows per intermediate page, using key width plus roughly 11 bytes of pointer overhead.

Non-leaf pages

456

Sum of the upper B-tree levels, approximated as a geometric series.

This is a planning estimate based on the documented page layout. Actual size varies with variable-length data, NULL columns, compression and fragmentation. Verify with sp_spaceused or sys.dm_db_index_physical_stats after you create the index.

Save results:

Index size (MB) versus fill factor

Sweeps Fill factor from 50 to 100, holding your other inputs fixed.

Index review on your ERP database

Optional: we can review your largest tables and index set read-only and send you a short sizing and cleanup list.

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

How to use the calculator

Pull the row count for the table from sys.dm_db_partition_stats. Add up the byte widths of your key columns, remembering that a nonclustered index also carries the clustering key. Enter the INCLUDE column widths separately, using average length for variable-width columns such as descriptions. Set the fill factor you plan to use, then read the size of one index and the combined size if you plan several similar indexes. The chart shows how much space you save or spend by moving the fill factor.

Use average, not maximum, width for varchar and nvarchar columns
Add 8 bytes for a bigint clustering key, 4 for an int
Enter the number of indexes only if they share a similar width

The method behind the estimate

SQL Server stores index rows on 8 KB pages with about 8096 bytes of usable row space and a 2 byte slot entry per row. The leaf row size is the key plus included columns plus a small header. Rows per page is the usable space, reduced by the fill factor, divided by row size plus the slot entry. Leaf pages equal rows divided by rows per page. Intermediate levels hold only the key and a pointer, so their count is the leaf pages divided by the fan-out minus one. Multiply pages by 8 KB for size.

Worked example and how to read it

A 20 million row transaction table, a 16 byte key and 24 bytes of included columns gives a 47 byte leaf row. At fill factor 90, about 150 rows fit per page, so roughly 133,000 leaf pages or about 1 GB per index. Four similar indexes add about 4 GB to the database, plus the same again in every backup copy and test refresh. If the index is on an append-only column, a fill factor of 100 is safe and saves around 10 percent.

Common mistakes

Most underestimates come from forgetting that wide INCLUDE lists are copied into every row of the index. Another is applying a low fill factor to every index by habit. Fragmentation, compression and partition alignment also change real numbers, so treat the result as a budget and confirm it on a restored copy of production.

Including large text columns by mistake
Ignoring the clustering key appended to the index
Using a low fill factor on ever-increasing keys

Frequently Asked Questions

Does this work for clustered indexes?

Not exactly. A clustered index leaf holds the whole data row, so set the key width to the full average row size and keep included columns at zero. The upper levels then use the key width only. The estimate is reasonable for planning but less precise than for nonclustered indexes.

Why does fill factor change size so much?

Fill factor reserves free space on each leaf page. At 70 percent, almost a third of every page is empty, so you need roughly 40 percent more pages than at 100 percent. The trade-off is fewer page splits on tables with random inserts or updates.

Is the estimate valid with page compression?

No. Row and page compression change the stored size of data and can often cut it by a third or more. Run the estimate for the uncompressed size, then use sp_estimate_data_compression_savings for a SQL Server specific compressed figure.

How accurate is the result?

For fixed-width keys the estimate is usually within 10 percent of reality. Variable-length columns, NULL bitmaps and fragmentation add noise. Always confirm with sp_spaceused on a test database before you commit storage for a large production index.

If you want sizing and index recommendations pulled directly from your ERP database, we can build a read-only agent that does this on your own servers.

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