# Estimating tables with fixed-length rows

You can estimate the size (in pages) of a table with fixed-length rows. A table with fixed-length rows has no columns of the VARCHAR or NVARCHAR data type.

Perform the following steps to estimate the size (in pages) of a table with fixed-length rows.

**To estimate the
page size, row size, number of rows, and number of data pages:**

- Use
**onstat -b**to obtain the size of a page.The

**buffer size**field in the last line of this output displays the page size. - Subtract 28 from this amount to account for the header
that appears on each data page. The resulting amount is
referred to as
*pageuse*. - To calculate the size of a row, add the widths of all the
columns in the table definition. TEXT and BYTE columns each use 56
bytes. If you have already created your table, you can
use the following SQL statement
to obtain the size of a row:
`SELECT rowsize FROM systables WHERE tabname = 'table-name';`

- Estimate the number of rows that the table is expected
to contain. This number is referred to as
*rows*. The procedure for calculating the number of data pages that a table requires differs depending on whether the row size is less than or greater than*pageuse*. - If the size of the row is less than or equal to
*pageuse*, use the following formula to calculate the number of data pages. The**trunc()**function notation indicates that you are to round down to the nearest integer.`data_pages =`

*rows*/ trunc(*pageuse*/(*rowsize*+ 4))The maximum number of rows per page is 255, regardless of the size of the row.

Important: Although the maximum size of a row that the database server accepts is approximately 32 kilobytes, performance degrades when a row exceeds the size of a page. For information about breaking up wide tables for improved performance, see Denormalize the data model to improve performance. - If the size of the row is greater than
*pageuse*, the database server divides the row between pages. The page that contains the initial portion of a row is called the*home page*. Pages that contains subsequent portions of a row are called*remainder pages*. If a row spans more than two pages, some of the remainder pages are completely filled with data from that row. When the trailing portion of a row uses less than a page, it can be combined with the trailing portions of other rows to fill out the partial remainder page. The number of data pages is the sum of the home pages, the full remainder pages, and the partial remainder pages.- Calculate the number of home pages.
The number of home pages is the same as the number of rows:

`homepages =`

*rows* - Calculate the number of full remainder pages.
First calculate the size of the row remainder with the following formula:

`remsize =`

*rowsize*- (*pageuse*+ 8)If

*remsize*is less than*pageuse*`- 4`, you have no full remainder pages.If

*remsize*is greater than*pageuse*`- 4`, use*remsize*in the following formula to obtain the number of full remainder pages:`fullrempages =`

*rows** trunc(*remsize*/(*pageuse*- 8)) - Calculate the number of partial remainder pages.
First calculate the size of a partial row remainder left after you have accounted for the home and full remainder pages for an individual row. In the following formula, the

**remainder()**function notation indicates that you are to take the remainder after division:`partremsize = remainder(`

*rowsize*/(*pageuse*- 8)) + 4The database server uses certain size thresholds with respect to the page size to determine how many partial remainder pages to use. Use the following formula to calculate the ratio of the partial remainder to the page:

`partratio =`

*partremsize*/*pageuse*Use the appropriate formula in the following table to calculate the number of partial remainder pages.

`partratio`ValueFormula to Calculate the Number of Partial Remainder Pages Less than .1 partrempages = *rows*/(trunc((*pageuse*/10)/*remsize*) + 1)Less than .33 partrempages = *rows*/(trunc((*pageuse*/3)/*remsize*) + 1).33 or larger partrempages = *rows* - Add up the total number of pages with the following formula:
`tablesize =`

*homepages*+*fullrempages*+*partrempages*

- Calculate the number of home pages.

**Parent topic:**Estimating data pages