space usage on sys.allocation_units and sp_spaceused It is a known fact that the DMVs dont hold accurate information regarding number of pages and count of rows.
However, when you have the stats updated, I can't see why they wouldn't.

I am working on a monitoring tool, want to know disk size of each index and data, etc.
Eventually I would like to find the right fill factor, and other things etc.

The space used by my function and the old sp_spaceused differs a little bit on the space usage, but not on record count.

Can you see if there is anything missing in my select?

this is the sp_spaceused (then I convert the numbers in MB):

sp_spaceused 'tblBOrderRelationship'
go

select 318008/1024.00 AS reserved,
140208/1024.00  AS data,
177048/1024.00 AS index_size,
752/1024.00    AS unused




But when I run my select, code below\picture below, I get slightly different figures.

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED

SELECT 
    schema_name(t.schema_id) as SchemaName,
    t.NAME AS TableName,
    t.type_desc,
    t.is_ms_shipped,
    t.is_published,
    t.lob_data_space_id,
    t.filestream_data_space_id,
    t.is_replicated,
    t.has_replication_filter,
    t.is_merge_published,
    t.is_sync_tran_subscribed,
    --t.is_filetable,
    i.name as indexName,
    i.type_desc,
    i.is_unique,
    i.is_primary_key,
    i.is_unique_constraint,
    i.fill_factor,
    i.is_padded,


    sum(p.rows)               OVER (PARTITION BY t.OBJECT_ID,i.index_id)  as RowCounts,
    sum(a.total_pages)        OVER (PARTITION BY t.OBJECT_ID,i.index_id)  as TotalPages, 
    sum(a.used_pages)         OVER (PARTITION BY t.OBJECT_ID,i.index_id)  as UsedPages, 
    sum(a.data_pages)         OVER (PARTITION BY t.OBJECT_ID,i.index_id)  as DataPages,

    (sum(a.total_pages)       OVER (PARTITION BY t.OBJECT_ID,i.index_id)  * 8) / 1024 as TotalSpaceMB, 
    (sum(a.used_pages)        OVER (PARTITION BY t.OBJECT_ID,i.index_id)  * 8) / 1024 as UsedSpaceMB, 
    (sum(a.data_pages)        OVER (PARTITION BY t.OBJECT_ID,i.index_id)  * 8) / 1024 as DataSpaceMB
FROM 
    sys.tables t
INNER JOIN      
    sys.indexes i ON t.OBJECT_ID = i.object_id
INNER JOIN 
    sys.partitions p ON i.object_id = p.OBJECT_ID AND i.index_id = p.index_id
INNER JOIN 
    sys.allocation_units a ON p.partition_id = a.container_id
WHERE 
    t.NAME NOT LIKE 'dt%' AND
    i.OBJECT_ID &gt; 255 
AND T.NAME = 'tblBOrderRelationship'


the figures



the bigger picture, including the index names


Now doing some calculations to check the results:

--==================================
-- the figures from sp_spaceused
--==================================
select 318008/1024.00 AS reserved,
140208/1024.00  AS data,
177048/1024.00 AS index_size,
752/1024.00    AS unused

--==================================
-- the figures from my select
--==================================
select 137+61+56+54 AS reserved,
       137 AS data,
       61+56+54 AS index_size




It is not so far off, really, apart the fact I did not calculate the unused space!

What can I do to make this accurate?

AFTER CHANGES:

After I replaced 1024 by 1024.00 the results are much more accurate. 
I noticed records have been inserted into the table in question, and obviously the stats are not so up to date, but still the results match (under 1 MB difference - which is all right for me)

The new result sets are:

--==================================
-- the figures from sp_spaceused
--==================================
select
318072 /1024.00 AS reserved,
140208 /1024.00 AS data,
177096 /1024.00 AS index_size,
768 /1024.00 AS unused
go

--==================================
-- the figures from my select
--==================================
select 137.7578125+61.7968750+56.4218750+54.6406250 as reserved,
       137.7578125 as data,
       61.7968750+56.4218750+54.6406250 as index_size
Even though you fixed the immediate rounding issue, the overall algorithm to get per-object / index stats is incorrect. It does not properly handle LOB and row-overflow data. It also excludes: Indexed Views, FullText indexes, XML indexes, and a few other cases. Hence, you might not be seeing all of your data.

The following is an adaptation of the code I posted to an answer on StackOverflow ( sp_spaceused - How to measure the size in GB in all the tables in SQL ) that handles all of the cases that sp_spaceused handles. That S.O. question was only concerned with per-object stats, not per index, so I have adjusted the code to handle things at the index level.

;WITH agg AS
(   -- Get info for Tables, Indexed Views, etc
    SELECT  ps.[object_id] AS [ObjectID],
            ps.index_id AS [IndexID],
            NULL AS [ParentIndexID],
            NULL AS [PassThroughIndexName],
            NULL AS [PassThroughIndexType],
            SUM(ps.in_row_data_page_count) AS [InRowDataPageCount],
            SUM(ps.used_page_count) AS [UsedPageCount],
            SUM(ps.reserved_page_count) AS [ReservedPageCount],
            SUM(ps.row_count) AS [RowCount],
            SUM(ps.lob_used_page_count + ps.row_overflow_used_page_count)
                    AS [LobAndRowOverflowUsedPageCount]
    FROM    sys.dm_db_partition_stats ps
    GROUP BY    ps.[object_id],
                ps.[index_id]
    UNION ALL
    -- Get info for FullText indexes, XML indexes, Spatial indexes, etc
    SELECT  sit.[parent_id] AS [ObjectID],
            sit.[object_id] AS [IndexID],
            sit.[parent_minor_id] AS [ParentIndexID],
            sit.[name] AS [PassThroughIndexName],
            sit.[internal_type_desc] AS [PassThroughIndexType],
            0 AS [InRowDataPageCount],
            SUM(ps.used_page_count) AS [UsedPageCount],
            SUM(ps.reserved_page_count) AS [ReservedPageCount],
            0 AS [RowCount],
            0 AS [LobAndRowOverflowUsedPageCount]
    FROM    sys.dm_db_partition_stats ps
    INNER JOIN  sys.internal_tables sit
            ON  sit.[object_id] = ps.[object_id]
    WHERE   sit.internal_type IN
               (202, 204, 207, 211, 212, 213, 214, 215, 216, 221, 222, 236)
    GROUP BY    sit.[parent_id],
                sit.[object_id],
                sit.[parent_minor_id],
                sit.[name],
                sit.[internal_type_desc]
), spaceused AS
(
SELECT  agg.[ObjectID],
        agg.[IndexID],
        agg.[ParentIndexID],
        agg.[PassThroughIndexName],
        agg.[PassThroughIndexType],
        OBJECT_SCHEMA_NAME(agg.[ObjectID]) AS [SchemaName],
        OBJECT_NAME(agg.[ObjectID]) AS [TableName],
        SUM(CASE
                WHEN (agg.IndexID &lt; 2) THEN agg.[RowCount]
                ELSE 0
            END) AS [Rows],
        SUM(agg.ReservedPageCount) * 8 AS [ReservedKB],
        SUM(agg.LobAndRowOverflowUsedPageCount +
            CASE
                WHEN (agg.IndexID &lt; 2) THEN (agg.InRowDataPageCount)
                ELSE 0
            END) * 8 AS [DataKB],
        SUM(agg.UsedPageCount - agg.LobAndRowOverflowUsedPageCount -
            CASE
                WHEN (agg.IndexID &lt; 2) THEN agg.InRowDataPageCount
                ELSE 0
            END) * 8 AS [IndexKB],
        SUM(agg.ReservedPageCount - agg.UsedPageCount) * 8 AS [UnusedKB],
        SUM(agg.UsedPageCount) * 8 AS [UsedKB]
FROM    agg
GROUP BY    agg.[ObjectID],
            agg.[IndexID],
            agg.[ParentIndexID],
            agg.[PassThroughIndexName],
            agg.[PassThroughIndexType],
            OBJECT_SCHEMA_NAME(agg.[ObjectID]),
            OBJECT_NAME(agg.[ObjectID])
)
SELECT sp.SchemaName,
       sp.TableName,
       sp.IndexID,
       CASE
         WHEN (sp.IndexID &gt; 0) THEN COALESCE(si.[name], sp.[PassThroughIndexName])
         ELSE N'&lt;Heap&gt;'
       END AS [IndexName],
       sp.[PassThroughIndexName] AS [InternalTableName],
       sp.[Rows],
       sp.ReservedKB,
       (sp.ReservedKB / 1024.0 / 1024.0) AS [ReservedGB],
       sp.DataKB,
       (sp.DataKB / 1024.0 / 1024.0) AS [DataGB],
       sp.IndexKB,
       (sp.IndexKB / 1024.0 / 1024.0) AS [IndexGB],
       sp.UsedKB AS [UsedKB],
       (sp.UsedKB / 1024.0 / 1024.0) AS [UsedGB],
       sp.UnusedKB,
       (sp.UnusedKB / 1024.0 / 1024.0) AS [UnusedGB],
       so.[type_desc] AS [ObjectType],
       COALESCE(si.type_desc, sp.[PassThroughIndexType]) AS [IndexPrimaryType],
       sp.[PassThroughIndexType] AS [IndexSecondaryType],
       SCHEMA_ID(sp.[SchemaName]) AS [SchemaID],
       sp.ObjectID
       --,sp.ParentIndexID
FROM   spaceused sp
INNER JOIN sys.all_objects so -- in case "WHERE so.is_ms_shipped = 0" is removed
        ON so.[object_id] = sp.ObjectID
LEFT JOIN  sys.indexes si
       ON  si.[object_id] = sp.ObjectID
      AND  (si.[index_id] = sp.IndexID
         OR si.[index_id] = sp.[ParentIndexID])
WHERE so.is_ms_shipped = 0
--so.[name] LIKE N''  -- optional name filter
--ORDER BY ????