You may also want to replace sysfiles with sys.database_files:
SELECT database_name = db_name()
,database_size = ltrim(str((convert(DECIMAL(15, 2), dbsize) + convert(DECIMAL(15, 2), logsize)) * 8192 / 1048576, 15, 2) + ' MB')
,'unallocated space' = ltrim(str((
CASE
WHEN dbsize >= reservedpages
THEN (convert(DECIMAL(15, 2), dbsize) - convert(DECIMAL(15, 2), reservedpages)) * 8192 / 1048576
ELSE 0
END
), 15, 2) + ' MB')
FROM (
SELECT dbsize = sum(convert(BIGINT, CASE
WHEN type = 0
THEN size
ELSE 0
END))
,logsize = sum(convert(BIGINT, CASE
WHEN type <> 0
THEN size
ELSE 0
END))
FROM sys.database_files
) AS files
,(
SELECT reservedpages = sum(a.total_pages)
,usedpages = sum(a.used_pages)
,pages = sum(CASE
WHEN it.internal_type IN (
202
,204
,211
,212
,213
,214
,215
,216
)
THEN 0
WHEN a.type <> 1
THEN a.used_pages
WHEN p.index_id < 2
THEN a.data_pages
ELSE 0
END)
FROM sys.partitions p
INNER JOIN sys.allocation_units a
ON p.partition_id = a.container_id
LEFT JOIN sys.internal_tables it
ON p.object_id = it.object_id
) AS partitions
-- Gianluca Sartori