sql server backup size

the output of the system command sp_spaceused gives the reserved value of the database which indicates the total size of all the pages in the database. we can use the same value to estimate the database backup size.
below query extracted from the sp_spaceused which gives the list of all the databases and their reserved size.

use master create table dbo.all_db_size (dbname varchar(256),reserved varchar(256),data varchar(256),index_size varchar(90),unused varchar(90)) EXECUTE sp_MSforeachdb ' use [?] declare @reservedpages  bigint ,@usedpages  bigint,@pages bigint     select @reservedpages = sum(a.total_pages),         @usedpages = sum(a.used_pages),         @pages = sum(                 CASE                     -- XML-Index and FT-Index internal tables are not considered "data", but is part of "index_size"                     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 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 insert into master.dbo.all_db_size (dbname,reserved,data,index_size,unused)     select db_name() as dbname,         reserved_kb = ltrim(str(@reservedpages * 8192 / 1024.,15,0) ),         data_kb = ltrim(str(@pages * 8192 / 1024.,15,0) ),         index_size_kb = ltrim(str((@usedpages - @pages) * 8192 / 1024.,15,0) ),         unused_kb = ltrim(str((@reservedpages - @usedpages) * 8192 / 1024.,15,0) )                 '         select * from dbo.all_db_size