Drive Offset and Sector Conversions
and
"What's the Sector-Size of Your HDD?"

( Copyright©2004 by Daniel B. Sedory )

The default sector-size for almost every single HDD is:
512 bytes per Sector. So that's what was used for each calculation below. Although most of this page is a Table of HexOffsets and Sector Numbers, you'll also find some important notes here about disk structures!

Formulas for converting a given Hexadecimal Offset to a Sector Number.

The Table: Hexadecimal OFFSETs and Absolute SECTOR Numbers.

How Many Sectors are there on Your  Hard Drive? -- The total number of sectors for some ideal-sized HDDs. Almost every drive produced will be a little more or less than these sizes.

 


FORMULAS for Converting OFFSETs to SECTOR Numbers and Vice Versa

Using the Calculator that comes with any Windows OS, you can easily convert between Hexadecimal and Decimal (when in 'Scientific View'). To find the sector that a given Hexadecimal (or Decimal) Offset byte resides in use these formulas:

Sector Number = Round Down to the Nearest Integer Number:
[ Hexidecimal Offset / 200 hex ] and convert to Decimal, or:
[ The Decimal Offset / 512 ].
If you're lucky enough to pick the very first byte of a Sector, it will divide exactly!
For example, converting the byte at Hex Offset 7C00  to  31,744 (in Decimal) and dividing it by 512 equals 62 exactly, so 7c00 is the first byte of Absolute Sector 62.

Decimal Offset = (Sector Number x 512) + offset within the sector.
[the "+ offset within the sector " simply means adding 0 through 511 depending upon where the byte is in a given 512 byte sector.]
To find the Hexadecimal Offset, either convert the Decimal answer above to Hex, or use the following:
Hexadecimal Offset = [ Hex conversion of: (Sector Number x 512) ] + Hex offset within the sector.
For example, (69 x 512 = 35,328)giving us an Offset of:  8A00 Hex  for the first byte of Absolute Sector 69.
 

 

The Table

Hexadeciamal  O F F S E T S  and  Absolute  S e c t o r s
H e x    O f f s e t s
Absolute
S e c t o r
Numbers
Range
Hex Offset
of First Byte
in the Sector

000 - 1FF
0 0 0 
0
Abs. Sector 0 is the Master Boot Record for all Hard Disk Drives (HDDs).
But for most Floppy Diskettes, this is their OS Boot Record.
(Floppy Diskettes never have a Master Boot Record!)
200 - 3FF
2 0 0 
1
Beginning of the 1st FAT copy ( Sectors 1-9 ) for
1440 kb Floppy Diskettes using the FAT12 File System.
400 - 5FF
4 0 0 
2
600 - 7FF
6 0 0 
3
800 - 9FF
8 0 0 
4
A00 - BFF
A 0 0 
5
C00 - DFF
C 0 0 
6
E00 - FFF
E 0 0 
7
1000 - 11FF
1 0 0 0 
8
1200 - 13FF
1 2 0 0 
9
1400 - 15FF
1 4 0 0 
10
Beginning of the 2nd FAT copy ( Sectors 10-18 ) for
1440 kb Floppy Diskettes using the FAT12 File System.
1600 - 17FF
1 6 0 0 
11
1800 - 19FF
1 8 0 0 
12
1A00 - 1BFF
1 A 0 0 
13
1C00 - 1DFF
1 C 0 0 
14
1E00 - 1FFF
1 E 0 0 
15
2000 - 21FF
2 0 0 0 
16
2200 - 23FF
2 2 0 0 
17
2400 - 25FF
2 4 0 0 
18
2600 - 27FF
2 6 0 0 
19
 Beginning of the Root Directory ( Sectors 19-32 ) for
1440 kb Floppy Diskettes using the FAT12 File System.
2800 - 29FF
2 8 0 0 
20
2A00 - 2BFF
2 A 0 0 
21
2C00 - 2DFF
2 C 0 0 
22
2E00 - 2FFF
2 E 0 0 
23
3000 - 31FF
3 0 0 0 
24
3200 - 33FF
3 2 0 0 
25
3400 - 35FF
3 4 0 0 
26
3600 - 37FF
3 6 0 0 
27
3800 - 39FF
3 8 0 0 
28
3A00 - 3BFF
3 A 0 0 
29
3C00 - 3DFF
3 C 0 0 
30
3E00 - 3FFF
3 E 0 0 
31
4000 - 41FF
4 0 0 0 
32
4200 - 43FF
4 2 0 0 
33
Beginning of the Data Area ( Sectors 33 - 2879 ) for
1440 kb Floppy Diskettes using the FAT12 File System.
4400 - 45FF
4 4 0 0 
34
4600 - 47FF
4 6 0 0 
35
4800 - 49FF
4 8 0 0 
36
4A00 - 4BFF
4 A 0 0 
37
4C00 - 4DFF
4 C 0 0 
38
4E00 - 4FFF
4 E 0 0 
39
5000 - 51FF
5 0 0 0 
40
5200 - 53FF
5 2 0 0 
41
Etc. etc. -- And so on...
7600 - 77FF
7 6 0 0 
59
7800 - 79FF
7 8 0 0 
60
7A00 - 7BFF
7 A 0 0 
61
7C00 - 7DFF
7 C 0 0 
62
7E00 - 7FFF
7 E 0 0 
63
For most HDDs, this is where the First Partition's Volume Boot Record begins.
NOTE: FAT32 File Systems use three sectors (Abs. Sectors 63-65)
  for each Boot Record, and have a Second Copy (at Abs. Sectors 69-71).
NOTE: NTFS File Systems have only one Boot sector, but it's normally
followed by the NTLDR code which usually takes up seven more sectors.
The Second Copy of an NTFS Boot sector is after the end of the volume!
Note: If you view a Hard Disk Logically (i.e., using drive letters, C:, D:, etc.), then Absolute Sectors 0 through 62 will be "hidden" from view in your disk editor and the first Logical Sector (0) will actually be Absolute Sector 63.
8000 - 81FF
8 0 0 0 
64
8200 - 83FF
8 2 0 0 
65
8400 - 85FF
8 4 0 0 
66
8600 - 87FF
8 6 0 0 
67
8800 - 89FF
8 8 0 0 
68
8A00 - 8BFF
8 A 0 0 
69
8C00 - 8DFF
8 C 0 0 
70
8E00 - 8FFF
8 E 0 0 
71
9000 - 91FF
9 0 0 0 
72
9200 - 93FF
9 2 0 0 
73
9400 - 95FF
9 4 0 0 
74
9600 - 97FF
9 6 0 0 
75
9800 - 99FF
9 8 0 0 
76
9A00 - 9BFF
9 A 0 0 
77
9C00 - 9DFF
9 C 0 0 
78
9E00 - 9FFF
9 E 0 0 
79

If an HDD has an NTFS File System in its First Partition, then this is normally where its $MFT Records begin. The $MFT file record area can be very long.
( NOTE: the exact length in sectors for the $MFT basically depends upon how large the volume is, but there are other factors as well. )

A000 - A1FF
A 0 0 0 
80
A200 - A3FF
A 2 0 0 
81
Etc. etc.
BA00 - BBFF
B A 0 0 
93
BC00 - BDFF
B C 0 0 
94
BE00 - BFFF
B E 0 0 
95

If an HDD has a FAT32 File System in its First Partition, then this is normally the first sector of its FAT. The first eight bytes of a FAT32 - FAT will begin with either:
F8 FF FF 0F FF FF FF FF   or:  F8 FF FF FF FF FF FF FF
or even:  F8 FF FF 0F FF FF FF 0F
(unless viewed with a disk editor after booting into a Windows 9x OS; in which case, it will usually begin with the bytes:
F8 FF FF 0F FF FF FF F7   or:  F8 FF FF FF FF FF FF F7
or even:  F8 FF FF 0F FF FF FF 07 instead. See my page about the "Dirty Shutdown Flag" for more on why these FAT bytes are changed by the Windows OS!)
( NOTE: the exact length in sectors for each copy of the FAT depends upon how large the partition is and what tool was used to format the drive!  However, I've observed that the same formatting tool may create slightly different FAT lengths for exactly the same partition! This would require much further study to understand why; any volunteers? )
C000 - C1FF
C 0 0 0 
96
C200 - C3FF
C 2 0 0 
97
C400 - C5FF
C 4 0 0 
98
C600 - C7FF
C 6 0 0 
99
C800 - C9FF
C 8 0 0 
100
Etc. etc.
FE00 - FFFF
F E 0 0 
127
10000 - 101FF
1  0 0 0 0 
128
Etc. etc. -- And so on...
7D000 - 7D1FF
7  D 0 0 0 
1000
Etc. etc.
FA000 - FA1FF
F  A 0 0 0 
2000
Etc. etc.
FFE00 - FFFFF
F  F E 0 0 
2047
100000 - 1001FF
1 0  0 0 0 0 
2048
Etc. etc.
167A00 - 167BFF
1 6  7 A 0 0 
2877
167C00 - 167DFF
1 6  7 C 0 0 
2878
167E00 - 167FFF
1 6 7 E 0 0 
2879
Last Sector of a 1440 kb Floppy Diskette using the FAT12 File System.
Etc. etc. -- And so on...
177000-1771FF
17 7000 
3,000
1F4000-1F41FF
1F 4000 
4,000
271000-2711FF
27 1000 
5,000
4E2000-4E21FF
4E 2000 
10,000
FFFE00-FFFFFF
FF FE00 
32,767
1000000-10001FF
100 0000 
32,768
30D4000-30D41FF
30D 4000 
100,000
FFFFE00-FFFFFFF
FFF FE00 
524,287
10000000-100001FF
1000 0000 
524,288
1E848000-1E8481FF
1E84 8000 
1,000,000
FFFFFE00-FFFFFFFF
FFFF FE00 
8,388,607
100000000-1000001FF
1 0000 0000 
8,388,608
1312D0000-1312D01FF
1 312D 0000 
10,000,000
2625A0000-2625A01FF
2 625A 0000 
20,000,000
4C4B40000-4C4B401FF
4 C4B4 0000 
40,000,000
989680000-9896801FF
9 8968 0000 
80,000,000
BEBC20000-BEBC201FF
B EBC2 0000 
100,000,000
FFFFFFE00-FFFFFFFFF
F FFFF FE00 
134,217,727
1000000000-10000001FF
10 0000 0000 
134,217,728
1312D00000-1312D001FF
13 12D0 0000 
160,000,000

 

How Many Sectors are on Your Hard Drive?

HDDs of exactly    10 GB  have:    19,531,250  sectors.
HDDs of exactly    20 GB  have:    39,062,500  sectors.
HDDs of exactly    30 GB  have:    58,593,750  sectors.
HDDs of exactly    40 GB  have:    78,125,000  sectors.
HDDs of exactly    60 GB  have:  117,187,500  sectors.
HDDs of exactly    80 GB  have:  156,250,000  sectors.
HDDs of exactly  100 GB  have:  195,312,500  sectors.
HDDs of exactly  120 GB  have:  234,375,000  sectors.
HDDs of exactly  140 GB  have:  273,437,500  sectors.
HDDs of exactly  160 GB  have:  312,500,000  sectors.
HDDs of exactly  180 GB  have:  351,562,500  sectors.
HDDs of exactly  200 GB  have:  390,625,000  sectors.
HDDs of exactly  250 GB  have:  488,281,250  sectors.
HDDs of exactly  300 GB  have:  585,937,500  sectors.
HDDs of exactly  320 GB  have:  625,000,000  sectors.
HDDs of exactly  500 GB  have:  976,562,500  sectors.

HDDs of exactly  640 GB  have: 1,250,000,000  sectors.
HDDs of exactly  750 GB  have: 1,464,843,750  sectors.
HDDs of exactly  860 GB  have: 1,679,687,500  sectors.
HDDs of exactly   1.0 TB  have: 1,953,125,000  sectors.
HDDs of exactly   1.5 TB  have: 2,929,687,500  sectors.
HDDs of exactly   2.0 TB  have: 3,906,250,000  sectors.
HDDs of exactly   3.0 TB  have: 5,859,375,000  sectors.
This data depends upon 1 GB = 1,000,000,000 bytes, and 512-byte sectors.


But real hard disks rarely contain the exact values above!
Drive manufacturers always round off to the nearest 10 or even 100 GB.
Thus, our WDC WD800JB-00ETA0 "80 GB" disk actually contains up to
156,301,488 accessible sectors; or, 80.026361856 GB.
However, due to the geometrical constraints of Microsoft's Basic Disk layout (of 255 heads and 63 sectors per head), there are 5,104 surplus sectors at the end of the disk that cannot be used by the Windows OS. But this still leaves 80.023748608 GB that can be used. Microsoft OSs often use a Binary GB of 1,073,741,824 bytes (1024 cubed), so it refers to the same size hard disk as having only about 74.53 GB (which should actually use the new standard abbreviation of GiB instead).

(Last update: June 7, 2008, and November 12, 2012.)

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