TTF Byte Layout Reference

TrueType (TTF) Byte Layout Reference

A minimal, quick-reference for the byte offsets and field layouts of every core TTF table. Big-endian throughout. For parsing strategy and walkthroughs, see Parse a Font.

File header

Offset table (sfnt header) — 12 bytes at byte 0:

offset	type	field	notes
0	u32	sfntVersion	`0x00010000` (TrueType) or `0x4F54544F` (OTTO)
4	u16	numTables	count of tables that follow
6	u16	searchRange	binary-search hint; recompute if modifying
8	u16	entrySelector	binary-search hint
10	u16	rangeShift	binary-search hint

Table directory — numTables records of 16 bytes each, starting at byte 12:

offset	type	field	notes
0	tag[4]	tableTag	e.g. `head`, `glyf` (ASCII)
4	u32	checksum	table checksum
8	u32	offset	from file start
12	u32	length	real length; table padded to 4-byte bound

Record i starts at byte 12 + i×16.

Tables

head — Font header (54 bytes, fixed)

Global font metrics and configuration.

offset	type	field	notes
0	u16	majorVersion	1
2	u16	minorVersion	0
4	i32	fontRevision	16.16 fixed-point
8	u32	checksumAdjustment	whole-file sum
12	u32	magicNumber	must be `0x5F0F3CF5`
16	u16	flags	bit field
18	u16	unitsPerEm	design grid (e.g. 1000, 2048)
20	i64	created	seconds since 1904-01-01 00:00 UTC
28	i64	modified	same epoch
36	i16	xMin	font bbox
38	i16	yMin
40	i16	xMax
42	i16	yMax
44	u16	macStyle	bit field
46	u16	lowestRecPPEM	smallest readable size in pixels
48	i16	fontDirectionHint
50	i16	indexToLocFormat	0 = short loca, 1 = long loca
52	i16	glyphDataFormat	0

maxp — Maximum profile

Memory requirements. Version 0.5 (CFF, 6 bytes):

offset	type	field	notes
0	u32	version	`0x00005000`
4	u16	numGlyphs	critical

Version 1.0 (TrueType, 32 bytes):

offset	type	field	notes
0	u32	version	`0x00010000`
4	u16	numGlyphs	glyph count
6	u16	maxPoints	max in simple glyph
8	u16	maxContours	max in simple glyph
10	u16	maxCompositePoints	max in composite glyph
12	u16	maxCompositeContours	max in composite glyph
14	u16	maxZones	1 or 2
16	u16	maxTwilightPoints
18	u16	maxStorage
20	u16	maxFunctionDefs
22	u16	maxInstructionDefs
24	u16	maxStackElements
26	u16	maxSizeOfInstructions
28	u16	maxComponentElements
30	u16	maxComponentDepth

hhea — Horizontal header (36 bytes, fixed)

Metrics for horizontal layout.

offset	type	field	notes
0	u16	majorVersion	1
2	u16	minorVersion	0
4	i16	ascender
6	i16	descender	usually negative
8	i16	lineGap
10	u16	advanceWidthMax
12	i16	minLeftSideBearing
14	i16	minRightSideBearing
16	i16	xMaxExtent
18	i16	caretSlopeRise	1 for vertical
20	i16	caretSlopeRun	0 for vertical
22	i16	caretOffset	0 for non-slanted
24	i16	reserved[4]	zero
32	i16	metricDataFormat	0
34	u16	numberOfHMetrics	sizes hmtx table

hmtx — Horizontal metrics (variable)

No header. Two back-to-back arrays:

Part 1: numberOfHMetrics (from hhea) entries of:

offset	type	field	notes
0	u16	advanceWidth
2	i16	lsb	left side bearing

Part 2: numGlyphs − numberOfHMetrics entries of:

offset	type	field	notes
0	i16	lsb	reuses last advance width

loca — Location index (variable)

Maps glyph index → byte offset in glyf table. numGlyphs + 1 offsets (the +1 marks end of last glyph).

Short format (head.indexToLocFormat == 0): uint16 offsets (multiply by 2 to get real byte offset).

Long format (head.indexToLocFormat == 1): uint32 offsets (use directly).

If loca[i] == loca[i+1], glyph i is empty (e.g. space character).

glyf — Glyph data (variable)

No header. Glyph blocks back to back, located by loca.

Glyph header (10 bytes):

offset	type	field	notes
0	i16	numberOfContours	≥ 0 = simple; < 0 = composite
2	i16	xMin	glyph bbox
4	i16	yMin
6	i16	xMax
8	i16	yMax

Simple glyph (numberOfContours ≥ 0):

offset	type	field	notes
10	u16[numberOfContours]	endPtsOfContours	last+1 = point count
…	u16	instructionLength	bytes of hinting
…	u8[instructionLength]	instructions	TrueType bytecode
…	u8[…]	flags	one logical flag per point, compressed
…	—	xCoordinates	delta-encoded
…	—	yCoordinates	delta-encoded

Flag bits (each point):

bit	name	meaning
0x01	ON_CURVE_POINT	on curve (else off-curve control point)
0x02	X_SHORT_VECTOR	x delta is 1 byte
0x04	Y_SHORT_VECTOR	y delta is 1 byte
0x08	REPEAT_FLAG	next byte = repeat count for this flag
0x10	X_IS_SAME_OR_POSITIVE	(see spec)
0x20	Y_IS_SAME_OR_POSITIVE	(see spec)
0x40	OVERLAP_SIMPLE	contours may overlap
0x80	reserved	0

Composite glyph (numberOfContours < 0):

Loop of components. Each:

offset	type	field	notes
0	u16	flags	component flags (below)
2	u16	glyphIndex	which glyph to reference
4	—	arg1, arg2	int8/uint8 or int16/uint16 per ARG_1_AND_2_ARE_WORDS
…	—	transform	0, 1, 2, or 4 × F2Dot14 per scale flags

Component flag bits:

bit	name	meaning
0x0001	ARG_1_AND_2_ARE_WORDS	args are 16-bit
0x0002	ARGS_ARE_XY_VALUES	args are offsets (else indices)
0x0004	ROUND_XY_TO_GRID
0x0008	WE_HAVE_A_SCALE	one F2Dot14 follows
0x0020	MORE_COMPONENTS	another component after this
0x0040	WE_HAVE_AN_X_AND_Y_SCALE	two F2Dot14 follow
0x0080	WE_HAVE_A_TWO_BY_TWO	four F2Dot14 (matrix)
0x0100	WE_HAVE_INSTRUCTIONS	after last component: u16 length + bytes
0x0200	USE_MY_METRICS
0x0400	OVERLAP_COMPOUND

cmap — Character to glyph mapping (variable)

Maps Unicode → glyph index. Multiple subtables; pick the best for your case.

Header (4 bytes):

offset	type	field
0	u16	version
2	u16	numTables

Encoding records (numTables of 8 bytes each):

offset	type	field	notes
0	u16	platformID	0=Unicode, 1=Mac, 3=Windows
2	u16	encodingID	platform-specific
4	u32	subtableOffset	from cmap table start

Common: (3,1) → format 4 (BMP); (3,10) → format 12 (full Unicode).

Format 0 (byte encoding, 256 glyphs):

offset	type	field
0	u16	format
2	u16	length
4	u16	language
6	u8[256]	glyphIdArray

Format 4 (segment mapping, BMP):

offset	type	field
0	u16	format
2	u16	length
4	u16	language
6	u16	segCountX2
8	u16	searchRange
10	u16	entrySelector
12	u16	rangeShift
14+	u16[segCount]	endCode
…	u16	reserved
…	u16[segCount]	startCode
…	i16[segCount]	idDelta
…	u16[segCount]	idRangeOffset
…	u16[…]	glyphIdArray

Format 12 (full Unicode coverage):

offset	type	field
0	u16	format
2	u16	reserved
4	u32	length
8	u32	language
12	u32	numGroups
16+	—	groups (numGroups entries, 12 bytes each)

Each group: { u32 startCharCode; u32 endCharCode; u32 startGlyphID }.

name — Font names (variable)

Human-readable strings (family, style, version, etc.). Stored once in a byte pool; records point in with metadata.

Header (6 bytes):

offset	type	field	notes
0	u16	version	0 or 1
2	u16	count	number of name records
4	u16	storageOffset	start of string pool from table start

Name records (count of 12 bytes each):

offset	type	field	notes
0	u16	platformID	0=Unicode, 3=Windows
2	u16	encodingID	platform-specific
4	u16	languageID	platform/language tag
6	u16	nameID	1=family, 2=subfamily, 4=full, 5=version
8	u16	length	string length in bytes
10	u16	stringOffset	from `storageOffset`

Then raw string bytes per platform (typically UTF-16BE for Windows).

post — PostScript data (variable)

Italic angle, monospace flag, PostScript names (v2.0 only).

Fixed header (32 bytes):

offset	type	field	notes
0	u32	version	`0x00010000`, `0x00020000`, or `0x00030000`
4	i32	italicAngle	16.16 fixed-point
8	i16	underlinePosition
10	i16	underlineThickness
12	u32	isFixedPitch	0 or nonzero
16	u32	minMemType42
20	u32	maxMemType42
24	u32	minMemType1
28	u32	maxMemType1

Version 2.0 only (after header):

offset	type	field
32	u16	numGlyphs
34	u16[numGlyphs]	glyphNameIndex
…	—	Pascal-string names for indices ≥ 258

OS/2 — OS/2 and Windows metrics (version-dependent)

Font classification, weight, width, coverage, PANOSE, embedding flags.

Version 0 (78 bytes):

offset	type	field	notes
0	u16	version	0–5
2	i16	xAvgCharWidth
4	u16	usWeightClass	100–900
6	u16	usWidthClass	1–9
8	u16	fsType	embedding flags
10	i16	ySubscriptXSize
12	i16	ySubscriptYSize
14	i16	ySubscriptXOffset
16	i16	ySubscriptYOffset
18	i16	ySuperscriptXSize
20	i16	ySuperscriptYSize
22	i16	ySuperscriptXOffset
24	i16	ySuperscriptYOffset
26	i16	yStrikeoutSize
28	i16	yStrikeoutPosition
30	i16	sFamilyClass
32	u8[10]	panose
42	u32	ulUnicodeRange1	bits 0–31
46	u32	ulUnicodeRange2	bits 32–63
50	u32	ulUnicodeRange3	bits 64–95
54	u32	ulUnicodeRange4	bits 96–127
58	tag[4]	achVendID
62	u16	fsSelection	style flags
64	u16	usFirstCharIndex
66	u16	usLastCharIndex
68	i16	sTypoAscender
70	i16	sTypoDescender
72	i16	sTypoLineGap
74	u16	usWinAscent
76	u16	usWinDescent

Version 1 adds (86 bytes):

offset	type	field
78	u32	ulCodePageRange1
82	u32	ulCodePageRange2

Version 2/3/4 add (96 bytes):

offset	type	field
86	i16	sxHeight
88	i16	sCapHeight
90	u16	usDefaultChar
92	u16	usBreakChar
94	u16	usMaxContext

Version 5 adds (100 bytes):

offset	type	field
96	u16	usLowerOpticalPointSize
98	u16	usUpperOpticalPointSize

kern — Kerning pairs (optional, variable)

Spacing adjustments between glyph pairs. Optional; modern fonts use GPOS instead.