diff --git a/internal/sync/kepub_cfi_converter.go b/internal/sync/kepub_cfi_converter.go
new file mode 100644
index 0000000..29bc49b
--- /dev/null
+++ b/internal/sync/kepub_cfi_converter.go
@@ -0,0 +1,283 @@
+package sync
+
+import (
+	"fmt"
+	"log"
+
+	"golang.org/x/net/html"
+)
+
+type KEPUBCFIConverter struct {
+	epubConverter  *CFIConverter
+	kepubConverter *CFIConverter
+}
+
+type KEPUBConversionResult struct {
+	CFI        string
+	Percentage float64
+	Precision  string
+}
+
+func NewKEPUBCFIConverter(epubPath, kepubPath string) *KEPUBCFIConverter {
+	return &KEPUBCFIConverter{
+		epubConverter:  NewCFIConverter(epubPath),
+		kepubConverter: NewCFIConverter(kepubPath),
+	}
+}
+
+func (k *KEPUBCFIConverter) ConvertKEPUBCFIToStandard(kepubCFI string, percentage float64, contextText string) (*KEPUBConversionResult, error) {
+	if !IsStandardEPUBCFI(kepubCFI) {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	spineIndex, steps, err := parseEPUBCFI(kepubCFI)
+	if err != nil {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	kepubDoc, _, docErr := k.kepubConverter.getContentDoc(spineIndex + 1)
+	if docErr != nil {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	kepubTextNode, kepubOffset, resolveErr := resolveCFIToNode(kepubDoc, steps)
+	if resolveErr != nil {
+		return k.kepubToStandardByPercentage(spineIndex, percentage)
+	}
+
+	var searchText string
+	if contextText != "" {
+		searchText = normalizeWhitespace(contextText)
+	} else if kepubTextNode != nil && kepubTextNode.Type == html.TextNode {
+		searchText = extractSurroundingText(kepubTextNode, kepubOffset, 80)
+	}
+
+	if searchText != "" {
+		epubDoc, _, docErr := k.epubConverter.getContentDoc(spineIndex + 1)
+		if docErr == nil {
+			epubBody := findBody(epubDoc)
+			if epubBody != nil {
+				matchNode, matchOffset := findTextInNode(epubBody, searchText)
+				if matchNode != nil {
+					cfi, buildErr := buildCFI(spineIndex, matchNode, matchOffset)
+					if buildErr == nil && cfi != "" {
+						return &KEPUBConversionResult{
+							CFI:        cfi,
+							Percentage: percentage,
+							Precision:  "exact",
+						}, nil
+					}
+				}
+			}
+		}
+	}
+
+	return k.kepubToStandardByPercentage(spineIndex, percentage)
+}
+
+func (k *KEPUBCFIConverter) ConvertStandardCFIToKEPUB(standardCFI string, percentage float64, contextText string) (*KEPUBConversionResult, error) {
+	if !IsStandardEPUBCFI(standardCFI) {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	spineIndex, steps, err := parseEPUBCFI(standardCFI)
+	if err != nil {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	epubDoc, _, docErr := k.epubConverter.getContentDoc(spineIndex + 1)
+	if docErr != nil {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	epubTextNode, epubOffset, resolveErr := resolveCFIToNode(epubDoc, steps)
+	if resolveErr != nil {
+		return k.standardToKEPUBByPercentage(spineIndex, percentage)
+	}
+
+	var searchText string
+	if contextText != "" {
+		searchText = normalizeWhitespace(contextText)
+	} else if epubTextNode != nil && epubTextNode.Type == html.TextNode {
+		searchText = extractSurroundingText(epubTextNode, epubOffset, 80)
+	}
+
+	if searchText != "" {
+		kepubDoc, _, docErr := k.kepubConverter.getContentDoc(spineIndex + 1)
+		if docErr == nil {
+			kepubBody := findBody(kepubDoc)
+			if kepubBody != nil {
+				matchNode, matchOffset := findTextInNode(kepubBody, searchText)
+				if matchNode != nil {
+					cfi, buildErr := buildCFI(spineIndex, matchNode, matchOffset)
+					if buildErr == nil && cfi != "" {
+						return &KEPUBConversionResult{
+							CFI:        cfi,
+							Percentage: percentage,
+							Precision:  "exact",
+						}, nil
+					}
+				}
+			}
+		}
+	}
+
+	return k.standardToKEPUBByPercentage(spineIndex, percentage)
+}
+
+func (k *KEPUBCFIConverter) kepubToStandardByPercentage(spineIndex int, percentage float64) (*KEPUBConversionResult, error) {
+	if percentage <= 0 {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	epubDoc, _, docErr := k.epubConverter.getContentDoc(spineIndex + 1)
+	if docErr != nil {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	epubBody := findBody(epubDoc)
+	if epubBody == nil {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	totalChars := countTextChars(epubBody)
+	if totalChars <= 0 {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	targetOffset := int(float64(totalChars) * percentage)
+	targetNode, foundOffset := findNodeAtCharOffset(epubBody, targetOffset)
+	if targetNode == nil {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	charInNode := targetOffset - foundOffset
+	cfi, err := buildCFI(spineIndex, targetNode, charInNode)
+	if err != nil || cfi == "" {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	return &KEPUBConversionResult{
+		CFI:        cfi,
+		Percentage: percentage,
+		Precision:  "percentage",
+	}, nil
+}
+
+func (k *KEPUBCFIConverter) standardToKEPUBByPercentage(spineIndex int, percentage float64) (*KEPUBConversionResult, error) {
+	if percentage <= 0 {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	kepubDoc, _, docErr := k.kepubConverter.getContentDoc(spineIndex + 1)
+	if docErr != nil {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	kepubBody := findBody(kepubDoc)
+	if kepubBody == nil {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	totalChars := countTextChars(kepubBody)
+	if totalChars <= 0 {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	targetOffset := int(float64(totalChars) * percentage)
+	targetNode, foundOffset := findNodeAtCharOffset(kepubBody, targetOffset)
+	if targetNode == nil {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	charInNode := targetOffset - foundOffset
+	cfi, err := buildCFI(spineIndex, targetNode, charInNode)
+	if err != nil || cfi == "" {
+		return &KEPUBConversionResult{Percentage: percentage, Precision: "percentage"}, nil
+	}
+
+	return &KEPUBConversionResult{
+		CFI:        cfi,
+		Percentage: percentage,
+		Precision:  "percentage",
+	}, nil
+}
+
+func extractSurroundingText(textNode *html.Node, offset int, window int) string {
+	if textNode == nil || textNode.Type != html.TextNode {
+		return ""
+	}
+
+	text := textNode.Data
+	runes := []rune(text)
+
+	start := offset - window
+	if start < 0 {
+		start = 0
+	}
+	end := offset + window
+	if end > len(runes) {
+		end = len(runes)
+	}
+
+	if start >= end {
+		return normalizeWhitespace(text)
+	}
+
+	return normalizeWhitespace(string(runes[start:end]))
+}
+
+func (k *KEPUBCFIConverter) ComputeKEPUBPercentage(kepubCFI string) (float64, error) {
+	if !IsStandardEPUBCFI(kepubCFI) {
+		return -1, fmt.Errorf("not a standard epubcfi")
+	}
+
+	spineIndex, steps, err := parseEPUBCFI(kepubCFI)
+	if err != nil {
+		return -1, err
+	}
+
+	kepubDoc, _, docErr := k.kepubConverter.getContentDoc(spineIndex + 1)
+	if docErr != nil {
+		return -1, docErr
+	}
+
+	kepubBody := findBody(kepubDoc)
+	if kepubBody == nil {
+		return -1, fmt.Errorf("no body in kepub doc")
+	}
+
+	textNode, textOffset, resolveErr := resolveCFIToNode(kepubBody, steps)
+	if resolveErr != nil {
+		return -1, resolveErr
+	}
+
+	charOffset := countTextCharsBefore(textNode) + textOffset
+
+	total := 0
+	kepubSpine, spineErr := k.kepubConverter.loadSpine()
+	if spineErr != nil {
+		return -1, spineErr
+	}
+
+	for i := range kepubSpine.items {
+		doc, _, dErr := k.kepubConverter.getContentDoc(i + 1)
+		if dErr != nil {
+			continue
+		}
+		b := findBody(doc)
+		if b != nil {
+			total += countTextChars(b)
+		}
+	}
+
+	if total <= 0 {
+		return -1, fmt.Errorf("no text in kepub")
+	}
+
+	return float64(charOffset) / float64(total), nil
+}
+
+func init() {
+	_ = log.Printf
+}