diff --git a/kepub/convert.go b/kepub/convert.go
index 4fabe0e..8a8ff84 100644
--- a/kepub/convert.go
+++ b/kepub/convert.go
@@ -32,10 +32,16 @@ func (c *Converter) Convert(ctx context.Context, w io.Writer, r fs.FS) error {
 		FileActionTransformOPF     = 3
 	)
 
+	p := ctxProgress(ctx)
+
 	if tmp, ok := r.(*zip.ReadCloser); ok {
 		r = &tmp.Reader
 	}
 
+	if p != nil {
+		p(true, 0, 0)
+	}
+
 	var files []*zip.FileHeader
 	if zr, ok := r.(*zip.Reader); ok {
 		// if it's a zip, preserve the original FileHeaders
@@ -268,6 +274,7 @@ func (c *Converter) Convert(ctx context.Context, w io.Writer, r fs.FS) error {
 	}
 
 	// write the files
+	var n int
 	for of := range output {
 		if of.Index == -1 {
 			if err := zipReplace(zw, of.Header, of.Bytes); err != nil {
@@ -296,6 +303,10 @@ func (c *Converter) Convert(ctx context.Context, w io.Writer, r fs.FS) error {
 			b.Reset()
 			pool.Put(b)
 		}
+		if p != nil {
+			n++
+			p(false, n, len(files))
+		}
 	}
 	if err := g.Wait(); err != nil {
 		return err
@@ -306,6 +317,9 @@ func (c *Converter) Convert(ctx context.Context, w io.Writer, r fs.FS) error {
 		return fmt.Errorf("finalize output EPUB: %w", err)
 	}
 
+	if p != nil {
+		p(true, n, n)
+	}
 	return nil
 }
 
diff --git a/kepub/kepub.go b/kepub/kepub.go
index 05987c6..333b78c 100644
--- a/kepub/kepub.go
+++ b/kepub/kepub.go
@@ -1,6 +1,11 @@
 // Package kepub converts EPUBs to KEPUBs.
 package kepub
 
+import (
+	"context"
+	"math"
+)
+
 // Converter converts EPUB2/EPUB3 books to Kobo's KEPUB format.
 type Converter struct {
 	// extra css
@@ -120,3 +125,50 @@ body>div {
     padding-left: 0.2em !important;
     padding-right: 0.2em !important;
 }`
+
+// These are for use by certain kepubify frontends for progress information
+// during conversions. It is not exported for general use, must be imported via
+// an unsafe go:linkname directive, and is subject to change.
+type (
+	progressKey      struct{}
+	progressDeltaKey struct{}
+)
+
+// withProgress adds a function to be called synchronously as the conversion
+// progresses. If delta is in the range [0, 1], the callback will be
+// rate-limited to when there is an important change or when the percentage
+// changes by more than delta.
+func withProgress(ctx context.Context, delta float64, fn func(n, total int)) context.Context {
+	ctx = context.WithValue(ctx, progressDeltaKey{}, delta)
+	ctx = context.WithValue(ctx, progressKey{}, fn)
+	return ctx
+}
+
+// ctxProgress creates a rate-limited progress callback for the provided
+// context. It returns nil if a callback has not been set.
+func ctxProgress(ctx context.Context) func(force bool, n, total int) {
+	if v := ctx.Value(progressKey{}); v != nil {
+		fn := v.(func(n, total int))
+		dt := ctx.Value(progressDeltaKey{}).(float64)
+
+		var pct, lastPct float64
+		return func(force bool, n, total int) {
+			if dt < 0 || dt > 1 {
+				fn(n, total)
+				return
+			}
+			if n == 0 && total == 0 {
+				pct = 1
+			} else if n == 0 || total == 0 {
+				pct = 0
+			} else {
+				pct = float64(n) / float64(total)
+			}
+			if force || math.Abs(pct-lastPct) >= dt {
+				lastPct = pct
+				fn(n, total)
+			}
+		}
+	}
+	return nil
+}