diff --git a/src/restic/backend/s3/s3.go b/src/restic/backend/s3/s3.go
index bf340e163..0a3fcccc1 100644
--- a/src/restic/backend/s3/s3.go
+++ b/src/restic/backend/s3/s3.go
@@ -3,6 +3,7 @@ package s3
 import (
 	"fmt"
 	"io"
+	"os"
 	"path"
 	"restic"
 	"strings"
@@ -84,6 +85,35 @@ type Sizer interface {
 	Size() int64
 }
 
+type Lenner interface {
+	Len() int
+}
+
+// getRemainingSize returns number of bytes remaining. If it is not possible to
+// determine the size, panic() is called.
+func getRemainingSize(rd io.Reader) (size int64, err error) {
+	if r, ok := rd.(Lenner); ok {
+		size = int64(r.Len())
+	} else if r, ok := rd.(Sizer); ok {
+		size = r.Size()
+	} else if f, ok := rd.(*os.File); ok {
+		fi, err := f.Stat()
+		if err != nil {
+			return 0, err
+		}
+
+		pos, err := f.Seek(0, io.SeekCurrent)
+		if err != nil {
+			return 0, err
+		}
+
+		size = fi.Size() - pos
+	} else {
+		panic(fmt.Sprintf("Save() got passed a reader without a method to determine the data size, type is %T", rd))
+	}
+	return size, nil
+}
+
 // Save stores data in the backend at the handle.
 func (be *s3) Save(h restic.Handle, rd io.Reader) (err error) {
 	if err := h.Valid(); err != nil {
@@ -91,12 +121,9 @@ func (be *s3) Save(h restic.Handle, rd io.Reader) (err error) {
 	}
 
 	objName := be.Filename(h)
-
-	var size int64
-	if r, ok := rd.(Sizer); ok {
-		size = r.Size()
-	} else {
-		panic("Save() got passed a reader without a method to determine the data size")
+	size, err := getRemainingSize(rd)
+	if err != nil {
+		return err
 	}
 
 	debug.Log("Save %v at %v", h, objName)
diff --git a/src/restic/backend/s3/s3_internal_test.go b/src/restic/backend/s3/s3_internal_test.go
new file mode 100644
index 000000000..3b0a7eb2c
--- /dev/null
+++ b/src/restic/backend/s3/s3_internal_test.go
@@ -0,0 +1,66 @@
+package s3
+
+import (
+	"bytes"
+	"io"
+	"io/ioutil"
+	"os"
+	"restic/test"
+	"testing"
+)
+
+func writeFile(t testing.TB, data []byte, offset int64) *os.File {
+	tempfile, err := ioutil.TempFile("", "restic-test-")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err = os.Remove(tempfile.Name()); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err = tempfile.Write(data); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err = tempfile.Seek(offset, io.SeekStart); err != nil {
+		t.Fatal(err)
+	}
+
+	return tempfile
+}
+
+func TestGetRemainingSize(t *testing.T) {
+	length := 18 * 1123
+	partialRead := 1005
+
+	data := test.Random(23, length)
+
+	partReader := bytes.NewReader(data)
+	buf := make([]byte, partialRead)
+	_, _ = io.ReadFull(partReader, buf)
+
+	partFileReader := writeFile(t, data, int64(partialRead))
+
+	var tests = []struct {
+		io.Reader
+		size int64
+	}{
+		{bytes.NewReader([]byte("foobar test")), 11},
+		{partReader, int64(length - partialRead)},
+		{partFileReader, int64(length - partialRead)},
+	}
+
+	for _, test := range tests {
+		t.Run("", func(t *testing.T) {
+			size, err := getRemainingSize(test.Reader)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			if size != test.size {
+				t.Fatalf("invalid size returned, want %v, got %v", test.size, size)
+			}
+		})
+	}
+}