diff --git a/lib/config/size.go b/lib/config/size.go
index 60081b7c9..0388941c5 100644
--- a/lib/config/size.go
+++ b/lib/config/size.go
@@ -87,11 +87,26 @@ func CheckFreeSpace(req Size, usage fs.Usage) error {
 	if req.Percentage() {
 		freePct := (float64(usage.Free) / float64(usage.Total)) * 100
 		if freePct < val {
-			return fmt.Errorf("%f %% < %v", freePct, req)
+			return fmt.Errorf("%.1f %% < %v", freePct, req)
 		}
 	} else if float64(usage.Free) < val {
-		return fmt.Errorf("%v < %v", usage.Free, req)
+		return fmt.Errorf("%sB < %v", formatSI(usage.Free), req)
 	}
 
 	return nil
 }
+
+func formatSI(b int64) string {
+	switch {
+	case b < 1000:
+		return fmt.Sprintf("%d ", b)
+	case b < 1000*1000:
+		return fmt.Sprintf("%.1f K", float64(b)/1000)
+	case b < 1000*1000*1000:
+		return fmt.Sprintf("%.1f M", float64(b)/(1000*1000))
+	case b < 1000*1000*1000*1000:
+		return fmt.Sprintf("%.1f G", float64(b)/(1000*1000*1000))
+	default:
+		return fmt.Sprintf("%.1f T", float64(b)/(1000*1000*1000*1000))
+	}
+}
diff --git a/lib/config/size_test.go b/lib/config/size_test.go
index 598c13981..c567b22c9 100644
--- a/lib/config/size_test.go
+++ b/lib/config/size_test.go
@@ -91,3 +91,42 @@ func TestParseSize(t *testing.T) {
 		}
 	}
 }
+
+func TestFormatSI(t *testing.T) {
+	cases := []struct {
+		bytes  int64
+		result string
+	}{
+		{
+			bytes:  0,
+			result: "0 ", // space for unit
+		},
+		{
+			bytes:  999,
+			result: "999 ",
+		},
+		{
+			bytes:  1000,
+			result: "1.0 K",
+		},
+		{
+			bytes:  1023 * 1000,
+			result: "1.0 M",
+		},
+		{
+			bytes:  5 * 1000 * 1000 * 1000,
+			result: "5.0 G",
+		},
+		{
+			bytes:  50000 * 1000 * 1000 * 1000 * 1000,
+			result: "50000.0 T",
+		},
+	}
+
+	for _, tc := range cases {
+		res := formatSI(tc.bytes)
+		if res != tc.result {
+			t.Errorf("formatSI(%d) => %q, expected %q", tc.bytes, res, tc.result)
+		}
+	}
+}