diff --git a/layout/style/nsComputedDOMStyle.cpp b/layout/style/nsComputedDOMStyle.cpp
index 723afa35fa21..d85f2dcbacd6 100644
--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -3541,9 +3541,11 @@ nsComputedDOMStyle::GetLineHeightCoord(nscoord& aCoord)
   // font->mSize as the font size.  Adjust for that.  Also adjust for
   // the text zoom, if any.
   const nsStyleFont* font = GetStyleFont();
-  aCoord = NSToCoordRound((float(aCoord) *
-                           (float(font->mSize) / float(font->mFont.size))) /
-                          mPresShell->GetPresContext()->TextZoom());
+  float fCoord = float(aCoord) / mPresShell->GetPresContext()->TextZoom();
+  if (font->mFont.size != font->mSize) {
+    fCoord = fCoord * (float(font->mSize) / float(font->mFont.size));
+  }
+  aCoord = NSToCoordRound(fCoord);
 
   return true;
 }