diff --git a/src/calibre/ebooks/html.py b/src/calibre/ebooks/html.py
index 7af13f21c0..03a0f7cbda 100644
--- a/src/calibre/ebooks/html.py
+++ b/src/calibre/ebooks/html.py
@@ -766,6 +766,8 @@ class Processor(Parser):
                 size = int(font.attrib.pop('size', '3'))
             except:
                 size = 3
+            if size and size.strip() and size.strip()[0] in ('+', '-'):
+                size = 3 + float(size) # Hack assumes basefont=3
             setting = 'font-size: %d%%;'%int((float(size)/3) * 100)
             face = font.attrib.pop('face', None)
             if face is not None: