sbcl (and other Common Lisps) uses what you want it to use: read-default-float-format (setf read-default-float-format 'single-float) (+ 0.1 0.2) => 0.3 (setf

sbcl (and other Common Lisps) uses what you want it to use: read-default-float-format (setf read-default-float-format 'single-float) (+ 0.1 0.2) => 0.3 (setf read-default-float-format 'double-float) (+ 0.1 0.2) => 0.30000000000000004 On LispWorks 7.0 I get 0.30000000000000005 for double-float. Hmm.