According to the APA Manual, 6th edition, 4.39 Policy on Metrication
"APA uses the metric system in its journals. All references to physical measurements, where feasible, should be expressed in metric units."
"In preparing manuscripts, use metric unites if possible. If you use instruments that record measurements in nonmetric units, you may report the nonmetric units, but also report the established SI equivalents in parentheses immediately after the nonmetric units." E.g. The rod was 3 ft (0.91 m) long.
For abbreviations of some units of measure using APA, click here.
