The union will tell the McCrone inquiry that teachers already work an average of 41 hours and 15 minutes a week, almost 12 hours beyond what they are contracted to.
The figure includes 39.2 hours of teaching duties and 2.05 hours for planned activity and parent meetings. Senior staff work between 46 and 51 hours, principal teachers and their assistants 41-44 hours and npromoted teachers 40 hours.
David Eaglesham, the association's general secretary, said: "Scottish secondary teachers undertake the great majority of preparation and correction, reporting and administration, and curriculum development outwith normal school hours.
"The figures will come as no surprise to Scottish secondary teachers. The extent of the burden of workload has been a repeated theme from teachers over many years, but as always, it is so difficult to quantify."