diff --git a/gtsam/nonlinear/NonlinearFactor.h b/gtsam/nonlinear/NonlinearFactor.h
index f6869140c..e1cfb2bc3 100644
--- a/gtsam/nonlinear/NonlinearFactor.h
+++ b/gtsam/nonlinear/NonlinearFactor.h
@@ -124,6 +124,9 @@ public:
    */
   virtual double error(const Values& c) const = 0;
 
+  /** get the dimension of the factor (number of rows on linearization) */
+  virtual size_t dim() const = 0;
+
 	/**
 	 * Checks whether a factor should be used based on a set of values.
 	 * This is primarily used to implment inequality constraints that
diff --git a/gtsam/slam/AntiFactor.h b/gtsam/slam/AntiFactor.h
index 890e5beee..1e0d64db7 100644
--- a/gtsam/slam/AntiFactor.h
+++ b/gtsam/slam/AntiFactor.h
@@ -78,6 +78,9 @@ namespace gtsam {
 	   */
 	  double error(const Values& c) const { return -factor_->error(c); }
 
+	  /** get the dimension of the factor (same as the original factor) */
+	  size_t dim() const { return factor_->dim(); }
+
 	  /**
 	   * Checks whether this factor should be used based on a set of values.
 	   * The AntiFactor will have the same 'active' profile as the original factor.