What is the standard ratio of lime, Portland cement and sand to make mortar?

The standard ratio for general purpose mortar, type N mortar, is one part Portland cement, one part lime and six parts sand. Type N mortar is typically used for exterior walls above ground level that are exposed to the elements. It is also appropriate for interior load-bearing walls.

Within about a month, the strength of type N mortar is between 1500 and 2400 pounds per square inch. Other types of mortar are appropriate to different situations. For instance, type S mortar is often used underground to deal with soil pressure or seismic vibrations. Type O works for non-load bearing walls, while type M is needed for walls that support heavy loads.