on the back of an envelope

on the back of an envelope

Calculated, prepared, or drafted in an overly quick, informal, or rough manner, as might be scribbled on a scrap of paper (such as the back of an envelope). It seems clear that they devised this legislation on the back of an envelope. Now, don't come up with these figures on the back of an envelope—I want you to be thorough and double check each sum and calculation.
See also: back, envelope, of, on

on the back of an envelope

If a piece of work was written or planned on the back of an envelope, it was prepared too quickly, with little thought or care. These proposals were sketched out on the back of an envelope. The screenplay sounds as if it's been written on the back of an envelope. Note: You can also talk about a back-of-an-envelope calculation or account to talk about something that is not accurate. According to his back-of-an-envelope calculation, a solar square of mirrors of this size would provide enough energy to supply the whole state.
See also: back, envelope, of, on