What is the Keynesian multiplier formula?

Multiplier = 1/(1-MPC) is the Keynesian multiplier formula. MPC is marginal propensity to consume.You can read about the Money Supply in Economy – Types of Money, Monetary Aggregates, Money Supply Control in the given link.

Further readings:

1. Monetary Policy – Objectives, Roles and Instruments (UPSC Indian Economy)
2. Topic-Wise GS 3 Questions for UPSC Mains