What is the formula for converting watts to foot-candles?

There is no formula for converting watts to foot-candles because the two measure different things. Watts are a unit of measurement for power while foot-candles are units of measurement for the amount of light something generates.

Watts describe the amount of work done over a certain amount of time, or power; 1 watt is 1 joule per second. Foot-candles refer to the amount of light given off from a light source at the distance of 1 foot; a typical tea candle or birthday candle gives off approximately 1 foot-candle at a distance of 1 foot. Because watts and foot-candles describe different things, they can't be equated to one another.