Larry Williams' WillTrend indicator, though, is not the one pointed out in the first post by: noumann, because this is the indicator (...it resembles a SuperTrend):



so the formula could be this:

-----------------------------------------------------------------------

{WillTrend - by Larry Williams}

Inputs:
Period(66),
ATRMultiplication(2.236),
MidPriceVar(10);

Variables:
upperBand(0), lowerBand(0), atr(0), trail(0),
midPrice1(0), midPrice2(0), midPrice(0);

atr = AvgTrueRange(Period) * ATRMultiplication;


midPrice1=Lowest(low,MidPriceVar);
midPrice2= Highest(high,MidPriceVar);
midPrice= (midPrice1 + midPrice2)/2;

lowerBand= midPrice + atr;
upperBand= midPrice - atr;


trail = IFF(Close > trail[1] and Close[1] > trail[1], MaxList(trail[1], upperBand),
IFF(Close < trail[1] and Close[1] < trail[1], MinList(trail[1], lowerBand),
IFF(Close > trail[1], upperBand, lowerBand)));


Plot1(trail);

if (Close > trail) then
SetPlotColor(1, green)
else
SetPlotColor(1, red);

------------------------------------------------------------------------

I know this is an old Post, but I have not been able to find an answer here or anywhere. And this post comes up first on my web search. I have the answer. Here it is:
upperBand = ((high-low)/2 + 2.236) * atr(66)
lowerBand = ((high-low)/2 - 2.236) * atr(66)

If your programming language does its math from left you right, you WON'T need the parenthesis.