Very similar to the natural gas pipe sizing calculator except this is for L.P.G. works with metric copper, stainless steel, and semi rigid pipes including 6,8,10,12,15,22,28 mm .

Input the gas rate for the appliances ,the meter lengths of pipe and number of elbows and this will find out if the pipe diameter you picked will have an allowable drop ... if it doesn't, then just change the diameter to see if that will work.
This also has conversion tables to change from any of the figures, m3/hr to kW gross to kW net to KG/hr and will also tell you how many gas bottles are required for an install dependant on their size.