how do i calculate voltage drop in landscape lighting?

Are the lights connected in series or parallel? If in series like Christmas tree lights, divide the voltage by the number of lights and that's your voltage drop per light. If in parallel, voltage drop is dependent mostly on the gauge of your wiring.
It's a little more complicated than just dividing the voltage by the number of lights hooked in series.You also need to consider the wattage of the lamps. A higher wattage lamp drops more voltage than a lower wattage lamp. So you can't divide by the number of lamps in series if you use lamps with different wattages. Instead, multiple the voltage left after you accounted for the wire's voltage drop, times the lamp's wattage divided by the total wattage. Voltage drop occurs in five places. 1) At the primary source 2) At the transformer 3) At the lamps 4) At the wire 5) At the connections The primary voltage drops that you need to be concerned with are at the lamp and on the wire. There is significant voltage drop on the wire in a low voltage system. The formula for voltage drop on the wire is: Vd = A * R * L * 2 Where Vd is voltage drop; A is Amps in the wire; R is resistance in the wire (AWG #12 - 0.00162, AWG #10 - 0.00108, AWG #8 - 0.00064); L is length of wire.