Short answer: four poles if you want a border, six if you want a canopy. A 20 ft side is slightly longer than the ideal 15 to 18 ft span, so plan for a pole part way along at least one edge.
Option one: the border, four poles
One pole at each corner, four runs around the perimeter. This is the quickest layout and it frames the space, but the light sits around the edge rather than over the middle. Good if your table is off to one side, or if the patio sits against the house and the house carries one edge.
On a 20 ft side the run is a little long, so expect a deeper droop than on a 15 ft span. If that bothers you, make it six poles with one part way along two opposite sides.
Option two: the canopy, six poles
Four at the corners plus two placed mid way on opposite sides, with runs crossing the middle. This is the one that actually lights the table. Five runs, light landing on the space instead of around it.
Adapting this to your space
- Measure each side you want lit.
- Divide each side by 18 and round up. That is the number of gaps for that side.
- Add the gaps, then add one for the final pole. Corners are shared, so subtract one per shared corner.
- Add two more poles for every run you want crossing the middle.
Before you order
Two things catch people out. First, string lights are usually not included with poles. The poles are the structure. Second, the ground fork needs soil, grass or soft ground. If your 20 x 20 is solid concrete, plan on clamping the poles to a fence, a wall or planters instead, and work out those fixing points before you buy.
Our poles come in packs of 2, 4, 6, 8 and 10, so a 20 x 20 patio is a 4 pack or a 6 pack depending on which layout you pick.