Can someone help me please with planting density. I am planting a large public park and the professional planting plan specifies 'Density' as plants/ square metre. How does this convert to plant spacing?

The densities given are 3, 4, 5 and 6 plants per square metre. I need to know how far apart to put the plants. I have tried to calculate this but don't believe my calculations. Is there a table anywhere?