The images below show 100 square metres (with a tennis court backdrop) to give some idea of scale. 10m x 10m is 100 square metres.
Each grid is 5m by 5m.
0.5 people per square metre (50 people in 100 square metres)
1 person per square metre (100 people in 100 square metres)
1.5 people per square metre (150 people in 100 square metres)
2 people per square metre (200 people in 100 square metres)
2.5 people per square metre (250 people in 100 square metres)
3 people per square metre (300 people in 100 square metres)
3.5 people per square metre (350 people in 100 square metres)
4 people per square metre (400 people in 100 square metres)
4.5 people per square metre (450 people in 100 square metres)
5 people per square metre (500 people in 100 square metres) - Maximum
5.5 people per square metre (550 people in 100 square metres) - HIGH RISK
6 people per square metre (600 people in 100 square metres) - HIGH RISK
From the links below you can view a 10m grid, against a tennis court backdrop, to assist in visualising crowd densities and crowd sizes. I'm going to produce a whole range of crowds against backdrops to act as both a training aid and to provide reference visuals for assessing both density and crowd numbers.
Tennis court - reference crowd 100 square metres 1
Tennis court - reference crowd 100 square metres 3
Tennis court - reference crowd 100 square metres 4
Tennis court - reference crowd 100 square metres 5
Tennis court - reference crowd 100 square metres 6
Tennis court - reference crowd 625 square metres 1
Tennis court - reference crowd 625 square metres 2
Tennis court - reference crowd 625 square metres 3
Tennis court - reference crowd 625 square metres 4
Tennis court - reference crowd 625 square metres 5