It cannot be done.
A millimetre is a measure of length or distance while a square metre is a measure of area. The two measure different things and, according to basic principles of dimensional analysis, conversion from one to the other is not valid.
To find out how many 20mm x 20mm squares fit in a square meter, first convert square meters to square millimeters. One square meter equals 1,000,000 square millimeters. Each 20mm x 20mm square has an area of 400 square millimeters. Dividing 1,000,000 by 400 gives you 2,500, so there are 2,500 of these squares in a square meter.
Multiply by one thousand1 meter = 1000 millimeters
1 meter = 1000 mm. Divide the millimeters by 1000 to convert to meters, then multiply.
You can't convert that. Square feet is a unit of area; meter is a unit of length. You can convert square feet to square meters, or feet to meters.
1 meter = 1000 millimeters = 1,000,000 microns = 0.001 kilometers. Or what do you want to convert it to?