The square area of any foursided space derives from the measurement and multiplication of the space's linear length by its linear width. However, if you need to measure other shapes and polygons (more than four sides or an irregularly shaped space) you must use the geometric formula for each of the area's polygons in order to calculate it. Calculating an area's square meters involves the application of some standard geometry and polygon formulas and simple arithmetic (multiplication, division, addition).
Things You'll Need
 Metric measuring device
 Calculator

Assess the area you intend to measure to determine whether or not you can reduce it to one or more regular shapes (square, rectangle, triangle, rhombus, trapezoid, parallelogram or regular polygon). Subdivide any irregular shapes into smaller regular shapes.

Select the scale for your area measurement. Use the lowest common denominator (for example, centimeters instead of meters, or meters instead of kilometers) as your scale in order to ensure the highest precision in your results.

Measure each regular shape using the metric measuring device and apply the geometric formula for it (see Thinkquest, in Resources). If there are multiple polygons that contribute to a single area, do the calculations for each polygon and then add all the calculated results together to find the complete square meter area.

Apply any necessary scaling conversion. If your scaling is centimeters, divide the result by 100 to find square meters. If your scaling is kilometers, multiply the result by 1,000 to find square meters.
References
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