To calculate the square meters of a triangle, you won't need a special measuring device. Square units gauge a triangle's area, the two-dimensional space inside its three sides. After measuring the triangle's sides in meters, you use a triangular area formula to determine the number of square meters in the triangle's area. One such formula is called Heron's formula, and it works for any type of triangle.
Measure all three of the triangle's sides. For an example, assume your triangle's three sides measure 6, 8 and 10 meters.
Add the measurements together to get the perimeter. Then halve that number to determine the semi-perimeter -- 6, 8 and 10 added together equals 24 meters, half of which is 12 meters.
Subtract the three sides separately from the semi-perimeter — 12 - 6 is 6 meters, 12 - 8 is 4 meters and 12 - 10 is 2 meters.
Multiply the three differences from the prior step, and then multiply that product by the semi-perimeter -- 6 * 4 * 2 equals 48 meters; 48 * 12 equals 576 meters.
Calculate the square root of the previous step's final product to find the triangle's area -- the square root of 576 meters is 24, so the area of the triangle is 24 square meters.
If your measurements are standard (feet or yards) rather than metric, you can convert them to metric units using an online conversion program.