Counting every organism in a large area is not practical, so we sample with quadrats and scale up. First, count the organisms in each quadrat. Then work out the mean number per quadrat: mean = total number counted divided by the number of quadrats.
For example, a quadrat with an area of 0.25 m2 gives counts of 4, 6, 3, 7 and 5 in five samples. The total is 25, so the mean is 5 per quadrat. This is 20 per square metre, because 5 divided by 0.25 is 20.
To estimate the population in the whole area, use: population = mean per quadrat × (total area divided by the area of one quadrat). If the field is 400 m2, it contains 400 divided by 0.25 = 1600 quadrat areas, so the estimate is 5 × 1600 = 8000 organisms. Similarly, 20 per square metre in an area of 50 m2 gives 20 × 50 = 1000 organisms.
Some plants, such as grass, are hard to count one by one. For these, estimate the percentage cover: the percentage of the quadrat covered by the plant. The more quadrats you sample, and the more randomly you place them, the more reliable the estimate. With a belt transect, the counts can be plotted against distance along the tape.