Estimating sizes of cells

Estimates can be made when doing calculations that need to be done quickly and without a calculator. They are usually less accurate than carrying out the full calculation, however, they still give a good idea of the answer to the question that is being asked.

Calculating the area of a plant cell

When looking down a microscope, a plant cell had a width of 10.3 µm and a length of 22.1 µm. Estimate the area covered by the cell.

The area of the plant cell is the width multiplied by the length because plant cells are roughly rectangular. This would be 10.3 × 22.1.

When estimating, round each number to its nearest whole number, or a more manageable number of significant figures. In this example we round 10.3 to 10 and 22.1 to 22.

The new calculation is more manageable:

10 × 22 = 220 µm2

Question

Estimate the area covered by an animal cell that has a radius of 5.6 µm.

A cell is roughly a circle. The area of a circle is given by πr2.

Rounding 5.6 to the nearest whole number gives 6.

π is 3 to the nearest whole number.

The calculation becomes:

area of cell = 3 × 62 = 3 × 36 = 108 µm2