In ancient times, the body was used as a form of measurement, and an inch was the width of a man's thumb. Since then, units of measurement have become more precise and objective. The British Standards Institution adopted an inch at 25.4 millimetres back in 1930.
An inch therefore equates to exactly 2.54 centimetres, or 0.0254 metres. 1/5 of an inch, expressed as a decimal, is 0.2 inches. It measures a little over half a centimetre - 0.508 cms. Expressed in metres, the figure is 0.00508.
A frozen pea is approximately 1/5 of an inch in diameter.