When we round a measurement, the greatest possible error is half the unit of measurement.

When we round to the nearest unit, the greatest possible error is 0.5.

When we round to the nearest tenth, the greatest possible error is 0.05.

When we round to the nearest hundredth, the greatest possible error is 0.005.

**Example**

The mass of a cake is 1.5 kg.

What is the greatest possible error?

**Answer**

The mass is given to the nearest tenth of a kg.

The greatest possible error is half of a tenth of a kg or half of 100g = **50 g.**

This means that the cake could have a mass as low as 1.45 kg and as high as 1.4999999... kg and still have a mass rounded to 1.5 kg.