Answer:
Take the correct value, subtract the value you got, and then divide that figure by the correct value. Then take the absolute value of that and multiply by 100. For example, say I weighed something and got that it was 2.5 grams, but it really was 2.7 grams. 2.7-2.5=.2. .2/2.7=.074. .074*100=7.4. Thus, I had 7.4% error. Another example: 16-15=-1. -1/15=-.067. .067*100=6.7% error.