$$\text{Absolute error} = |\text{approximate value} - \text{exact value}|$$ $$\text{Relative error} = \frac{\text{absolute error}}{\text{exact value}} \qquad \text{Percentage error} = \frac{\text{absolute error}}{\text{exact value}} \times 100\%$$

• A measurement given to the nearest unit $u$ lies in the range $\text{value} \pm \dfrac{u}{2}$.
• Maximum possible error $= \dfrac{1}{2}\times$ (smallest unit of measurement).
• Sum: upper bound $= \sum$ upper bounds; lower bound $= \sum$ lower bounds. Difference $a - b$: upper $= a_{\max} - b_{\min}$, lower $= a_{\min} - b_{\max}$.