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# How to Calculate Relative Error

When you make a measurement, it is important to know how accurate it is. Use relative error to express accuracy as a relative quantity.

### Instructions

- Step 1:
**Understand relative error**Understand that relative error is an indication of how accurate a measurement is relative to the size of what is being measured. - Step 2:
**Make a measurement**Make a measurement. You might, for example, decide to measure the length of an object or its weight. - Step 3:
**Determine accuracy**Determine how accurate your measurement is. For example, if you use a meter stick to measure an object's length, you might reasonably estimate the accuracy of the measurement to be plus or minus 1 millimeter. Accuracy can be converted to absolute error. - Step 4:
**Calculate absolute error**Convert accuracy units to units of absolute error by making sure the measurement and accuracy units are consistent. - TIP: For example, if an object determined to be 1 meter long is measured to within an accuracy of plus or minus 1 millimeter, the absolute error 1 millimeter times 1 meter divided by 1000 millimeter or .001 meter.
- Step 5:
**Calculate relative error**Calculate the relative error by dividing the absolute error by the measured value of the object. In the example given, the relative error is .001 meter divided by 1 meter or .001. It's that easy! - FACT: The biblical cubit was the length of a man's forearm or the distance from the tip of the elbow to the end of his middle finger.

### You Will Need

- Measurement
- Accuracy
- Calculation