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## 4. Error-Free numbers

There are two particular categories of numbers which are error free and therefore have an infinite number of sig fig. Using these numbers does not introduce any error beyond what is already in the measurement.

- Counts - These are whole integers and represent “how many” of something. If we measure an angle twice direct and reverse there are a total of 4 measurements: 4 exactly, not 3.99 or 4.01.
- Many conversion factors - These allow us to convert a measurement from one unit to another without introducing error. There are exactly 12 inches in exactly 1 foot. There are exactly 43560 sq ft in exactly 1 acre. As mentioned above, when using scientific notation the base is also exact.