Short answer
What you need to know
Keep an unrounded working value during the calculation, then round the reported answer to a precision supported by the supplied measurements and your course method. Exact definitions and counted quantities do not limit precision in the same way as measured inputs.
Precision belongs to the information, not the screen
A calculator can display many digits generated by binary floating-point arithmetic. Those digits do not add information to a measured input. A result should not imply more precision than the measurements and method justify.
Recognize significant digits
Leading zeros locate the decimal point and are not significant. Zeros between nonzero digits are significant. Trailing zeros after a decimal marker normally communicate significance; a value such as 1200 can be ambiguous unless scientific notation or uncertainty is supplied.
Keep working and display values separate
Rounding after every intermediate step can accumulate avoidable error. Keep a higher-precision working value and round the final display once. The converter exposes both so the rounded answer can be checked against the actual computation.
Know what is exact
Defined conversion relationships such as 1 inch = 0.0254 metre are exact. Counted objects are also exact. A measured length written as 2.4 m is not exact, even if a calculator internally represents it with more digits.
Use the course rule
Different courses may teach specific rules for multiplication, division, addition, subtraction, uncertainty, or intermediate rounding. Select a practical display setting, but follow the stated method and show enough working for another person to reproduce the answer.
Try these before revealing the answers
How should 1.20 m be interpreted?
Show worked answer
The trailing zero after the decimal indicates three significant digits.
Should you round every conversion step?
Show worked answer
Usually keep higher precision in working and round the reported result once, unless the required method says otherwise.
Important limitation
Verify important findings
A significant-figures selector is not an uncertainty analysis. It cannot infer the quality of an instrument, the uncertainty of an input, or a course's required convention.
Try a conversion with controlled precision Review the methodology
Further reading
Authoritative references
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