Foot, Inch, and Ounce Unit Symbols (ft, in, oz)
The strings ft, in, and oz function as compact unit symbols in NIST conversion material. They are useful after numbers in technical or tabular measurement notation, while the spelled-out unit names remain clearer in prose when space is not constrained. Punctuation and symbol treatment should follow the governing technical or house style.
In measurement notation, NIST conversion tables use “ft” for foot, “in” for inch, and “oz” for ounce. Treat these as unit symbols in technical notation, not as ordinary plural words such as feet, inches, or ounces.
Key details
Important caveats
Punctuation Boundary
NIST SP 330 explains the general technical principle that unit symbols are not ordinary abbreviations and, for SI symbols, are not followed by a period except at the end of a sentence. For U.S. customary symbols such as ft, in, and oz, follow the governing technical or house style rather than inventing a universal prose-punctuation rule.
Further guidance
Name Vs Symbol
Use foot/feet, inch/inches, and ounce/ounces when spelling the unit name out. Do not treat the plural word and the unit symbol as competing spellings of the same grammatical form.
Reader Check
Are you writing compact measurement notation after a number, or ordinary prose that would be clearer with the unit name spelled out?
Symbol Forms
NIST conversion tables use ft for foot, in for inch, and oz for ounce. These forms stay compact after a number, for example 6 ft, 12 in, or 8 oz.
Worked Example
Technical notation: “The board is 6 ft long and 2 in thick.” Prose: “The board is six feet long.” Keep the selected notation consistent within the same context.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- Approximate Conversions from U.S. Customary Measures to Metric (opens in a new tab)National Institute of Standards and Technology · Directly supports ft as the symbol used for foot in NIST conversion tables; later copy must distinguish symbols from ordinary prose abbreviations.
- SP 330 Section 5 — Rules and Style Conventions for Printing and Using Units (opens in a new tab)National Institute of Standards and Technology · Authoritative SI style rules: unit symbols are not pluralized or followed by periods, and sec is not the permissible symbol for second