length · International · Modern
SI base unit of length. Its definition fixes the speed of light in vacuum at exactly 299,792,458 m/s, with the second defined from the caesium-133 hyperfine frequency.
Converter
length · Ancient Roman · Republic/Empire and later
palmus (Roman palm)
reference-only
- Record
- M121
- Definition quality
- Historically documented
- Conversion mode
- reference only
- Calculator
- Reference only
Definition
Core Roman palmus = 4 digiti = 1/4 foot; representative ≈74 mm. Later writers also used a larger 'palmus' sense.
History & use
Smith notes two Roman uses of palmus: the proper four-digit palm and a later larger palm corresponding to a span.
Modern use
Modern use is context-dependent and varies by region or measurement system.
Sources & evidence
Source tier — · confidence high
Related measurements
length · International · Modern
1 km = 1000 m by the SI prefix kilo-.
length · International · Modern
1 cm = 0.01 m by the SI prefix centi-.
length · International · Modern
1 mm = 0.001 m by the SI prefix milli-.
inch
inlength · UK / US · Modern / historical
1 international inch = exactly 0.0254 m.
foot
ftlength · UK / US · Modern / historical
1 international foot = exactly 0.3048 m.
yard
ydlength · UK / US · Modern / historical
1 international yard = exactly 0.9144 m.
mile
milength · UK / US · Modern / historical
1 international mile = exactly 1609.344 m = 1760 yd.