"The tail is five meters long!"
Translation:De staart is vijf meter lang!
Yeah, it's kind of a weird exception. For "uur" (hour), "jaar" (year) and "kwartier" (quarter of an hour), you use the singular when describing a specific duration of time.
- Ik heb drie jaar in Canada gewoond. - I lived in Canada for three years.
- Het duurt drie kwartier. - It takes 45 minutes. / It takes three quarters of an hour.
- Ik stond twee uur lang in de rij - I was in line for two hours.
Strangely enough, the same is not true for "dagen" (days), "minuten" (minutes) or "eeuwen" (centuries).
(Thanks Simius, I copied the text above from: https://www.duolingo.com/comment/4662641)
And this goes for other measures as well, e.g. meter (meter), gram (gram). Plural is only used when the individual instances are meant e.g. "er zijn twee slechte meters in de weg" (this means that there are two lengths of a meter in the road that both are bad). If you say "er zijn 2 kilogrammen" people will expect that there are two weights of a kilogram.
- Het boek weegt twee kilo. - The book weighs two kilos.
- Hij sprong drie meter en veertig centimeter. - He jumped three metres and forty centimeters.
- Het geluid was 600 Herz. - The sound was 600 Herz.
In general, you would use meter even for units more than 1, e.g. twintig meter.
When you say vijf meters you are putting emphasis on the separate units.
Another cases with numbers for which meters are used is when an adjective comes before meters e.g.:
- vijf lange meters - five long meters.