Strong's numbers

κάλαμος (G2563)

kálamos

kal'-am-os

of uncertain affinity; a reed (the plant or its stem, or that of a similar plant); by implication, a pen:--pen, reed.

κάλαμος, -ου, ὁ, [in LXX chiefly for קָנֶה, Is 42:3, Ez 40:3 ff., al.;]

a reed: Mt 11:7 12:20(LXX), Lk 7:24; hence, acc. to its various uses,

(a) a reed-pipe, flute;

(b) a reed-staff, staff (cf. IV Ki 18:21, ῥάβδος καλαμίνη): Mt 27:29, 30, 48, Mk 15:19, 36;

(d) a writing reed, a pen: III Jo 13 (cf. Milligan, NTD, 9, 7).†

From G. Abbott-Smith, A Manual Greek Lexicon of the New Testament (1922), in the public domain.

Shown in the King James Version (KJV), whichever translation you read in — it is the text these numbers are tagged against.

Revelation 21:15
And he that talked with me had a golden reed to measure the city, and the gates thereof, and the wall thereof.
Revelation 21:16
And the city lieth foursquare, and the length is as large as the breadth: and he measured the city with the reed, twelve thousand furlongs. The length and the breadth and the height of it are equal.