The Outer Mizbe’ach (altar) was five amos (cubits) wide (see A) and five amos long (see B). [Shemos 27:1]. It had four karnos (horns) [Shemos (Exodus) 27:2]. Rambam tells us that these horns were box-like protrusions that sat on top of each corner of the mizbe’ach. Each box was one amah (cubit) long (see C), one amah wide (see D), and five tefachim high (a tefach is the width of a hand). [Rambam, Hil. Bais haBechirah 2:5].
How much space was there between each of the horns (see E)?
5 – 2 = 3 amos
How much of the area of the top of mizbe’ach was not covered by the horns?
5 x 5 = 25
1 x 1 = 1, 4 x 1 = 4
25 – 4 = 21 square cubits