Fix a square-free monomial
m
∈
S
=
K
[
x
1
,
…
,
x
n
]
m \in S = \mathbb {K}[x_1,\ldots ,x_n]
. The square-free principal Borel ideal generated by
m
m
, denoted
sfBorel
(
m
)
\operatorname {sfBorel}(m)
, is the ideal generated by all the square-free monomials that can be obtained via Borel moves from the monomial
m
m
. We give upper and lower bounds for the Waldschmidt constant of
sfBorel
(
m
)
\operatorname {sfBorel}(m)
in terms of the support of
m
m
, and in some cases, exact values. For any rational
a
b
≥
1
\frac {a}{b} \geq 1
, we show that there exists a square-free principal Borel ideal with Waldschmidt constant equal to
a
b
\frac {a}{b}
.