For any
ε
>
0
\varepsilon > 0
we derive effective estimates for the size of a non-zero integral point
m
∈
Z
d
∖
{
0
}
m \in \mathbb {Z}^d \setminus \{0\}
solving the Diophantine inequality
|
Q
[
m
]
|
>
ε
\lvert Q[m] \rvert > \varepsilon
, where
Q
[
m
]
=
q
1
m
1
2
+
…
+
q
d
m
d
2
Q[m] = q_1 m_1^2 + \ldots + q_d m_d^2
denotes a non-singular indefinite diagonal quadratic form in
d
≥
5
d \geq 5
variables. In order to prove our quantitative variant of the Oppenheim conjecture, we extend an approach developed by Birch and Davenport to higher dimensions combined with a theorem of Schlickewei. The result obtained is an optimal extension of Schlickewei’s result, giving bounds on small zeros of integral quadratic forms depending on the signature
(
r
,
s
)
(r,s)
, to diagonal forms up to a negligible growth factor.