Title: A Fast, Well-Founded Approximation to the Empirical Neural Tangent Kernel. Abstract: Empirical neural tangent kernels (eNTKs) can provide a good understanding of a given network's representation: they are often far less expensive to compute and applicable more broadly than infinite-width NTKs. For networks with $O$ output units (e.g. an $O$-class classifier), however, the eNTK on $N$ inputs is of size $NO \times NO$, taking $\mathcal{O}\big( (N O)^2\big)$ memory and up to $\mathcal{O}\big( (N O)^3 \big)$ computation. Most existing applications have therefore used one of a handful of approximations yielding $N \times N$ kernel matrices, saving orders of magnitude of computation, but with limited to no justification. We prove that one such approximation, which we call ``sum of logits,'' converges to the true eNTK at initialization. Our experiments demonstrate the quality of this approximation for various uses across a range of settings.