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We present an analytical approximation formula for the growth function in aspatially flat cosmology with dust and a cosmological constant. Our approximateformula is written simply in terms of a rational function. We also show theapproximate formula in a dust cosmology without a cosmological constant,directly as a function of the scale factor in terms of a rational function. Thesingle rational function applies for all, open, closed and flat universes.  Our results involve no elliptic functions, and have very small relative errorof less than 0.2 per cent over the range of the scale factor $1/1000 \la a \lid1$ and the density parameter $0.2 \la \Omega_{\rmn{m}} \lid 1$ for a flatcosmology, and less than $0.4$ per cent over the range $0.2 \la\Omega_{\rmn{m}} \la 4$ for a cosmology without a cosmological constant.

An analytical approximation of the growth function in Friedmann-Lema\^itre universes