A derivation is given for a simple algorithm which yields the quasi steady burning rate of a vertical panel of non-charring, non-melting material in an enclosure with stratification of temperature and oxygen concentration. The algorithm requires the solution of an transcendental equation; it is suggested that this be done by successive approximations. Among the thermophysical data which are needed, are the mean flame temperature and the height dependent absorption coefficient K(Z). It is found from experiment that K(Z) is well described for PMMA by a two-parameter expression linear in l/Z. Comparison with a transient experiment yields good agreement over much of the ranges.