  We do a complete calculation of the stochastic gravitational wave background
to be expected from cosmic strings. We start from a population of string loops
taken from simulations, smooth these by Lorentzian convolution as a model of
gravitational back reaction, calculate the average spectrum of gravitational
waves emitted by the string population at any given time, and propagate it
through a standard model cosmology to find the stochastic background today. We
take into account all known effects, including changes in the number of
cosmological relativistic degrees of freedom at early times and the possibility
that some energy is in rare bursts that we might never have observed.
