[en] The fusion cross-sections have always been of great interest to plasma physics since they are needed for predicting as well as for interpretating fusion rates. Especially for predicting the fusion rate and power gain Q in future deuterium-tritium plasmas it is important to know the fusion cross-sections as accurately as possible. Therefore, since 1945 a lot of measurements have been made for the reactions D(d,n)³He, D(d,p)T, D(t,n)α, and D(³He,p)α. For most applications, however, it is necessary to have analytical approximation formulas for the cross-sections, as derived by B.H. Duane in 1972, and by A. Peres in 1979. M. Hively clculated a formula for the reactivity <σ·v> based on Duane's formula, as did Peres for his formula. With new measurements, however, it becomes clear that these formulas do not describe σ as accurately as needed. Especially at the low energies Duane's formula is a factor of 10 too low, while Peres' formula is up to 20% too low. This is shown in a comparison of the data available with the formulas mentioned. New experimental data and the results of R-matrix fits result in a much better fit, for which an analytical formula will be given. (author) 9 refs., 1 fig., 1 tab