Many blends of HFCs both with other HFCs and also with other substances are supplied for refrigeration and air-conditioning applications. Annex IV gives the method for calculating the total GWP of a mixture:
Σ (Substance X % x GWP) + (Substance Y % x GWP) + … + (Substance N % x GWP)
where % is the contribution by weight with a weight tolerance of +/- 1 %.
For example: applying the formula to a blend of gases consisting of 60 % dimethyl ether,
10 % HFC-152a and 30 % isobutane:
Σ (60 % x 1) + (10 % x 124) + (30 % x 3)
=> Total GWP = 13,9