How much material is in that silage bag? One way to estimate this value is to calculate the volume in the bag and multiply by its density. Although the bag is initially round, the shape after filling is more like half an ellipse. Therefore, the volume of the bag can be estimated as:

 

V = 0.79 H W L
where: V = Volume (ft³)
H = Height of bag above ground (ft)
W = Width of bag at widest (ft)
L = Length of silage (ft)

When full length bags are used, the length of silage is the bag length minus the unused portion needed to seal each end of the bag. Since the end of the bag often slopes, measure using a point that is about half-way (lengthwise) between where the bag touches the ground and where the slope starts at the top of the bag.

The dry matter density can vary from bag to bag and is based on machine type and adjustment as well as forage type. Typical densities range between 11-15 lb. DM per cubic foot.

Example: You have used a bag that had a 10 ft. diameter (specified by the manufacturer). After filling you estimate that the top of the bag is 3.5 ft. above ground (H=3.5) and the width of the bag at its widest point is 14 ft. (W=14). The length of the bag is measured at 80 ft. Therefore, the volume of the bag is 0.79(3.5)(14)(80) = 3,097 cubic feet.

If the density of the silage is assumed to be 13 lb DM per cubic foot, then the bag holds 3,097(13) = 40,261 lb. DM or about 20 tons DM.

If that same volume were packed to only 11 lb. DM per cubic foot, then the bag capacity would be 3,097(11) or about 17 tons DM.