Calculations for Multiwall Bags

  • Basis weight determination:  Weight in pounds of 500 sheets (1 ream) of paper 24 x 36 inches (6 sq ft x500 sheets = 3000 sq ft).
  • Trim width x Tube length x basis weight divided by 432 = Theoretical weight per thousand bags.
  • To calculate a sack size based on product density, see Chapter 10.
  • To size a sack for a new weight based on the present sack, see Chapter 10.
  • To calculate a new sack size based on a random sack, see Chapter 10
  • To estimate filled sack dimensions

- Width = empty sack face width minus 80% of one end width
- Length = empty sack length or finished bag length (FBL) +½ to 1 inch for bag rounding
- Thickness = end (top or bottom) width

  • To adjust bag size to fit pallet while retaining same cubic content

 One can use a general formula to adjust bag sizes while retaining approximate cubic volume.   The general rule is:

   1”F = 2”L = 1”G

To change one inch on the sack face, change the length by two inches
  
To change the thickness or gusset by one inch, change the length by two

However, this is only an approximate formula and should be used only as a guide.  Because of all of the other factors involved in determining a bag size interact with each other, additional adjustments to the sack size may be required to develop the properly sized sack.

 

  • To calculate a sack size based on product density:
    • Establish the desired weight in pounds
    • Obtain product density in pounds per cubic foot (lbs/cu ft)
    • To calculate the required sack volume:

a. Divide the desired bag weight by the product density to obtain sack volume in cubic feet.

        

                             Desired sack weight  (in lbs)
Sack volume =  -------------------------------------------  = cubic feet
                             Density of product (in lbs/cu ft)

Example:  50 lb desired sack weigh for product with density of 35 lbs/cu ft.

          

                           50 lbs   
Sack volume =  ---------------   = 1.43 cu ft
                        35 lbs/cu ft 

  • Using the Sack Size versus Volume of Sack chart (Appendix IV), find the bag volume in the left hand column.  If calculated volume is between two volumes listed select the sack size of the larger volume.
Select the style of sack desired, and obtain the estimated sack size from the appropriate column.
  
   For 1.43 cu ft, select 1.48 cu ft from chart
    SOM = 16 x 4 x 35
    PVE = 20 x 27 x 5 
  • To size a sack for a new weight based on the present sack:

Assume the present sack is a pasted valve sack with a net weight of 80 lbs
The current valve sack size is 24 x 32 x 5
The desired net weight of the new pasted valve sack is 50 lbs,

  • Find the volume for the current size bag on the Sack Size versus Volume of Sack chart (Appendix IV).     
    24 x 32 x 5 = 2.43 cu ft
  • Calculate the volume of the new sack using the formula:
New Sack Weight
------------------------   x  Volume of Old Sack  =  Volume of New Sack
Old Sack Weight

  

                    50 lb
Example:      -------     x 2.43 cu ft  = 1.52 cu ft (volume of new sack)
                    80 lb

 

  • Using the Sack Size versus Volume of Sack chart (Appendix IV), find 1.53 cu ft (value is between 1.48 and 1.55.  Select larger value and find dimension of new pasted valve sack.   New sack size is 20 x 28 x 5.
  • To calculate a new sack size based on a random sack:

The calculations are based on whatever size sack is available that is closest to the size desired and is of the same style.

 Example:
   Available sack is an open mouth sack 16 x 4 x 32
   The available sack filled to the proper freeboard holds 41 lbs
   The desired bag weight is 50 pounds.

  • Find the volume of available sack in Sack Size versus Volume of Sack Chart ( Appendix IV)
       16 x 4 x 32  =  1.30 cu ft
  • Calculate the volume of the desired bag using the formula:

   

Desired Sack Weight
-----------------------------  x  Actual Volume  =  1.58 cu ft
Actual Sack Weight

   

50 lb
------   x 1.30 cu ft  =  1.58 cu ft
41 lb

  • Using the Sack Size versus Volume of Sack Chart (Appendix IV), find 1.58 cu ft (the value is between 1.55 and 1.61).  Select the larger value and find the new dimensions of the open mouth sack as 16 x 4 x 37.

As mentioned before, when resizing a sack or developing a new size of a   sack, always verify the estimated sack size by filling a sample sack with the product at the packing location.

  
Estimating Filled Sack Sizes

  • Valve Sack Styles

Even though rounding occurs on all surfaces of a filled sack, a valve sack usually approaches a rectangular, brick-like shape.

However, to estimate the filled valve sack size, assume that all surfaces are flat and that the corners are square.

- The width of the filled sack is equal to the empty sack face width minus 80% of the width of one end for rounding.
     - The length of the filled valve sack is equal to the length of the empty bag length (finished bag length) + ½” to 1” for rounding
     - The thickness of the filled sack is equal to the width of the sack ends (top and bottom)



Filled sack dimensions are:

    • Width = Empty sack face width minus 80% of one end width
    • Length = Empty bag length or finished bag length (FBL) + ½ to 1 inch for rounding
    • Thickness = End (top or bottom) width
       
  • Sewn Gusseted and Pinch Bottom Valve Sacks

The SOM, SV and PBV sacks all become shorter when they are filled.  The face width and gusset become the filled width and thickness.  The length becomes shorter by about ½ the width of the gusset.

    •    The width of the filled sack is equal to the empty sack face + ½” to 1” for rounding.
    • The filled length is equal to the empty bag length (FBL) minus ½ the width of a gusset.
    • The filled thickness equals the width of the gusset.
  • Pasted Open Mouth Stepped-End Sack (also called Pinch Bottom Sack)

These Sacks also become shorter when they are filled.  The sack is shortened because of the folding as sealing of the stepped ends.  The length is shortened by about ½ the width of the gusset plus the length of the fold over. The sack width and thickness are the same as the SOM sacks.

    • The width of the filled PBOM is equal to the empty face width plus ½” to 1” for rounding.
    • The filled length is equal to the empty finished bag length (FBL) minus ½ the width of the gusset plus 1½” for rounding.
    • The filled thickness is equal to the width of the gusset.
  • Sack Direct Size Conversions from One Sack Style to Another
        
    SOM to Other Bag Styles



Source: PSSMA
  • Sack Direct Size Conversions from One Sack Style to Another
        
        POM to Other Bag Styles



Source: PSSMA
  • Sack Direct Size Conversions from One Sack Style to Another

PVE to Other Bag Styles
 




Source: PSSMA
  • Sack Direct Size Conversions from One Sack Style to Another

SOS to Other Bag Styles




Source: PSSMA