The Fundamentals of Fiberglass - Fibre Glass Developments: "Estimating Material Weights And Cost
Accurate material estimates are necessary for two reasons.First, they obviously are needed for proper ordering, material stocking, and bidding of projects. More importantly though, estimates offer the opportunity to calculate the weight or cost of the part using a variety of laminating schedules before beginning to build.
Unlike estimating coverage when painting, resin usage will vary depending on the type of reinforcement being used. The heavier the fabric, the more resin it will take to wet it out. A good hand laminate consists of about 50% fabric and 50% resin by weight. For example, if an application requires 3 sq yds of a 4 oz/sq yd fabric (total fabric weight = 12 ounces), 12 oz of resin will also be needed. However, if 3 yards of 10 oz/sq yd fabric is chosen (total fabric weight = 30 ounces), 30 oz. of resin will be needed.
Glass mat requires a minimum of 2 ounces of resin for each ounce of mat. Therefore, it the application calls for 20 sq feet of 1-1/2 oz/sq ft mat, it will require a minimum of 60 ounces of resin. Remember that mat is specified in ounces per square foot, where fabrics are specified in ounces per square yard. 1-1/2 oz/ sq ft chopped mat actually weighs 13.5 oz/ sq yd!
Since there are so many possible combinations of materials, one should calculate the weight and cost of a single layer using a variety of reinforcements. These can then be added or subtracted form the theoretical laminate until the design properties are achieved."