Resin to Fiber Ratio Explained

Customers often ask us how much
resin is needed for a part. Hopefully this guide will help on that topic and
answer some of the questions that may be out there. Warning though, there will
be math!

When it comes to figuring out how
much resin is needed for a given project, planning is key. Investigating how
much resin and fabric are needed, design and measurement of the pieces needed
are important to gather for buying the right amount of material.

Start by writing down the
estimates and measurements needed for the repair, skinning, or fabrication of
the part. Figure up how much fabric will be used using the surface area of
which plys will be laid. For example, having a surface area of 2 ft2 for each ply of material with a
need of 3 plys.

Each fabric has a density
associated with it. This is the calculation given by the fabric’s weight (grams
or ounces) over its surface area, usually measured in m2 (square meters) or ft2 (square ft). Using Carbon Fiber
Fabric, Plain weave 6K for example, its density is 370 gsm (Grams per square
meter). Through conversion to gsf (grams per square foot) it equals 34.37 grams
per square foot.

CF Fabric Density: 370 gsm

(1) Gram / Square meter =
(0.092903) Grams / Square foot

370 * 0.093 = 34.37 grams per
square foot (Density of CF fabric)

Having known the need of 2 ft2 per ply for 3 plys, multiply the
34.37 gsf of the fiber by 2 ft2 for multiple of 3 plys. Giving the
total fiber volume of 206.22 grams of total fiber weight.

3 Plys * 2ft2 = 6ft2 (Total material area for

6ft2 * 34.37 (Density of CF Fabric) =
206.22 grams of total estimated fiber weight

For hand Layups, the fiber / resin
ratio is usually ~50% at best. For exact science, one would say 207 grams of
fiber is the same as 207 grams of mixed resin. But if you are estimating, it is
better to use a higher figure to ensure there is enough resin for a job. The
ratio is always going to be proportional to this ratio.

Example 1:
50/50 Fiber / Resin Ratio: 207 g Fiber & 207 g of Resin

Example 2: 40/60
Fiber / Resin Ratio: (207 g Fiber * 60%) / 40% = 310.5 Grams of resin

Example 3:
60/40 Fiber / Resin Ratio: (207 g Fiber * 40%) / 60% = 138.0 Grams of resin

For estimating purposes, most
heavier epoxies density is approximately 1.4 g/cm3 which converts to .04734 g/oz.
Different resins come in a wide variety of densities, many manufacturer’s
specifications should give a good idea for calculation. Using Example 1 Figures
from above, the need of 207 grams of resin, 207g multiplied by .04734 g/oz equals
~9.8 oz of mixed resin needed for a 50/50 resin to fiber ratio.

Given Epoxy Density: 1.4 g/cm3

Conversion factor: 1 g/cm3 = 29.5735 grams / US fluid Ounce

1.4 / 29.5735 = 0.04734 g/oz

Resin needed: 207 grams

0.04734 g/oz * 207 g = 9.8 oz of
mixed resin (50/50 Fiber to Resin Ratio)

When estimating for needed resin
amount, having more is better than having not enough. For all accounts of
variation in composite processing, getting 30% more resin than designed is a
safe bet. Still, for this job, a quart of resin once mixed would

Generalized: Most one-off jobs for
a single part can be handled using a quart of resin. However, for larger hand
layups a gallon would be needed. It should be noted that Chopped Fiberglass
(CSM) is going to need more resin than most any other material. Woven material
generally weighs less (smaller density) than CSM.

Real world example – Skinning a Carbon Fiber Hood: For
a single ply of material of CF fabric

Weight / Density of Fabric: 285

285 gsm * 0.092903 (Conversion
Factor) = 26.4774 Grams / ft2

Car hood rough dimensions: 65” x

Square In” to Square ft’:
(65”/12) * (48”/12) = 21.6667 ft2

26.4774 Grams / ft2* 21.667 ft2 = 573.67 grams of CF

(1) Ply for skinning, so there is
still a total of 573.677 grams of CF fabric that will need to be impregnated
with resin. Accounting for a rough estimate of a Fiber to Resin ratio of a
traditional layup with no vacuum bagging, a 50/50 ratio will be a safe bet and
ensure underestimating is eliminated.

[(573.677 grams of CF fabric) *
50%] / 50% = 573.677 grams of resin

(No, the math wasn’t needed but
practice makes perfect)

Resin needed: 573.677 grams

General Heavy Epoxy Density:
.04734 g/oz

0.04734 g/oz * 573.677 g = 27.15
oz of mixed resin (50/50 Fiber to Resin Ratio)

Using a 4:1 fast hardener / epoxy
stem, this skinning would fit using a quart of mixed resin.

For many basic applications, the
math is going to result in the accurate numbers. As mentioned at the beginning
of this post, planning is the key to success in composites. Spending the time
to make ply templates, making layouts of the ply shapes and doing the math
before the resin and fabric are bought are going to make the difference in
making the proper selections. The more data that is gathered before buying, the
greater chance at making a successful estimate on fiber and resin