Fiberglass epoxy calculator

Hand-laid woven cloth takes roughly its own weight in epoxy to wet out, plus more for the fill coats that hide the weave. Glass weight is the number to start from.

Resin-to-glass ratio

Fiberglass cloth is sold by weight per area — ounces per square yard in the US, grams per square meter elsewhere. Epoxyworks (the WEST SYSTEM technical magazine) puts a carefully hand-laid laminate at about 50% resin by weight, which is where the rule of thumb comes from: wet out the cloth with about its own weight in epoxy. Vacuum bagging squeezes that down to around 38% resin. Chopped strand mat soaks up more, commonly two to two-and-a-half times its weight.

glass weight = area × layers × cloth weight
wet-out resin = glass weight × ratio
fill coats = area × coats × ≈5 mils (0.13 mm)
volume = resin weight ÷ specific gravity

Example: a 4 × 2 ft area (0.89 yd²) with two layers of 6 oz cloth is 10.7 oz of glass. Wet-out takes about 10.7 oz of epoxy; two fill coats add about 7.3 oz more. With 15% waste that’s roughly 21 oz by weight — about 18 fl oz, or just over half a quart, of mixed epoxy.

A full sheet is a common job: see how much epoxy to fiberglass a 4×8 sheet of plywood for one layer of 6 oz cloth with fill coats, and how the cloth width decides whether you end up with a seam.

Fill coats

Wet-out saturates the cloth, but the weave texture still shows. Most jobs get one to three thin fill coats, rolled on while the previous one is still tacky, to bury the weave before sanding. The calculator assumes about 5 mils (thousandths of an inch, 0.13 mm) per rolled coat. As a cross-check, WEST SYSTEM’s published fabric formula — gallons = area in sq ft × [(cloth oz/yd² × 0.00085) + 0.0075] — assumes three thinner fill coats and about 15% waste, and gives about 1.61 fl oz per square foot for 6 oz cloth. Two of this calculator’s coats land close to that.

Counting pump strokes

Laminating epoxies such as WEST SYSTEM 105 are often measured with calibrated metering pumps: one full stroke of the resin pump and one full stroke of the hardener pump give the right ratio. WEST lists its 300 mini pumps at about 0.8 fl oz (about 24 ml) of mixed epoxy per pair of strokes with 205 or 206 hardener (5:1), and about 0.9 fl oz (about 27 ml) with 207 or 209 (3:1). Choose 5:1 or 3:1 as the mix ratio and the calculator divides the mixed volume by that figure. Other brands’ pumps deliver different amounts, so check yours — and pump a few strokes into a cup at a time rather than the whole job at once.

Tips for glassing

• Mix small batches for laminating. Epoxy in a deep cup heats up and cures faster than epoxy spread on the cloth.
• Use a squeegee to push resin through and pull excess off. Resin-rich laminates are heavier without being stronger.
• White, opaque patches mean dry cloth — add a little more resin there.
• Many marine epoxies use 5:1 or 3:1 with metering pumps. Stick to the product’s pump or weight ratio — pick it under More options and the result counts WEST mini-pump strokes for you.
• Cloth to buy includes about 10% for overlaps between pieces; add more for complex shapes.