Estimate resin for a fiberglass layup from the glass you will wet out. Enter the area, layers, fabric weight, and your resin-to-glass ratio; get the glass weight, resin weight, and gallons.
Use this when planning a hull or deck repair, a new laminate for a hatch or locker, or sheathing a panel, and you want to buy enough resin without leaving a gallon to expire on the shelf.
Chopped strand mat is sold by ounces per square foot (1.5 oz mat is common); woven roving and cloth are sold by ounces per square yard. Pick the fabric type so the page converts correctly.
The resin-to-glass ratio depends on the fabric and how well you roll it out. The defaults are typical hand-layup starting points; your resin supplier’s data sheet is the better source.
Resin quantity follows the weight of glass you wet out. Fabric weight per square foot times area times layers gives ounces of glass; divide by 16 for pounds. Woven fabrics are sold per square yard, so their weight is divided by 9 first.
The resin-to-glass ratio by weight sets how much resin that glass holds. Hand-laid mat holds much more resin than woven cloth because the binder and random fibers soak it up. Multiply by the ratio, add waste, and divide by the resin’s density for gallons.
oz/ft² = mat weight, or woven weight / 9
glass (lb) = area × layers × oz/ft² / 16
resin (lb) = glass × resin:glass ratio × (1 + waste%)
gallons = resin (lb) / density (lb/gal)
Each row below starts from the default inputs and changes only the columns shown. Recompute any row by hand with the formula above.
| Fabric | Weight | Area | Layers | Ratio | Glass lb | Resin lb | Gal |
|---|---|---|---|---|---|---|---|
| Woven | 10 | 20 | 3 | 1 | 4.17 | 4.79 | 0.52 |
| Woven | 10 | 20 | 1 | 1 | 1.39 | 1.60 | 0.17 |
| Woven | 6 | 20 | 3 | 1.2 | 2.50 | 3.45 | 0.37 |
| Mat | 1.5 | 20 | 3 | 2 | 5.63 | 12.94 | 1.39 |
| Mat | 1.5 | 8 | 4 | 2.5 | 3.00 | 8.63 | 0.93 |
| Woven | 17 | 12 | 2 | 0.9 | 2.83 | 2.93 | 0.32 |
With the calculator defaults (Fabric type = Woven; Fabric weight = 10; Area to cover (ft²) = 20; Layers = 3; Resin : glass by weight = 1; Waste allowance (%) = 15; Resin density (lb/gal) = 9.3), the arithmetic runs like this:
fabric weight = 10 / 9 = 1.1111 oz/ft²
glass = area × layers × oz/ft² / 16 = 20 × 3 × 1.1111 / 16 = 4.167 lb
resin = glass × ratio × (1 + waste) = 4.167 × 1 × 1.15 = 4.79 lb
gallons = resin / density = 4.79 / 9.3 = 0.515 gal
Result: 4.79 lb resin. About 0.52 US gal (1.95 L) at 9.3 lb/gal. Glass weight 4.17 lb; 6.67 yd² of fabric before overlaps.
Now change Layers to 1 and leave everything else alone:
fabric weight = 10 / 9 = 1.1111 oz/ft²
glass = area × layers × oz/ft² / 16 = 20 × 1 × 1.1111 / 16 = 1.389 lb
resin = glass × ratio × (1 + waste) = 1.389 × 1 × 1.15 = 1.60 lb
gallons = resin / density = 1.60 / 9.3 = 0.172 gal
Result: 1.60 lb resin. About 0.17 US gal (0.65 L) at 9.3 lb/gal. Glass weight 1.39 lb; 2.22 yd² of fabric before overlaps.
Resin scales directly with layers: one layer of the same cloth needs a third of the resin. Build-ups on a repair are usually several layers of graduated size, so add each layer’s area separately if they differ.
Roll the laminate out thoroughly. Excess resin adds weight without adding strength, and resin-rich laminates are more brittle. A well-rolled woven laminate can come in below 1:1; a puddled one can be much higher.
For repairs, layers usually step out in size — each one larger than the last to give a long taper onto sound laminate. Work out the area of each layer and add them rather than multiplying the largest by the layer count.
Mix only what you can wet out within the pot life. On a big job, mixing several smaller batches wastes less than one large batch that kicks in the bucket.
Units: Square feet for area; oz/yd² for woven, oz/ft² for mat; pounds and US gallons for resin.
Where the model stops: It estimates resin from glass weight and a ratio you supply. Core materials, fairing compound, gelcoat, and peel ply are not included.
Not a substitute: Follow the resin maker’s data sheet for mix ratio, pot life, and laminate schedule, and the designer’s or surveyor’s requirements for structural repairs.
| Cloth | 1 layer | 2 layers | 4 layers |
|---|---|---|---|
| 6 oz/yd² | 0.48 lb | 0.96 lb | 1.92 lb |
| 10 oz/yd² | 0.80 lb | 1.60 lb | 3.19 lb |
| 12 oz/yd² | 0.96 lb | 1.92 lb | 3.83 lb |
| 18 oz/yd² | 1.44 lb | 2.88 lb | 5.75 lb |
| 24 oz/yd² | 1.92 lb | 3.83 lb | 7.67 lb |
Multiply by your area in tens of square feet. A 1:1 resin-to-glass ratio by weight is a typical hand-layup figure for woven cloth; chopped strand mat soaks up more, often 2 to 2.5 parts resin to one of glass.
Resin and hardener are skin sensitisers: wear nitrile gloves and long sleeves, and never wash resin off skin with solvent. Polyester and vinyl ester give off styrene; work with ventilation or a respirator. Sanding glass needs a dust mask and eye protection.
Hand-laid chopped strand mat commonly runs around 2:1 to 2.5:1 resin to glass by weight. The resin supplier’s data sheet may give a better figure for their product.
Roughly 1:1 by weight for hand layup is a common starting point. Vacuum bagging can bring it lower.
That is how chopped strand mat is sold in the U.S. — 1.5 oz mat weighs 1.5 oz per square foot. Woven fabrics are listed per square yard.
Only through density and the ratio you choose. Enter the density from the data sheet.