Epoxy mix ratio converter (volume and weight)

Why the two ratios differ

Resin and hardener have different densities, so the same product has one ratio by volume and a different one by weight. TotalBoat gives the example of a system that mixes 1:1 by volume but 100:83 by weight, and warns that off-ratio epoxy can stay tacky and never reach full strength (TotalBoat). Using the volume ratio on a scale, or the weight ratio in measuring cups, is one of the easiest ways to ruin a pour.

This converter keeps the two apart and tells you which published number each result comes from.

Formula and assumptions

Splitting a total by a ratio a:b:

A = total × a ÷ (a + b)    B = total × b ÷ (a + b)

Converting a volume ratio to a weight ratio needs the specific gravity (SG) of each part:

weight ratio = (a × SG_A) : (b × SG_B)

The converter uses data in this order:

1. Published weight ratio, if the data sheet has one. It is used as printed.
2. Computed weight ratio, from the published volume ratio and the published SG of Part A and Part B. The result is labelled “computed” so you know it is our arithmetic, not the manufacturer’s figure.
3. Nothing. If neither is available, the by-weight answer is withheld with a note explaining what is missing.

To convert a total between volume and weight, the converter uses the published density of the mixed resin if there is one, or the density implied by the two part SGs and the volume ratio. Specific gravity is treated as grams per millilitre (water = 1 g/mL). That ignores the small difference between water’s density and exactly 1 g/mL: the USGS lists 0.99802 g/cm³ at 70 °F (21 °C), a difference of 0.2 % (USGS). It also assumes the volumes of the two parts simply add up when mixed.

US units use exact definitions: 1 lb = 453.59237 g and 1 US gal = 3,785.411784 mL (NIST Handbook 44, Appendix C).

Worked example

A product publishes a mix ratio of 2:1 by volume, SG 1.15 for Part A and 0.95 for Part B, and no weight ratio. You need 1 US gallon of mixed resin. These numbers are for illustration; pick a real product in the converter to use its data.

1. By volume: A = 3,785 mL × 2/3 = 2,524 mL (85.3 fl oz); B = 1,262 mL (42.7 fl oz).
2. Weight ratio: (2 × 1.15) : (1 × 0.95) = 2.30 : 0.95 = 100 : 41.3.
3. Mass of the mix: 2,524 × 1.15 + 1,262 × 0.95 = 2,902 g + 1,199 g = 4,101 g (9.04 lb).
4. By weight: A = 2,902 g (6.40 lb); B = 1,199 g (2.64 lb).

Note that 2:1 by volume became roughly 100:41 by weight, not 100:50.

Using a scale

• Tare the empty cup, pour Part A to its weight, then tare again and add Part B. TotalBoat notes that a digital gram scale, or a food scale set to grams, works for small quantities (TotalBoat). Resolution matters most for small batches: a 1 g error is 5 % of a 20 g Part B but only 0.2 % of a 500 g one.
• Pour both parts at similar temperature. TotalBoat warns that resin that is too cold turns thick and is hard to measure accurately (TotalBoat).
• Mix for as long as the data sheet says, scraping the sides and bottom. The soft or tacky spots guide explains why.