An epoxy mix ratio tells you the relative amount of Component A and Component B required by one exact system. For a ratio A:B, add the ratio parts, divide the planned total by that sum, then allocate A and B in their stated proportions.

The arithmetic is simple. The critical work is preserving what the manufacturer actually specified: component order, weight or volume basis, permitted batch size, measurement precision, and mixing procedure.

Read A:B literally—but verify the labels

For a ratio of 2:1, every complete three-part batch contains two proportional parts of A and one proportional part of B. The “parts” can be milliliters, fluid ounces, grams, pounds, or another consistent unit only if that measurement mode is the one documented.

Do not assume A always means resin or B always means hardener. Check the exact labels and TDS. Do not assume 2:1 is by volume because the containers look that size.

The batch formula

For ratio A:B and target total T:

A amount = T × A ÷ (A + B)
B amount = T × B ÷ (A + B)

These are proportioning formulas, not chemistry recommendations. The ratio must come from the manufacturer.

Stated ratio Example target total Component A Component B
1:1 600 mL 300 mL 300 mL
2:1 600 mL 400 mL 200 mL
3:1 800 g 600 g 200 g

The epoxy mix-ratio calculator performs this division for a manufacturer-specified ratio and keeps volume and weight modes explicitly separate.

Weight and volume ratios are different specifications

Volume measures space occupied; weight measures mass. Density connects them. If resin and hardener have different densities, the numbers in a volume ratio will differ from the numbers in the corresponding weight ratio.

System Three documents MirrorCoat as 2:1 by volume and 100:44 by weight. WEST SYSTEM documents some resin/hardener combinations whose published weight and volume ratios differ. These examples demonstrate the principle; their values do not apply to another product.

Safe decision rule

  • If the instructions give volume only, measure by volume.
  • If they give weight only, measure by weight.
  • If they give both, use the complete ratio for the chosen mode.
  • If they conflict across label, TDS, and web page, pause and ask the manufacturer for the current controlling document.

Never convert a volume ratio to weight without current densities for both components and manufacturer confirmation that partial-batch conversion is allowed. A mathematically correct density conversion is not automatically an approved process.

Why mix ratio is a cure requirement

The ratio represents the formulated balance of reactive components. It is related to stoichiometry—the proportions needed for the intended chemical network—but a commercial formulation can also contain fillers, diluents, pigments, and other ingredients. Users should follow the finished product ratio, not attempt to calculate a new one from raw chemistry.

Too much or too little of either component can leave resin-rich or hardener-rich material and prevent full property development. Possible evidence includes soft or tacky material, localized uncured streaks, surface residue, or later performance loss. Similar symptoms can have other causes, so the sticky-or-soft epoxy diagnostic guide also inspects component identity, mixing, temperature, contamination, and cure conditions.

Adding extra hardener is not a valid way to accelerate cure. WEST SYSTEM explicitly warns against it. The resin-versus-hardener guide explains why a curing agent in a two-part epoxy is not simply a speed control.

Scaling a batch without changing the ratio

You can scale a permitted ratio up or down by multiplying both components by the same factor. A 2:1 batch could be 200 mL + 100 mL or 2 L + 1 L. It must not become 200 mL + 110 mL because a little more B seems convenient.

Before scaling, confirm:

  • whether partial kits are allowed;
  • the minimum amount the measuring device can resolve reliably;
  • any manufacturer minimum or maximum batch size;
  • the mixer’s usable capacity and required geometry;
  • working time at the expected material temperature;
  • how quickly the mixed material must be poured out or placed.

Larger contained masses can retain reaction heat and accelerate. The epoxy exotherm guide explains this mass effect. This article does not prescribe a universal safe batch size; exact product instructions control it.

Plan from a known component amount

If A is already measured and the ratio is A:B, calculate:

B required = known A × B ÷ A

If B is known:

A required = known B × A ÷ B

This is useful for planning, not for rescuing a contaminated or uncertain pour. Do not estimate what remains in a dirty cup or add the other component later to a partially reacting batch.

Measurement precision is system- and batch-specific

No universal acceptable percentage error applies to all epoxy. Small batches can make the same absolute measuring error a much larger fraction of the component amount. A scale displaying 0.1 g is not necessarily accurate to 0.1 g across every load; capacity, readability, repeatability, linearity, calibration, environment, and container stability all matter.

Choose a method capable of meeting the manufacturer’s stated process. The measuring-tools and scales guide compares volume containers, balances, system-specific pumps, and batch-control tools by task rather than declaring one universal best device.

Pumps can hide the ratio math

With a matched calibrated pump set, one full stroke of each pump can deliver unequal component volumes in the correct proportion. WEST SYSTEM, for example, sells specific pump sets for specific resin/hardener families and container groups.

“One pump each” therefore means one complete cycle from the correct paired pumps—not equal volume from two generic dispensers. Prime and verify the system as instructed. Partial strokes, air, clogged outlets, a wrong container group, or mismatched pumps can change output.

Measuring is not mixing

After accurate proportioning, follow the documented mixing procedure. Important variables include container material and shape, tool or paddle, speed, time, scraping, temperature, and whether transfer mixing is required.

System Three advises scraping sides and bottom and describes a two-container method for its systems. That does not make double-cup mixing a universal requirement. If the selected product specifies a different sequence, that instruction controls.

Do not scrape visibly unmixed residue from the original container onto the work. Do not add solvent, water, filler, or pigment until the system instructions allow it.

Common ratio mistakes

  1. Reading a 2:1 volume ratio as 2:1 by weight.
  2. Reversing which component receives two parts.
  3. Calculating two parts plus one part, then calling two parts the total.
  4. Using unequal “one pump each” dispensers that were not calibrated as a pair.
  5. Measuring below the device’s practical resolution.
  6. Taring incorrectly or including container mass.
  7. Using sloped containers as if height changes represented equal volumes.
  8. Splitting a kit the manufacturer requires to be mixed complete.
  9. Changing ratio to alter working time.
  10. Measuring correctly but mixing incompletely.

A controlled batch record

For repeatable work, record:

  • product, lot, and component identities;
  • stated ratio and measurement mode;
  • planned total and actual component amounts;
  • measuring device and calibration/verification status;
  • material, air, and substrate temperatures where relevant;
  • mix start/end, transfer, pour-out, and application times;
  • unexpected appearance, heat, or cure behavior.

Records cannot prove cure, but they make failure investigation more informative than memory.

Safety boundary

Mixing exposes workers to uncured components and can generate heat. Use both component SDSs and task-specific controls. NIOSH warns that odor is not a reliable safety indicator and that glove material must be appropriate to the chemicals. Do not select PPE from a generic epoxy ratio.

Bottom line

Use the exact product’s ratio in the exact basis stated. Scale both components proportionally, choose measuring equipment suitable for the batch, and follow the documented mixing process. Ratio arithmetic does not set cure speed, establish an allowable tolerance, or override minimum and maximum batch limits.

Future visual opportunity

A three-part visual could show ratio parts, equal proportional scaling, and why density makes 2:1 by volume differ from 2:1 by weight without assigning universal densities.

Sources and technical references

Accessed September 2026. Product documents can change; verify the current revision before specifying or installing a system.

  1. Dispensing and Mixing — WEST SYSTEM Epoxy
  2. Epoxy Problem Solver — WEST SYSTEM Epoxy
  3. Measuring and Mixing — System Three Resins
  4. MirrorCoat Product and Application Information — System Three Resins
  5. About Epoxies and Resins and Reproductive Health — National Institute for Occupational Safety and Health