What does epoxy mix ratio mean?

A mix ratio like 2:1 (resin : hardener) by volume means for every 2 mL of Part A you add 1 mL of Part B, giving 3 mL of combined mix. The ratio is set by the manufacturer to control curing chemistry — deviating from it leads to a tacky, undercured surface or a resin that never sets. Always use the ratio printed on your kit.

Should I mix by weight or by volume?

Most consumer epoxy systems specify ratios by volume, which is what this calculator uses. Some professional systems — especially those used in boatbuilding or composites — specify ratios by weight. Check your product data sheet. If yours is by weight, multiply the Part A volume result by the Part A density (g/mL) and the Part B result by the Part B density to convert.

Why add a waste allowance?

A small amount of mixed resin always remains in the mixing cup after pouring (typically 5–15 mL depending on cup size), and some moulds require slight overfilling to account for surface tension and any air loss. A 10% allowance is standard for small pours; use 5% for large river tables where cup waste is proportionally smaller.

How thick can I pour epoxy in one go?

Standard table-top and coating epoxies generate heat as they cure (exothermic reaction). Pours deeper than roughly 6–9 mm in a single pass can overheat, yellow, crack or even smoke. Deep-pour casting resins are formulated with slower hardeners that allow single pours up to 50–75 mm. Check the maximum single-pour depth on your product data sheet before planning a river table or thick casting.

What is the density of cured epoxy?

Most standard bisphenol-A epoxy systems cure to a density of approximately 1.10–1.20 g/mL (the calculator uses 1.15 g/mL as a representative average). Specialty filled systems, flexible epoxies and high-temperature resins may differ. The mass figure here is an estimate useful for shipping calculations; verify with your product spec sheet for precision work.

How do I measure small quantities accurately?

For volumes under 50 mL use calibrated plastic syringes or a jewellery-grade digital scale (accuracy 0.01 g). Graduated mixing cups introduce more error than syringes for small batches. Keep Part A and Part B in separate syringes and never double-dip — cross-contamination in the bottle will cause premature gelling.

Epoxy Resin Calculator

Epoxy resin is a two-part material: Part A (resin) and Part B (hardener) must be combined in the exact ratio printed on the kit. Pour too little hardener and the surface stays permanently sticky; add too much and the exothermic reaction can overheat and crack your piece. This calculator works out the precise volumes of each component for any mould shape, so you mix only what you need and nothing cures in the cup.

How it works

The calculation has two steps.

Step 1 — mould volume. The tool calculates the internal volume of your mould from the shape and dimensions you enter (all in cm or inches, converted to cm internally):

Rectangle / slab: V = L × W × D
Circle / disc: V = π × (D⁄2)² × depth
Ring / annulus: V = π × (R⊂outer;² − R⊂inner;²) × depth
Custom: enter the volume directly in mL (or cubic inches for imperial users)

Step 2 — mix split. Total mixed volume = mould volume × (1 + waste %). The calculator then splits that total in the ratio A : B:

Part A = Total × a ⁄ (a + b)

Part B = Total × b ⁄ (a + b)

where a : b is your selected mix ratio (e.g. 2 : 1). Cured mass is estimated at 1.15 g/mL — a good average for standard bisphenol-A systems.

Worked example

You are casting a 60 cm × 40 cm river table slab at a pour depth of 0.3 cm (3 mm) using a 2:1 casting epoxy with a 10% waste allowance.

Mould volume = 60 × 40 × 0.3 = 720 mL
Total mix = 720 × 1.10 = 792 mL
Part A = 792 × 2/3 = 528 mL
Part B = 792 × 1/3 = 264 mL
Approx. cured mass = 792 × 1.15 = 911 g

Mould	Ratio	Depth	Part A	Part B	Total
60 × 40 cm rectangle	2:1	3 mm	528 mL	264 mL	792 mL
30 cm diameter circle	1:1	5 mm	177 mL	177 mL	354 mL
20 cm outer / 10 cm inner ring	3:1	10 mm	222 mL	74 mL	296 mL

Every figure is calculated instantly in your browser — no data is sent anywhere.

Formula note

Volume is in cubic centimetres (cm³), which is numerically equal to millilitres (mL) — a convenient unit for measuring resin because most mixing cups are graduated in mL. The waste multiplier accounts for residue left in the mixing cup and any overfill needed to compensate for surface tension at the mould edges. Ten percent is the industry-standard starting point; experienced casters trim this to 5% for large pours once they know how much their specific mixing cups retain.