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.