Running out of epoxy halfway through a pour is a frustrating way to discover that your estimate was too small. Buying far more than you need is not ideal either. A few measurements before you order can make the whole project easier.
To work out how much epoxy resin you need, calculate the volume of the space you want to fill, then allow for normal mixing and application losses. The result is the total amount of resin and hardener combined. For a rectangular space, multiply length × width × depth. Use the inside dimensions of a mould, the open channel of a river table, or the intended coating thickness on a tabletop.
You can enter rectangular project dimensions into the Magic Resin epoxy volume calculator. The examples below explain what to measure, how to handle round or irregular shapes, and how to turn the result into the right amount of each component.
Start with the space the epoxy will occupy
A table that is 5 cm thick does not necessarily need a 5 cm pour. If you are coating its surface, you only calculate the thin layer on top. If you are filling a river between two slabs, you calculate the gap between the wood.
- Tabletop coating: measure the top surface and your intended coating thickness. Account for edges, drips and a seal coat separately.
- River table: measure the empty channel, excluding the wood. Include any cracks or voids you also plan to fill.
- Silicone mould: measure the internal cavity to your intended fill line. The outside dimensions include silicone that will never hold resin.
- Several small castings: calculate one cavity, then multiply by the number of pieces.
Keep every measurement in the same unit. In Canada, it is common to buy resin in litres while measuring wood in inches; either system works as long as you convert the final volume correctly.
The epoxy resin volume formulas
For a rectangle or square
Centimetres: length × width × depth = volume in millilitres. Divide by 1,000 for litres.
Inches: length × width × depth × 16.387 = volume in millilitres. Alternatively, divide the volume in cubic inches by 231 to get US gallons.
For example, a rectangular cavity measuring 30 cm × 20 cm × 1 cm needs 600 mL of mixed epoxy before an allowance for losses. Remember that 5 mm is 0.5 cm, not 5 cm.
For a round mould or circular tabletop
Volume = 3.1416 × radius × radius × depth. The radius is half the diameter. With all measurements in centimetres, the answer is in millilitres.
A round coaster cavity with a 10 cm diameter and a 1 cm fill depth needs about 78.5 mL. Four identical coasters need about 314 mL before losses. This assumes a flat, straight-sided cavity; decorative edges, taper and raised details change the actual capacity.
Quick examples: coasters, coatings and a river table
The estimates below are for mixed resin and hardener. The final column adds a 10% planning allowance and rounds up to a practical whole millilitre. This is an estimating choice, not a required mixing ratio or a guarantee of coverage.
| Project | Dimensions to fill | Calculated volume | With 10% allowance |
|---|---|---|---|
| Four round coasters | Each 10 cm diameter × 1 cm deep | 314 mL | 346 mL |
| Rectangular casting | 30 × 20 × 1 cm | 600 mL | 660 mL |
| Small tabletop coating | 24 × 18 inches × 1/8 inch | 885 mL | 974 mL |
| River channel | 120 cm long × 10 cm average width × 4 cm deep | 4.8 L | 5.28 L |
These are geometric estimates, not instructions to mix or pour the entire quantity at once. The coating example covers the flat top only. Seal coats, vertical edges and runoff require additional product.
How to estimate an irregular river table
Live edges rarely leave a perfectly rectangular channel. Measuring only its widest point can overestimate the amount of resin; measuring only its narrowest point can leave you short.
- Position the slabs exactly as they will sit in the sealed form.
- Divide the channel length into several equal sections.
- Measure a representative gap width in each section. Use more sections where the shape changes sharply.
- Average those widths, then multiply by the channel length and pour depth.
- Add a separate estimate for large knots, cracks and spaces underneath the slabs that will fill with epoxy.
If the depth varies significantly, calculate smaller sections individually and add their volumes. A rough river channel needs a more generous allowance than a simple silicone cavity. Inspect and seal the form carefully: buying extra resin is no substitute for preventing a leak.
How much extra resin should you allow?
For a straightforward mould, budgeting roughly 5–10% above the calculated volume is a useful starting point. Some resin stays on the mixing cup and stir stick, and real moulds are not always the exact size advertised.
For a tabletop with flowing edges, porous wood or an irregular casting, estimate those extra demands separately instead of assuming 10% will cover everything. A seal coat is its own step and should have its own product allowance.
Extra product to have on hand does not mean extra product to mix immediately. Plan batches around the resin's working time, recommended batch size and pour depth. Once resin and hardener are combined, the reaction begins; a large mass left in a cup can heat up quickly.
Does the calculated amount include the hardener?
Yes. If your project needs 600 mL of mixed epoxy, that means 600 mL total, not 600 mL of Part A plus additional hardener.
The split depends on your product's mixing ratio by volume:
- 1:1 by volume: 300 mL of Part A + 300 mL of Part B = 600 mL total.
- 2:1 by volume: 400 mL of Part A + 200 mL of Part B = 600 mL total.
Use these Magic Resin mixing ratios, expressed as Part A (resin) : Part B (hardener):
- Table Top & Coating: 1:1 by volume, or 100:85 by weight.
- Art & Craft: 1:1 by volume, or 100:90 by weight.
- 2-inch and 4-inch Deep Pour: 2:1 by volume, or 100:45 by weight.
For example, if you measure 100 g of Table Top Part A on a scale, add 85 g of its Part B. For Art & Craft, 100 g of Part A needs 90 g of Part B; for either Deep Pour formula, it needs 45 g of Part B. Check the label and the instructions for your exact resin before measuring.
Do not use a volume ratio as a weight ratio. Resin and hardener can have different densities, so choose the ratio that matches your measuring method: volume for graduated cups, weight for a scale. The weight ratios tell you how to proportion a batch; they do not convert your project's millilitres into grams. Adding extra hardener will not make up for a shortage of resin.
Choose the resin for the depth, then choose the kit size
The volume tells you how much to buy. The application and pour depth tell you which formula to buy.
- Table Top & Coating Resin is designed for surface coatings and pours up to 1/4 inch.
- Art & Craft Resin suits many moulds and small castings, with pours up to approximately 3/4 inch.
- Deep Pour & Casting Resin comes in 2-inch and 4-inch versions for deeper projects.
These depth limits do not replace the product instructions: total pour volume, shape and temperature also matter. For a large casting near a product's maximum depth, confirm the plan before mixing.
Compare your estimate, including any separate coats, with the kit's listed combined volume. Use the litre amount on the product listing when comparing sizes. One US gallon is approximately 3.785 L, while an imperial gallon is approximately 4.546 L; confusing the two can leave your order short.
Frequently asked questions
How much epoxy does a 1-gallon kit cover?
A combined US gallon theoretically covers about 12.8 square feet at 1/8 inch thick. Actual usable coverage is lower when you allow for cup residue, edges, drips or uneven surfaces. This is a tabletop volume calculation. For a rolled garage-floor system, use the product's stated coverage and the Magic Resin floor epoxy calculator.
Can I use water to measure a silicone mould?
For an empty, removable, watertight silicone mould, you can fill it with water to the intended level and measure that water in a graduated container. Empty the mould and let it dry completely before using epoxy. Do not use this method on wood, porous inclusions or an assembled river-table form.
What if my mould has flowers, wood or other objects inside?
Solid inclusions displace resin, so the empty mould's capacity is an upper estimate. For small decorative inclusions, keeping that estimate can provide a useful margin. For large wood pieces, calculate the remaining empty space. Avoid guessing a large deduction that could leave you short.
Should I mix all the resin I bought?
No. Buying enough for the whole project and mixing an appropriate batch are separate decisions. Measure only what you can apply within the product's instructions and working time, maintaining the correct ratio in every batch.
Measure once more before you pour
Write down the cavity dimensions, calculated mixed volume, allowance and Part A/Part B split before opening your kit. That small checklist keeps unit conversions and last-minute arithmetic out of your working time.
Start with the epoxy resin volume calculator, then choose the formula suited to your project's depth. If the shape is unusual or the pour is large, contact the Magic Resin team with the dimensions, photos and planned pour depth so we can help you check the estimate.