Where these numbers come from: TotalBoat recommends adding 5-10% for resin lost on the stir stick, in the corners of the mixing cup and to drips. Craft Resin recommends 5-15% and calls a 10% allowance "a good starting point". The MakersMath resin calculator adds 10% by default. Lean to the top of the range on small pours, the bottom on large ones.
Every resin quantity starts as a volume in cubic inches and ends as a number of fluid ounces or gallons. This guide is the unit-conversion and worked-example page: it shows the arithmetic behind the Resin Calculator so you can check any result by hand. For choosing the waste percentage see the resin waste buffer guide, and for per-layer depth limits see the resin pour calculator guide.
Volume Formulas
Measure in inches, then multiply.
- Rectangular mold or tray: Length × Width × Depth. A 12 × 8 in tray filled 0.5 in deep is 48 cubic inches.
- Round mold: π × Radius² × Depth. A 10 in diameter mold filled 0.25 in deep is 3.1416 × 25 × 0.25 = 19.6 cubic inches.
- Irregular shapes: split them into rectangles and circles and add the pieces, or fill the mold with water and measure the water.
- Embedded objects: an object takes up space that resin would have filled. Measure its volume (water displacement works) and subtract it.
Unit Conversions
A US gallon is defined as 231 cubic inches and holds 128 US fluid ounces, so 1 cubic inch = 0.554113 fl oz (the factor the calculator uses) and 1 fl oz = 1.805 cubic inches. The guides round that to 1.8 for hand math.
| US measure | Fluid ounces | Cubic inches | Millilitres | |------------|--------------|--------------|-------------| | 1 cubic inch | 0.554 | 1 | 16.4 | | 1 fl oz | 1 | 1.805 | 29.6 | | 1 cup | 8 | 14.44 | 236.6 | | 1 pint | 16 | 28.88 | 473.2 | | 1 quart | 32 | 57.75 | 946.4 | | 1 gallon | 128 | 231 | 3,785 |
To convert: cubic inches × 0.554 = fluid ounces, or cubic inches ÷ 1.8 for a quick estimate.
Coverage per Gallon
For a coating, the volume is surface area × thickness. One gallon (231 cubic inches) spreads this far:
| Thickness per coat | Square feet per gallon | Fluid ounces per square foot | |--------------------|------------------------|------------------------------| | 1/16 in (0.0625) | 25.7 | 5.0 | | 1/8 in (0.125) | 12.8 | 10.0 | | 1/4 in (0.25) | 6.4 | 20.0 | | 3/8 in (0.375) | 4.3 | 29.9 | | 1/2 in (0.5) | 3.2 | 39.9 |
TotalBoat quotes about 12 sq ft per mixed gallon at 1/8 in for common tabletop and bar top epoxies, which is consistent with the 12.8 sq ft above once some waste is allowed for. The calculator's coat mode reports this number for your thickness. For how thick a single layer may be, see the resin pour calculator guide.
Mix Ratio Split
The calculator's Part A / Part B figures are measured by volume. Divide the total by the sum of the ratio parts. For 32 fl oz of mixed resin:
| Ratio (resin : hardener) | Part A | Part B | |--------------------------|--------|--------| | 1:1 | 16 fl oz | 16 fl oz | | 2:1 | 21.3 fl oz | 10.7 fl oz | | 3:1 | 24 fl oz | 8 fl oz | | 4:1 | 25.6 fl oz | 6.4 fl oz |
For example, 27 fl oz at 2:1 splits exactly into 18 fl oz resin and 9 fl oz hardener. No rounding up is needed. Use the ratio on your product's label, and check whether it is by volume or by weight.
Step-by-Step Process
- Calculate volume in cubic inches (length × width × depth, or π × r² × depth).
- Subtract the volume of anything embedded.
- Add your waste allowance. The calculator uses 10%, inside the 5-10% (TotalBoat) and 5-15% (Craft Resin) ranges.
- Multiply by 0.554 to get fluid ounces, then divide by 128 for gallons.
- Split into Part A and Part B using the ratio.
Worked Examples
All examples use the 10% allowance. Fluid ounces use the 0.554113 factor.
| Project | Base volume | With allowance | Mixed resin | |---------|-------------|----------------|-------------| | 4 round coasters, 4 in diameter, 1/4 in deep | 4 × 3.14 = 12.57 in³ | 13.8 in³ | 7.7 fl oz | | 12 pendants, 1 × 1 in, 1/8 in deep | 1.5 in³ | 1.65 in³ | 0.9 fl oz | | Tray river channel, 18 in long × 2 in wide × 3/8 in deep | 13.5 in³ | 14.9 in³ | 8.2 fl oz | | Side table top coat, 24 × 18 in, 1/4 in | 108 in³ | 118.8 in³ | 65.8 fl oz (just over 2 qt) | | 36 × 24 in painting, 1/8 in coat | 108 in³ | 118.8 in³ | 65.8 fl oz | | Coffee table river, 48 × 20 in, 3/8 in deep | 360 in³ | 396 in³ | 219.4 fl oz (1.71 gal) | | Dining table river, 72 in long × 4 in avg wide × 1.75 in deep | 504 in³ | 554.4 in³ | 307.2 fl oz (2.40 gal) | | Countertop, 120 × 25 in, 1/8 in coat | 375 in³ | 412.5 in³ | 228.6 fl oz (1.79 gal) |
Without the allowance the countertop is 207.8 fl oz (1.62 gal). For tiny items such as pendants the percentage rule stops meaning much, because what stays in the cup can outweigh the pour itself. Mix the smallest batch you can measure accurately.
Common Calculation Mistakes
- Thickness typos: 1/4 in versus 1/8 in is a 100% difference in resin.
- Area instead of volume: square inches ignore depth. Multiply by thickness.
- Wrong conversion factor: 1 fl oz is 1.8 cubic inches, not 1.0. Mixing up millilitres (29.6 per fl oz) with cubic inches (1.8 per fl oz) is an easy slip.
- Forgetting embedded objects: a piece of driftwood displaces resin. Measure it and subtract.
- Counting the whole top for a river table: only the channel gets filled. Add the wood surface only if you are also flood-coating.
Key Takeaways
- Gallon = 231 cubic inches = 128 fl oz; 1 cubic inch = 0.554 fl oz.
- Add a waste allowance. 10% is the default, within the 5-15% manufacturer range.
- Split by the mix ratio instead of rounding the total up.
- Coverage at 1/8 in is about 12.8 sq ft per gallon before waste.
Ready to run your own numbers? Try our free Resin Calculator and compare it to the tables above.