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.
Running a few ounces short mid-pour ruins a project, and so does mixing far more than you need. This guide is about one decision: what waste percentage to add to your calculated volume. For the volume arithmetic itself see the epoxy resin volume calculator guide, and for per-layer depth limits see the resin pour calculator guide.
What a Waste Buffer Covers
A waste buffer is the extra resin you add on top of the calculated volume. The manufacturers name these sources:
- Resin left on the stir stick and in the bottom corners of the mixing cup (TotalBoat).
- Drips, including resin running over the edges of the piece (TotalBoat).
- Surface absorption and edges (Craft Resin). Craft Resin suggests a higher allowance for very porous wood or live-edge projects.
- A seal coat on porous surfaces, which uses additional resin (TotalBoat).
Both manufacturers' ranges overlap at 5-10%, and TotalBoat's advice for uncertain cases is simply to buy more.
The Formula
Total Resin Needed = Base Amount × (1 + Buffer Percentage)
If a river table needs 1.5 gallons and you use the default 10%: 1.5 × 1.10 = 1.65 gallons.
Choosing a Percentage
The calculator uses 10%, which sits inside both manufacturers' ranges. The sizing tiers below are a MakersMath rule of thumb, not manufacturer guidance. They rest on one idea: whatever stays on the stir stick and cup is roughly the same amount whatever the batch size, so it is a larger share of a small batch.
| Batch size | Suggested buffer | Reason | |------------|------------------|--------| | Under 32 fl oz | Top of the range (about 15%) | Cup and stick losses are a larger share | | 32 fl oz to 1 gallon | 10% | The default | | Over 1 gallon | 5-10% | The same losses are a smaller share | | Porous wood or live edge | Top of the range or higher | Craft Resin notes these may need more |
The illustration below is hypothetical: it assumes 1 fl oz stays in the cup and on the stick, whatever the batch.
| Batch | 1 fl oz as a share of the batch | |-------|-----------------------------------| | 8 fl oz | 12.5% | | 32 fl oz | 3.1% | | 128 fl oz (1 gallon) | 0.8% |
What You Would Buy at Each Buffer
Computed from the formula above, in fluid ounces of mixed resin:
| Calculated volume | +5% | +10% | +15% | |-------------------|-----|------|------| | 8 fl oz | 8.4 | 8.8 | 9.2 | | 32 fl oz | 33.6 | 35.2 | 36.8 | | 128 fl oz (1 gal) | 134.4 | 140.8 | 147.2 | | 192 fl oz (1.5 gal) | 201.6 | 211.2 | 220.8 |
Do not stack percentages. If you already added 10% for the buffer, measure natural wood at several points to get the real volume instead of adding another 10% for uneven slabs.
Reducing Waste
- Pick a mixing cup that fits the batch. A very large bucket for a small pour leaves more resin on its walls.
- Scrape the cup with a flexible silicone spatula and pour the scraped resin in right away.
- Contain the pour. Plastic sheeting under the piece catches drips. Hold the stir stick over the cup or the piece.
- Measure the actual channel or mold. Water displacement gives an exact volume for odd shapes.
Leftover Mixed Resin: Do Not Store It
Mixed epoxy begins curing as soon as Part A and Part B combine, so it cannot be kept for later. If you have extra after a pour:
- Pour it promptly into small silicone molds (coasters, keychains) while it is still workable.
- Have those molds ready before you start mixing.
- Do not pour leftover mixed resin down a drain. Follow the disposal instructions on your product's safety data sheet (SDS).
Keep unmixed, unopened Part A and Part B in their original containers and check the product's data sheet for storage.
Tracking Your Own Waste
Keep a short log for each pour: the calculated volume, the buffer used, the total mixed, and the amount left over. After a few pours you can see whether your own number belongs at the top or bottom of the 5-15% range.
- Leftover each time: lower the buffer a little.
- Running short: first recheck the volume math, then raise the buffer toward 15%.
Key Takeaways
- Start with the calculator's 10% default, which is inside the manufacturer ranges (TotalBoat 5-10%, Craft Resin 5-15%).
- Small batches are the ones that benefit from the top of the range. Large batches can use the lower end.
- The batch-size tiers on this page are a MakersMath rule of thumb, not manufacturer guidance.
- Mixed resin cannot be stored, so plan a mold for any surplus before you mix.
Ready to run your own numbers? Try our free Resin Calculator and toggle the 10% allowance to see the difference.