Resin Calculation Formula
All epoxy quantity math comes down to two situations: surface coating (floors, countertops, tabletops) and volume casting (molds, channels, embedments). This page collects every formula you need in one place — imperial and metric — with the 1.805 constant explained from first principles.
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Every epoxy volume formula in imperial units uses the constant 1.805. Here's where it comes from:
1 US fluid ounce = 231 ÷ 128 = 1.80469 cubic inches
Therefore: cubic inches ÷ 1.805 = fluid ounces
This constant is just a unit conversion — it does not change based on the density or brand of epoxy. Any liquid poured into a space of X cubic inches requires X ÷ 1.805 fluid ounces to fill it.
Formula 1: Volume Casting (Molds & Channels)
Use this for any project where you're filling a defined 3D space — molds, river table channels, embedments, geode trays.
Step 2: fl oz = cubic inches ÷ 1.805
Step 3: gallons = fl oz ÷ 128
Step 4: Add 10% waste buffer
Rectangular: V = L × W × D
Cylinder: V = π × r² × h
Sphere: V = (4/3) × π × r³
Hemisphere: V = (2/3) × π × r³
Cone: V = (1/3) × π × r² × h
Worked example: River table channel, 72 in long × 8 in wide × 1.5 in deep, + 15% for wood edges:
72 × 8 × 1.5 = 864 cu in ÷ 1.805 = 478.9 fl oz × 1.15 = 550.7 fl oz ≈ 4.3 gallons
Formula 2: Surface Coating (Floors, Tables, Countertops)
Use this when you're applying a layer of epoxy over a flat surface.
fl oz = Area (sq ft) × 144 × Depth (in) ÷ 1.805
gallons = fl oz ÷ 128
Method B — Using coverage rate:
gallons = Area (sq ft) ÷ Coverage rate (sq ft/gal)
Standard coverage rates:
1/32 in = 400 sq ft/gal | 1/16 in = 200 sq ft/gal
1/8 in = 100 sq ft/gal | 1/4 in = 50 sq ft/gal
Worked example — Method B (recommended for surface coatings): 200 sq ft garage floor, 2 coats at the standard "1/8 inch" floor thickness:
200 sq ft ÷ 100 sq ft/gal = 2 gal/coat × 2 coats = 4 gal + 10% = 4.4 gallons
Method B is the reliable choice for surface coating because the industry coverage rates (100–200 sq ft/gal) already account for the actual wet film thickness used in practice. Those rates are measured at 8–16 wet mils (0.008–0.016 inches) — not a literal 1/8 inch pour. Use Method A (cu in ÷ 1.805) only for volume casting projects where you know the exact pour depth in inches.
When to use each method: Method B (area ÷ coverage rate) for any surface coating — floors, countertops, tabletops. Method A (volume in cu in ÷ 1.805) for any volume cast — molds, river table channels, embedments, castings.
Formula 3: Metric Calculations
Metric epoxy math is much simpler because 1 cm³ = 1 mL exactly.
Surface coating: Area (m²) × Depth (mm) = litres
Example: 3 m² table at 3mm flood coat = 3 × 3 = 9 litres
Add 10% buffer = 9.9 litres → buy 10 litres
Metric Shape Formulas
Use cm for all dimensions — result is in mL (same as cm³).
- Box: L × W × D = mL
- Cylinder: π × r² × h = mL
- Sphere: (4/3) × π × r³ = mL
- Surface coating: area (m²) × depth (mm) = litres
Unit Conversion Reference
| From | To | Operation |
|---|---|---|
| Cubic inches | Fluid ounces | ÷ 1.805 |
| Fluid ounces | US gallons | ÷ 128 |
| US gallons | Litres | × 3.785 |
| Litres | US gallons | × 0.2642 |
| Litres | Fluid ounces | × 33.81 |
| mL (cm³) | Fluid ounces | ÷ 29.57 |
| sq ft | sq in | × 144 |
| sq m | sq ft | × 10.764 |
| inches | mm | × 25.4 |
Common Mistakes in Resin Calculations
- Mixing unit systems: Using sq ft for area and inches for depth without converting to sq in first. Either convert all dimensions to inches before multiplying, or use Method B (area ÷ coverage rate) for surface calculations.
- Forgetting the waste buffer: Always add at least 10%. Mixing epoxy leaves residue in the mixing cup, drips occur off the edge, and the first application on bare wood absorbs more than expected.
- Not accounting for multiple coats: Multiply single-coat amounts by the number of coats. Don't forget the seal coat if using one — it adds 20–25% more product to the total.
- Using diameter instead of radius: Sphere and cylinder formulas require radius (diameter ÷ 2). Using diameter instead of radius gives a result 4× too large.
- Not adjusting for porous surfaces: Bare wood and uncoated concrete absorb 10–25% more than a sealed surface. Add extra buffer for first-pour-on-raw-substrate applications.
Frequently Asked Questions
What is the formula to calculate epoxy resin amount?
For volume casting: Volume (cu in) ÷ 1.805 = fluid ounces. For surface coating: Area (sq ft) ÷ coverage rate = gallons (standard rates: 200 sq ft/gal at 1/16 in, 100 sq ft/gal at 1/8 in). Always add 10% waste buffer to either result.
Where does the 1.805 constant come from?
1 US gallon = 231 cubic inches (by legal definition). 1 fluid ounce = 1/128 of a gallon = 231/128 = 1.80469 cubic inches. So dividing cubic inches by 1.805 converts volume to fluid ounces. It's a pure unit conversion — valid for any liquid, not specific to epoxy.
How do I calculate epoxy resin in metric units?
In metric: 1 cm³ = 1 mL exactly, so volume in cm³ equals mL of resin. For surfaces: area (m²) × depth (mm) = litres needed. For a 2m × 1m table at 3mm: 2 × 1 × 3 = 6 litres. Add 10% buffer = 6.6 litres. No conversion constant needed.