Estimate how much thickened epoxy a fillet will take. Choose a radiused cove or a straight 45° fillet, enter its size and length, and get the mixed volume with a waste allowance.
Use this when bonding bulkheads, stringers, or furniture into a hull, or filleting a stitch-and-glue seam, and you want to mix enough thickened epoxy without a lot left to harden in the cup.
For a cove fillet, enter the radius of the tool you will shape it with (a 3/4 in radius is a common tongue-depressor size; a 1-1/2 in radius is roughly a spoon). For a straight fillet, enter the leg length along each face. Length is the total run of joint in feet.
The volume is the mixed, thickened epoxy. How much neat resin and hardener that takes depends on the filler, which bulks the mix.
A cove fillet in a right-angle corner fills the square of side r minus a quarter circle of radius r — the shape left after the round tool sweeps through. Its area is r² × (1 − π/4), about 0.215 r². A straight 45° fillet is a right triangle with two legs of length L, area L²/2.
Multiply the cross-section by the length in inches to get cubic inches, add the waste allowance, and convert: 1 US fluid ounce is 1.8047 in³, 1 mL is 0.0610 in³, and 1 US gallon is 231 in³.
cove area = r² × (1 − π/4) ≈ 0.2146 r²
straight area = leg² / 2
volume (in³) = area × length (ft) × 12 × (1 + waste%)
fl oz = in³ / 1.8047 · mL = in³ × 16.387 · gal = in³ / 231
Each row below starts from the default inputs and changes only the columns shown. Recompute any row by hand with the formula above.
| Shape | Size | Feet | Waste % | Area in² | fl oz | Gallons |
|---|---|---|---|---|---|---|
| Cove | 0.75 | 10 | 15 | 0.1207 | 9.2 | 0.072 |
| Cove | 1 | 10 | 15 | 0.2146 | 16.4 | 0.128 |
| Cove | 1.5 | 10 | 15 | 0.4829 | 36.9 | 0.288 |
| Straight | 0.75 | 10 | 15 | 0.2813 | 21.5 | 0.168 |
| Straight | 0.5 | 24 | 15 | 0.1250 | 22.9 | 0.179 |
| Cove | 0.5 | 40 | 10 | 0.0537 | 15.7 | 0.123 |
With the calculator defaults (Fillet shape = Cove; Radius or leg length (in) = 0.75; Total fillet length (ft) = 10; Waste allowance (%) = 15), the arithmetic runs like this:
cross-section = r² × (1 − π/4) = 0.75² × 0.214602 = 0.12071 in²
volume = area × length × 12 = 0.12071 × 10 × 12 = 14.49 in³
with waste = 14.49 × 1.15 = 16.66 in³
fluid ounces = in³ / 1.8047 = 9.2 fl oz
Result: 9.2 fl oz thickened mix. 16.7 in³ (273 mL, 0.072 US gal) for 10 ft of 0.75 in radius cove fillet, including 15% waste.
Now change Fillet shape to Straight and leave everything else alone:
cross-section = leg² / 2 = 0.75² / 2 = 0.28125 in²
volume = area × length × 12 = 0.28125 × 10 × 12 = 33.75 in³
with waste = 33.75 × 1.15 = 38.81 in³
fluid ounces = in³ / 1.8047 = 21.5 fl oz
Result: 21.5 fl oz thickened mix. 38.8 in³ (636 mL, 0.168 US gal) for 10 ft of 0.75 in leg straight fillet, including 15% waste.
A straight 45° fillet with the same 3/4 in legs holds more than twice the epoxy of a 3/4 in cove, because the cove’s curved face removes most of the triangle. Coves are lighter and spread load more evenly; straight fillets are quicker to shape.
Mix fillets in small batches. A large cup of thickened epoxy heats up and can kick before you have used it; several smaller batches are easier to control, and the waste allowance covers the residue in each cup.
Thicken to a peanut-butter consistency that holds its shape on a vertical surface without sagging. Wet out both faces with unthickened epoxy first so the fillet bonds rather than sitting on a dry surface.
If the fillet will be glassed over, shape it with a radius large enough for the cloth to lie into without bridging. A cove radius of at least the cloth’s comfortable bend is easier than trying to force glass into a tight corner.
Units: Inches for fillet size, feet for length; volume in cubic inches, fluid ounces, milliliters, and US gallons.
Where the model stops: It assumes a 90° corner and a uniform fillet. Corners at other angles, uneven fillets, and the extra used to wet out faces are not modeled beyond the waste allowance.
Not a substitute: Follow the epoxy maker’s mix ratio, pot life, and filler guidance, and wear gloves and eye protection.
| Size | Cove (fl oz) | Straight 45° (fl oz) |
|---|---|---|
| 0.25 in | 1.0 | 2.4 |
| 0.375 in | 2.3 | 5.4 |
| 0.5 in | 4.1 | 9.6 |
| 0.75 in | 9.2 | 21.5 |
| 1 in | 16.4 | 38.2 |
| 1.5 in | 36.9 | 86.0 |
A straight 45° fillet of the same leg uses about 2.3 times the epoxy of a cove fillet whose radius equals that leg, because the cove is hollow.
A common starting point is a radius about equal to the thickness of the thinner panel being joined, larger where the joint carries load such as a bulkhead tabbing or a stringer. Too small a fillet concentrates stress at the edge; too large adds weight and resin without much strength.
Mixed, thickened volume. Filler adds bulk, so the neat resin and hardener needed is usually less than the fillet volume; how much less depends on the filler and how thick you mix it.
It depends on the joint and the designer’s plans. Larger radii spread load over more of the panel; many small-boat plans specify the fillet size by the tool used to shape it.
Add up every joint that gets a fillet, on both sides of a bulkhead if both are filleted. Measure in feet.
Some mix stays in the cup, some is squeezed out and scraped off, and some goes to tidy-up. Fifteen percent is a reasonable start; adjust with experience.