A step-by-step guide using a real river table as the example. Follow this to get surveyor-grade accuracy from the Precision Calculator. Switch units anytime using the toggle in the header.
Before we touch any numbers, let's be clear on one thing: the reason the Precision Calculator is more accurate than the basic one is that you feed it more data points. The standard calculator asks for one "average width" — which is always a guess. This calculator asks for the width at multiple specific points, so the math becomes integration rather than estimation.
Here's exactly what that looks like in practice, using a river table as our example.
Start with your table placed flat, pre-pour. Your slabs are in the mold with the river gap between them. Here's what you're about to measure:
You're measuring 9 numbers total: the length, plus the width at each of 7 points evenly spaced along that length.
How to set up the measurement points: If your river is and you pick 7 measurement points, that's 6 equal segments of . Point 1 is at one end , Point 2 is at , Point 3 at , Point 4 at (the midpoint), Point 5 at , Point 6 at , and Point 7 at (the other end).
Mark the measurement points with painter's tape before you start. Stick a small piece of tape at each division along the edge of your slab. This way your measuring tape stays still and every width is taken at exactly the right place.
Measuring each width: At each marked point, measure straight across the river from the edge of one slab to the edge of the other — perpendicular to the length of the table, not at an angle. A square against the slab's edge makes this easy.
For our example table, here's what we wrote down:
| Point | Position along length | Measured width |
|---|---|---|
| P1 | ||
| P2 | ||
| P3 | ||
| P4 | ||
| P5 | ||
| P6 | ||
| P7 |
The second thing you need is the depth of your pour — how thick the epoxy layer will be. Look at your table from the side, and measure vertically from the base of the mold up to where you want the epoxy surface to sit.
For a river table, depth is almost always uniform — you're pouring epoxy to a flat level across the whole river. Use the "Uniform depth" option in the calculator and enter that single number ( in our example).
Use variable depth only if your mold has a stepped or sloped base (rare), or if you're pouring the river in layers of different thicknesses and want to calculate just one layer at a time.
Now plug everything in. Open the Precision Calculator and fill in:
For reference, if you had used the basic calculator with "average width × length × depth," you'd get a different answer — and most of the time it would be wrong by 15-30% because real river shapes are not uniform rectangles. The trapezoidal math handles that variation properly.
Our result is . Never buy exactly what the calculator says — buy the next kit size up. Running out of mixed epoxy mid-pour is the most painful mistake in this hobby, because you cannot add fresh epoxy to a partially cured layer without visible lines.
For , buy a (or as a buffer). The waste factor already added of buffer, but temperature, absorption into porous wood, and the inevitable "oh I spilled some" should push you up one more tier.
Mix in batches. Most deep-pour epoxies have a pot life of 30-45 minutes. For , split into two batches so you don't race the clock during a complex pour.
Head to the Precision Calculator and plug in your measurements. It takes about 2 minutes.
Open Precision Calculator →