Why do container candles develop a sinkhole?

Molten wax is less dense than solid wax, so it occupies more volume when poured. As it cools and crystallises it contracts, and the contraction concentrates around the wick where the wax cools last, leaving a dip or sinkhole on the surface.

How much does each wax type shrink?

Soy wax typically shrinks 3 to 10 percent, paraffin 10 to 15 percent, and coconut, beeswax, and blends fall in between. The tool uses a sensible default per wax type and lets you enter a custom value if your wax behaves differently.

How is the top-up pour calculated?

The shrinkage void equals the first-pour volume times the shrinkage percentage. Because the top-up wax also shrinks as it cools, the tool divides the void by one minus the shrinkage fraction so the cooled second pour lands exactly at your original fill line.

How do I convert wax volume to weight?

Most candle waxes have a density near 0.9 grams per millilitre when solid, so a millilitre of wax weighs roughly 0.9 grams. The tool multiplies the required volume by your density to give a weight you can measure on a kitchen scale.

When should I do the second pour?

Wait until the candle has fully cooled and the sinkhole has clearly formed, usually a few hours. Pour the top-up slightly hotter than the first pour, around 5 to 10 degrees Celsius warmer, to melt the surface edge and blend for a smooth, seamless top.

Candle Container Wax Shrinkage & Top-Up Calculator

Container candles almost always sink around the wick as the wax cools and contracts, leaving an unsightly dip that needs a second pour. This calculator works out exactly how much wax to set aside for that top-up so your candle finishes flush with the fill line instead of overflowing or coming up short.

How it works

The void to fill comes from how much the first pour shrinks, and the top-up is scaled up because it shrinks too:

first pour   = container volume × (1 − pour-short %)
shrink void  = first pour × shrinkage %
top-up pour  = shrink void / (1 − shrinkage %)
total wax    = first pour + top-up
weight (g)   = volume (mL) × density (g/mL)

Dividing by 1 − shrinkage is the key step: a naive top-up equal to the void would itself sink slightly, so the formula compensates so the cooled second pour reaches your original line.

Example and tips

An 8 oz jar (about 236 mL) of soy wax poured 10 percent short fills to roughly 212 mL. At 7 percent shrinkage the cooling wax leaves about a 15 mL void, and the top-up needs to be about 16 mL to refill it after its own shrinkage — close to 14 grams of wax. Pour the top-up only after the candle has fully set and the sinkhole is obvious, and warm it a few degrees above the first pour so it melts the surface edge and blends seamlessly.