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.
Why container candles shrink
Molten wax is less dense than solid wax. When you pour and the wax begins to cool, the outer edges and surface set first while the wax around the wick stays liquid longest. As that inner core finally solidifies and contracts, it pulls the surface down, creating the characteristic funnel-shaped sinkhole. The effect is more pronounced with waxes that have a large density change between liquid and solid — paraffin shrinks 10–15% compared to 3–10% for soy, which is why soy candles still need top-ups despite being marketed as “clean-pouring.”
How the calculation works
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.
Shrinkage by wax type
| Wax type | Typical shrinkage | Notes |
|---|---|---|
| Soy (single-pour blends) | 2–5% | Some soy blends are formulated to shrink less |
| Soy (natural 100% soy) | 5–10% | Higher shrinkage; better fragrance throw |
| Paraffin | 10–15% | Highest shrinkage; may need large top-up |
| Coconut wax | 3–7% | Slower cooling helps reduce sinkholes |
| Beeswax | 5–9% | Dense and slow to solidify |
| Paraffin/soy blend | 5–10% | Depends on blend ratio |
Use the custom field in the calculator if your wax behaves outside these ranges — every supplier’s formulation differs.
Worked example
An 8 oz jar (approximately 236 mL) of natural soy wax:
- Pour-short setting: 10% → first pour volume ≈ 212 mL
- Shrinkage: 7% → void after cooling ≈ 212 × 0.07 = 14.8 mL
- Top-up needed: 14.8 / (1 − 0.07) ≈ 15.9 mL, about 14.3 g of wax at 0.9 g/mL density
Without the correction, pouring exactly 14.8 mL would underfill by about 1 mL after the top-up shrinks. The formula ensures you heat exactly the right amount.
Timing and technique for the top-up
The top-up pour works best when:
- The first pour is fully set — wait until the sinkhole has fully formed, typically 4–8 hours depending on ambient temperature and jar size. Pouring too early risks blending without actually filling the void.
- Top-up wax is slightly warmer than the first pour — warming it 5–10°C above the first pour temperature helps it melt the surface edge slightly, blending the two layers invisibly.
- Pour slowly and into the centre — directing the top-up at the deepest point of the sinkhole avoids creating air pockets or a visible pour line at the edges.
- Use a heat gun sparingly — a brief pass with a heat gun after the top-up sets can smooth any remaining surface irregularities without disturbing the wick.