How Much Can a 5 mm Difference in Candle Jar Diameter Affect the Wick Size and Melt Pool?

The worked maths: 7.0 vs 7.5 cm, and every size either side

Melt pool area is π × r², using the inside radius at the height the pool sits. A 7.0 cm inside diameter has a 3.5 cm radius: 3.14159 × 3.5 × 3.5 = 38.48 cm². At 7.5 cm, 3.14159 × 3.75 × 3.75 = 44.18 cm². The difference is 5.69 cm², which is 14.8% of the original: the wick has to melt that much more surface, with the same flame, from the same centre. Going the other way, 6.5 cm gives 33.18 cm², 5.3 cm² or 13.8% less.

The length change is only 7%, because 5 mm is that share of 70 mm. The area change is about double that, because both the width and the depth of the disc grow. That squared effect is why a difference you can barely see side by side shows up in a burn.

Read the last column as the wick's point of view. Every direction from the wick, the glass has moved 2.5 mm. On a 7 cm jar that is 7.1% more reach; on a 10 cm jar, 5.0%. Small jars are sensitive because the wick's reach is short to begin with, and 2.5 mm is a big share of it.

If your change is 1 cm rather than 5 mm, the numbers roughly double (7 to 8 cm is about 31% more), and the question becomes how much of the burn-testing process to repeat, covered in the 7 to 8 cm retest guide. How to adjust a starting wick for a deliberately wider jar is the step-up guide.

What that extra (or missing) area does to the burn

5 mm wider: the ring at the edge

Your wick melts a pool that stops somewhere. In the old jar it stopped at the glass. In a jar 5 mm wider inside, the same pool can stop 2.5 mm short, leaving a thin ring of solid wax round the edge. On the first burn that looks harmless. Many makers find soy candles "remember" the first pool: the next burn tends to stop at the same ring, the centre sinks, and over several burns the ring becomes a wall. That is tunnelling, and it starts with a pool a few millimetres too small. The fix order for tunnelling in a wider jar is set out in the narrow-fine, wide-tunnels guide.

It is not certain. A wick with spare reach, or a wax and oil that melt freely, may still reach the glass in the wider jar, just later. That is why the check below compares time to full pool, not only whether it got there.

5 mm narrower: the deeper, hotter pool

In a jar 5 mm narrower, the wick has 13.8% less wax surface to melt on a 7 cm jar but produces a similar amount of heat. The pool tends to reach the glass early, then keeps deepening past the 0.5 to 1 cm many makers aim for; the flame tends to stand taller and may flicker or soot; and in the last third, with the flame closer to the base and less wax around it, the glass tends to run warmer than it did in the old jar. None of these is certain. All of them are things to watch in a burn-down, never unattended, stopping with about 1 cm of wax left (common practice).

The deciding factor: how much margin your wick had

A wick is rarely exactly right. It is usually either comfortably inside what the jar needs or sitting at the edge. If you kept burn logs when you wicked the old jar, they tell you which, and that decides how seriously to take 5 mm.

The logic runs both ways. A wick that was slightly strong in the old jar can be just right in a jar 5 mm wider; one that was slightly weak may be just right in a jar 5 mm narrower. That is why we never say "5 mm means one wick size". It means one wick size of pressure on whatever margin you had.

Margin is also shared with other variables. A heavier fragrance or a higher load can eat into it; a change of oil at the same time as a change of jar makes it impossible to tell which one moved the burn. Change one thing, test, then change the next (load and wick size explains the load side).

Where a 5 mm difference comes from

The last row is worth pausing on. A wick that drifts 2.5 mm while the wax sets gives one side of the pool exactly the extra reach a 5 mm wider jar would, and the other side 2.5 mm less. If your test candles are not centred identically, you cannot measure a 5 mm effect at all. A Wick Centering Device costs ₹23.60 a holder, reusable.

The 5 mm check: seven candles, two rungs

When the inside differs by about 5 mm and your margin reading says "check", this is the proportionate test. It is smaller than a full re-bracket because you already know the wick family, the wax, the oil and the load work; only the jar has moved.

If neither rung wins cleanly, the change is bigger than it looked, or the jar has a shape effect: go to a full bracket as in the first-three-wicks guide. Rolling the check out across several jar sizes at once is a standardisation question, covered in the one-wick-across-jars guide.

Wicks and tools for a 5 mm check

Chosen on stated wording, price and stock on 26 September 2026. None of these pages states a jar-diameter range we would stand behind as a fitting rule, and CSI publishes no burn data; these are the rungs and tools the check needs.

The other thing 5 mm changes: fill weight and cost

At the same fill height, a straight jar's volume grows with the same square as the pool. So if a jar 7.0 cm inside gives you the series' planning candle of about 150 g, the same height in a jar 7.5 cm inside holds about 172 g (14.8% more), and a jar 6.5 cm inside about 129 g. Worked example only: weigh your own, as in the fill-weight hub.

At 8%, the wider candle carries about 13.8 g of oil instead of 12 g and about 20.4 g more wax. At CSI 464's 10 kg price, the extra wax is ₹10.36 a candle, or ₹5,181.57 across 500 candles, before the extra oil. It is not a reason to avoid the wider jar; it is a reason to re-cost it and re-weigh your label, and not to discover it when the wax runs out mid-batch. Costs exclude labour, freight, packaging and GST.

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