I'm Using the Same Amount of Wax in a Tall Narrow Jar and a Short Wide Jar — Why Do They Need Different Wick Sizes?

Diameter is squared: the maths of the pool

Everything in this article follows from one formula. The area of the pool is π × r², where r is half the inside diameter at the height of the wax. Because the radius is squared, area grows much faster than width. That is the part people underestimate: a jar that looks "a bit wider" is often a lot more pool.

Read down the table: each half centimetre of width adds more area than the last, while the depth needed for the same candle shrinks from about 6 cm to 3 cm. The wax did not change. The oil did not change. The grams did not change. Only the shape of the space did, and with it the job the wick has to do.

Why 170 ml? A 150 g candle, by the common 0.86-to-0.9 rule of thumb for wax against water, needs about 167 to 174 ml of space; we use 170 ml as a round middle. Your jar's real figure comes from a weighed test pour (see the 200 ml fill-weight hub). None of this changes the conclusion: equal volume is not equal area.

Distance, heat and the edge of the pool

A candle flame heats the wax immediately around the wick first. Heat then spreads outward through the liquid pool and the softening wax towards the glass. At the same time the glass and the air above it are losing heat to the room. The pool reaches the wall when the flame supplies heat fast enough to win that race at the edge.

In the narrow jar in our example, the edge is 3.00 cm from the wick; in the wide jar it is 4.25 cm, 41.7% farther. The wide jar also has more pool to keep liquid: at a common working depth of 0.5 to 1 cm, about 14.1 to 28.3 ml of molten wax in the narrow jar against 28.4 to 56.8 ml in the wide one. A wick sized for the narrow jar's job simply does not produce enough heat to melt the wide jar's edge in 3 to 4 hours. The wax at the glass stays solid; the pool drills down around the wick; and after a few burns you have a tunnel.

Why the fix is not "burn it longer"

A small wick in a wide jar can sometimes creep to the edge if you leave it burning for many hours. That is not a solution. Very long burns are poor practice, the flame can grow as carbon builds on the wick, and your customer will not burn for six hours to make up for your wick. The rough rule some makers use, about one hour per 2.5 cm of width for a full pool, gives about 2.4 hours for 6 cm and 3.4 hours for 8.5 cm. If a wick needs much more than that, it is under-sized for the jar.

And the reverse problem

Now take the wick that makes a full pool in the wide jar and put it in the narrow one. It delivers heat for 56.75 cm² into a pool of 28.27 cm². The pool reaches the glass early and keeps deepening; the flame can grow tall and flicker; soot on the glass is common; and the glass near the flame runs hotter than it needs to. None of that proves the wick is "bad". It proves it belongs in a wider jar.

What the height changes: hours, descent and the last third

If diameter decides the wick, what does the tall jar's extra height do? Three things, all of which show up in testing rather than wick choice.

1. How fast the flame sinks

Every gram the candle burns comes off the top. In the narrow jar, 10 g of wax lowers the surface by about 0.40 cm (using a 0.88 mid-point factor); in the wide jar, about 0.20 cm. So in the tall jar the flame moves down into the glass about twice as fast for the same grams burned. Deeper in a narrow jar, there is less air movement around the flame and the glass has been warming for longer. A wick that looked calm at the top can behave differently halfway down.

2. How many hours the candle lasts

Total burn time is roughly grams of candle divided by grams burned per hour. You can measure the second number yourself: weigh the candle before and after a 3-hour session. Because the wide jar needs a larger wick (or two) to melt its larger pool, it usually burns more grams per hour, so the same 150 g tends to finish in fewer hours. We do not publish burn-rate figures for CSI wicks; weigh your own sessions and you will have the number for your wax and oil.

3. Where the final third starts

Makers watch the last third of a candle closely, because heat builds as the wax gets shallow and the flame nears the base. In the tall jar the last third is the bottom 2.0 cm or so; in the short jar it is the last 1.0 cm, and the whole candle is only 3.00 cm deep. The short jar reaches its "final third" conditions after very little wax has gone, and the base glass is never far from the flame. Stopping the burn with about 1 cm of wax left is common practice; in a 3 cm candle that is a third of the candle, so plan fill weight and label claims accordingly. A thick base makes this worse; see the thick-base post.

A real pair from CSI's listings

Two pairs of CSI listings state the same wax capacity in very different shapes. We quote them as written, verified 26 September 2026; none of the diameters is stated as inside, so the comparison is on listed figures only.

On the listed figures, the 130 g ceramic's pool would be 56.3% larger than the 130 g shot glass's, and the 120 g Shiny jar's 44% larger than the 120 g tall ceramic's. Ceramic walls are often thicker than glass, which would narrow the difference on the inside, but it would take a lot of wall to close a gap of that size. Both of these ceramic containers are out of stock on 26 September 2026 (CSI's separate Ceramic Jar for Candle Making is in stock), so treat them as an illustration, not a purchase.

Notice the reverse case in the same table: the in-stock Shot Glass (130 g, 8 cm tall) and Shiny jars (120 g, 6.5 cm tall) both list 6 cm. If their insides measure alike, they are one wick bracket despite different heights and capacities. That is a hypothesis to prove with a burn test, and the business logic behind it is the fewer-wick-sizes post.

Proving it: a paired test in twelve candles

The fastest way to see the physics is to pour the same batch into both jars and bracket each. Twelve candles: three rungs per jar, two jars per rung. Every candle gets the same wax, oil, load and cure, so any difference you see comes from diameter and height.

If the results surprise you (C1 over-melting the narrow jar, say, or Cotton Braided under-melting the wide one), the oil or load is shifting the wick more than the jar is. That is a fragrance effect, not a diameter effect, and it is covered in the wick-works-with-one-fragrance post. Whether to go to one big wick or two small ones in the wide jar is the single-versus-double post.

Wicks and tools for a paired test

Five items, chosen on page wording, price and stock on 26 September 2026. None of them has been burn-tested by CSI in your jars.

What the paired test costs, and what the wrong wick costs

Worked example: twelve 150 g candles at 8% (12 g oil + 138 g wax each), your own jars. Prices are CSI live pricing on 26 September 2026; costs exclude labour, freight, packaging and GST.

With your own oil the kit is ₹1,538.20. Six grams of Lavender left over will not cover a re-pour; add another 50 g (₹270.00) if a rung needs repeating. Five holders will not hold all twelve at once; pour in two rounds, or use sticks for the singles. Lavender is a costing example only; every size is in stock on 26 September 2026, and 8% sits inside both its stated 6-10% range and CSI 464's 10% ceiling.

Per candle, the jar shape moves the wick cost only

Wax and oil for one 150 g candle at the 10 kg wax and 1 kg oil prices: ₹70.02 + ₹62.65 = ₹132.67, identical in both jars. If C1 wins the narrow jar and Cotton Braided wins the wide one, the materials are ₹138.57 and ₹144.67: the wide candle costs ₹6.10 more, all of it wick. Two C1s would cost ₹11.80 per candle instead of ₹12.00. Neither difference is worth a tunnelled candle. The expensive mistake is the refund, or the whole batch melted out and re-poured, when a narrow-jar wick goes into a wide jar because "it's the same candle".

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