Why there is no universal millimetre tolerance

You will find confident figures online for how far jar diameters can differ before a wick stops working. We are not going to add one, for three reasons you can check yourself.

1. The pool grows with the square of the diameter

A wick has to melt a circle of wax, and the area of a circle is π × r² using the inside radius. Area does not rise in step with diameter; it rises with its square. Going from 6.3 cm to 7.0 cm inside is an 11% wider jar and 23.5% more wax surface. Worse, the same gap in millimetres means more in a small jar than a large one, which kills any single mm rule before it starts:

Read across the +5 mm column: 19.0% more pool in a small jar, 11.0% in a large one. A "5 mm tolerance" would be generous for your smallest jars and fussy for your largest. The worked maths for a single 5 mm step is in the 5 mm difference post.

2. The wick's window depends on the rest of your formula

How wide a pool one wick can hold depends on wax, fragrance oil and load as well as the wick. A wick that reaches the glass in a 6.8 cm jar with one oil may stop short with another; that is why different fragrance oils can need different wicks in the same jar. So even a proven band is proven for one formula. Change the oil or the load and the band's edges can move.

3. Diameter is not the only thing that has to match

Two jars can share an inside diameter and still burn differently: a 10 cm-tall jar keeps the flame deeper for longer than an 8 cm one, a jar that narrows towards the bottom shrinks the pool late in the burn, and a thick base shortens the burn column. Diameter bands group jars that might share a wick. Shape and depth decide whether they do.

Grouping CSI jars on paper: what the listings allow

Of CSI's 72 container listings, 21 state some diameter (outer, base or unspecified) and 20 give only a width; none states an inside diameter. The table covers every in-stock glass jar listing with a usable figure, sister colours grouped, quoted as written, with the paper group it falls into. Heights matter as much as widths here, so they are included.

Three things stand out. The 7 cm group is written three different ways (width, outer rim, plain diameter), and nothing tells you whether the three mean the same measurement. Inside each group the heights differ: 6.5 and 8 cm in the 6 cm group, 8 and 10 cm in the 7 cm group. And the gap between groups is large: if the 6 and 7 cm figures are both outside measurements with similar walls, the 7 cm jars carry about 41.3% more pool than the 6 cm ones. Across that gap, sharing a wick would be a surprise; inside a group it is a question worth testing. Whether an outside figure can stand in for an inside one is taken apart in The Supplier Gives the External Diameter of the Candle Jar — Can I Use That for Wick Selection or Do I Need the Internal Diameter?.

Measure the spread inside one jar type first

Mass-produced glass varies from piece to piece and batch to batch. Before you ask whether the 200 ml Matte and the 300 g Iridescent can share a wick, find out how much three Matte jars differ from each other. That spread is the floor: if your own Matte jars differ by 2 mm inside, no band built on them can pretend to be tighter.

How to measure the inside diameter

Use callipers with inside jaws if you have them; otherwise lay a steel ruler across the mouth and read the inside edges, or cut a strip of card to fit snugly across the inside and measure the strip. Measure at three heights: just below your fill line, where the first pools form; half-way down; and about 1 cm above the inside floor, near where you will stop the burn. Turn the jar 90 degrees and repeat, because a jar can be slightly oval. If you only have the outside figure, measure the wall at the rim and take off two wall thicknesses; the full method is in the diameter-and-wick hub.

The edge-jar method: prove a band with two jars

A wick that works in the narrowest jar of a band and in the widest has shown it can cope with both ends. The jars between are only covered if they sit between the edges on everything else that matters: depth, shape and fill line. That is the whole idea; the steps make it repeatable.

Pass, fail and what else must match

Soot, heat and flame size are the narrow-edge signals; tunnelling and a wax ring are the wide-edge signals. If one wick shows both across a band (hot in the narrow jar, rings in the wide one), the band is too wide for that wick, and no single wick will close it.

What else has to be identical across a band

A band is a promise that the wick sees the same job in every jar. That holds only if these match: wax (switching from CSI 464 to a coconut-soy blend is a new test), fragrance oil and load (a band proved at 8% of one oil is not proved at 10% of another), fill line (a jar filled closer to the rim burns at a different height relative to the rim), shape class (straight-sided with straight-sided), and depth within the range your edges covered. Colour, frosting and label do not change the wick; the glass under them can, so measure one jar of every colour you sell. If your jar narrows towards the bottom, the diameter to band on is its own question, answered in My Jar Narrows Toward the Bottom — Should I Select the Wick Based on the Top, Middle or Bottom Internal Diameter?.

Worked example: one wick for the 7 cm group, costed

Suppose you sell in the Matte 200 ml ("width 7 cm", 8 cm tall), the Clear 150 g (outer rim 7 cm, base 6.5 cm, 8 cm tall) and the Green Iridescent 300 g ("Diameter 7cm", 10 cm tall), and C2 already works in the Matte. For costing only, suppose your measurements put the Clear 150 g narrowest at pool height and the Iridescent widest (it is also the deepest, so it would be an edge anyway). Your jars may sort differently; measure before you pour.

The fill weights are the listings' own figures and the 150 g planning figure from the 200 ml fill-weight hub; weigh your own test pours. The oil is Woody Oud Fragrance Oil as a costing example only: its page states "candles (6-10%)", and at 100 g and above it is flat at ₹4.53 per gram, so the 100 g bottle is the sensible test size. Wax is Luxury Soy Wax CSI 464 (page: up to 10%, so 8% sits inside it) at ₹0.51 per gram.

What to buy, and what is left over

Materials: CSI 464 1 kg (₹507.40) plus 500 g (₹260.00), because 966 g of wax leaves too little in a single 1 kg bag for pitcher loss and top-ups; 100 g of Woody Oud (₹453.12); C1, C2 and Cotton Braided packs of 10 (₹59.00, ₹94.40, ₹120.00). Total ₹1,493.92. If you own none of the jars, add a Matte pack of 5, two Iridescent jars and a Clear 150 g pack of 10 (₹1,598.00), for ₹3,091.92 in all; eight Clear jars and four Matte jars remain for measuring or production.

Watch the oil. The five candles use 84 g of a 100 g bottle, leaving 16 g. If the wide edge fails and you retest two Iridescent candles on Cotton Braided, they need 48 g of oil (₹217.50) and 552 g of wax (₹280.08), a little more than the 534 g left from 1.5 kg. Neither the bottle nor the bag covers the retest, and it needs two more Iridescent jars (₹300.00), so buy a second 100 g of oil then, or a 500 g if the band is likely to become production (at ₹4.53 per gram it is no cheaper per gram, only fewer orders).

Why the edges, not everything

The alternative is a full bracket on every jar: three wick rungs, two candles each, three jar types, eighteen candles and 3,600 g of candle. That is ₹1,680.51 of wax and ₹1,304.99 of oil, ₹2,985.49 together, against ₹870.77 for the band test: ₹2,114.73 less in wax and oil. It also takes more than three times the candle-burn hours. A full bracket is still the right move for a jar with no working wick at all. For a band where one jar already works, test the edges.

What to buy to prove a band

Wicks first, because they are the variable; then jars from the two paper groups most likely to be merged. Chosen on what the pages state, price and stock on 26 September 2026, not on burn performance: CSI publishes no burn data for any jar and wick pairing.

Keeping a proven band honest

A band proved once is proved for that batch of glass, that oil and that load, so write down which. Record each band on one sheet: the jar types in it, their measured inside diameters at three heights, the spread within each type, the edge jars, the wick, wax, oil and load, and the burn logs. It is short, and it is what you check against when something changes.

If a new, wider jar arrives for a band that was proved on 7 cm glass, the question is no longer "does the band stretch" but "does this jar need its own wick"; the retest scope for a 7 cm to 8 cm move is in I Already Finalised My Wick for a 7 cm Jar — If My New Jar Is 8 cm Wide, Do I Have to Repeat the Entire Burn-Testing Process?.

Frequently asked questions