Candle vessels: sizes, wax volume and wicks
What a 4, 8 or 12 oz jar means in millimetres, how to turn the capacity the studio computes into grams of wax, what the ⌀23 mm wick socket is for, which wick goes with which cavity, and the safety rules that apply to concrete around a flame.
Updated 6 November 2026 · 8 min read
Jar sizes: ounces to millimetres
Candle jars are sold by the volume they hold to the brim, in US fluid ounces: 1 oz is 29.57 ml. The wax that goes in is less, because you stop 10 to 15 mm below the rim, and the net weight printed on a label is lower still, because wax weighs about 0.9 g per millilitre. An 8 oz jar typically carries 170 to 185 g of wax, which a label calls 6 oz.
The sizes below are the ones makers ask for most, with a wall of 5 to 6 mm and a floor of about 10 mm. Capacities are for a straight cavity; the studio computes the real volume with the floor fillet, which is a few millilitres less. The two 80 mm candle presets correspond to the 6 oz and 7 oz rows.
| Format | Outer size (mm) | Cavity (mm) | Brim capacity | Wax | Wick |
|---|---|---|---|---|---|
| Tealight | ⌀55 × 25, wall 4 | ⌀47 × 18 | ≈ 30 ml (1 oz) | ≈ 20 g | pre-tabbed tealight wick, ⌀12.5 mm tab |
| 4 oz | ⌀65 × 60, wall 5 | ⌀55 × 50 | ≈ 120 ml (4 oz) | ≈ 85 g | cotton wick on a ⌀12.5 or 15 mm tab, or a 9.5 mm wood wick |
| 6 oz square | 80 × 80 × 60, wall 5 | ellipse 70 × 67, depth 49 | ≈ 183 ml (6.2 oz) | ≈ 130 g | 15 mm wood wick in the ⌀23 mm socket |
| 7 oz round | ⌀80 × 70, wall 6 | ⌀68 × 58 | ≈ 210 ml (7.1 oz) | ≈ 150 g | 15 mm wood wick, or cotton on a 15 mm tab |
| 8 oz | ⌀80 × 80, wall 6 | ⌀68 × 68 | ≈ 245 ml (8.3 oz) | ≈ 180 g | 15 mm wood wick |
| 12 oz | ⌀90 × 85, wall 6 | ⌀78 × 73 | ≈ 350 ml (11.8 oz) | ≈ 250 g | 16–19 mm wood wick, or two cotton wicks |
Reading the capacity the studio shows
The studio updates the cavity volume as you change the dimensions, in millilitres and in ounces (ml divided by 29.57). That figure is the brim capacity, and the README repeats it in the vessel table. To turn it into wax:
- Subtract the headroom: cavity area × 10 to 15 mm. On a ⌀68 mm cavity, 12 mm of headroom is π × 34² × 12 ≈ 44 ml.
- Multiply what is left by 0.9 g/ml. Soy, paraffin and coconut blends all sit close to that figure; check your wax datasheet if you need the exact one.
- Convert to label ounces by weight if you sell in the US: grams divided by 28.35.
The wick socket
The cavity floor carries a recess, ⌀23 × 2 mm by default, formed by the silicone sleeve, so it comes out as crisp as the rim. It is sized for the standard metal clip of a wood wick, which is what the reference kit was designed around: the clip sits flush and centred and cannot slide when you pour. Set the diameter to 15 or 12.5 mm in the studio for a cotton wick on a sustainer tab instead; both are standard tab sizes. Set it to zero for planters and trays.
Fix the clip or tab in the socket with a wick sticker, a dab of hot glue or a drop of molten wax. Because the socket is 2 mm deep, the first pour of wax covers the tab completely, and the wick stays centred for the whole burn.
Choosing the wick for the cavity
Wick size follows the diameter of the cavity, not the outside of the vessel, and the charts differ with the wax. Wood wicks, from supplier charts: 9.5 mm for cavities of 50–63 mm, 12.7 mm for 63–76 mm, 15.9 mm for 76–88 mm, 19 mm for 88–101 mm, all about 1 mm thick. The reference kit pairs a 15 mm wood wick with a ⌀70 mm cavity. Cotton wicks: use the maker's chart for your wax and the cavity diameter; above about 85 mm, use two wicks rather than one oversized one.
Every new size needs a test burn: the melt pool should reach the wall within a few hours, with a calm flame and no soot. Tunnelling means the wick is too small; a tall, smoking flame or a pool that gets dangerously deep means too big.
The centring tool and the pour
Candle kits include a printed bow-tie with a 16 × 2 mm slot that rests across the rim and holds a 15 mm wood wick upright while the wax sets. For a cotton wick, a clothes peg or a wick bar does the same. Pour at the temperature your wax maker specifies, let the candle set fully, then trim the wick to about 5 mm for wood, 6 mm for cotton.
Before any wax goes in, the vessel must be bone dry and sealed inside. Wax soaks into bare concrete and darkens it, and moisture trapped in the wall is what cracks a concrete candle later. Both points are in the safety list below.
Safety: concrete around a flame
- Wall thickness of at least 5 mm, 6 mm for comfort; floor of at least 8 mm. The studio warns below 4 mm, but a 4 mm wall is for tealights, not for a jar that burns for hours.
- Dry the vessel fully before sealing: 48 hours after demoulding at the very least, a week is better. Trapped water expands when heated and can crack the vessel.
- Seal the inside with a heat-resistant, water-based sealer and let it cure before pouring wax. Sealing keeps the wax out of the concrete and the concrete's moisture away from the wax.
- Never let the candle burn to the bottom. Stop with 10 to 12 mm of wax left: the last centimetre heats the floor and the socket, and that is where concrete cracks.
- Burn on a heat-resistant surface, away from draughts, and trim the wick to 5 mm before each burn.
- Label every candle you sell with the standard cautions: burn within sight, keep away from things that catch fire, keep away from children and pets, stop burning when 12 mm of wax remains. Check your country's candle labelling rules and keep a record of your test burns.
- Test-burn every new vessel design to the end before selling it, with the wax and wick you will actually use.