How much wax do I need?

The short version: an 8 fl oz jar takes about 6.9 oz of wax, not 8 oz. A 300 ml jar takes about 243 g, not 300 g. If that seems wrong, it is worth understanding why, because the same confusion causes short batches and wasted orders.

Volume and weight are different things

Vessels are sold by the volume they hold — millilitres, or fluid ounces. Wax is sold by weight — grams, or ounces. The unit names overlap in imperial, which is where most of the confusion comes from: a fluid ounce and an ounce are not the same measurement at all.

Water is the reference point. One millilitre of water weighs one gram. Wax is lighter, at roughly 0.9 grams per millilitre, so any given space holds about 10% less wax by weight than it would water.

Quantities on this page follow the unit switch in the header — tap g or oz and everything below converts.

Then take off the headspace

You never pour to the brim. Headspace keeps the wick clear of the rim, leaves room for a lid, and stops molten wax spilling if the jar is moved. Around 90% full is normal.

So the full calculation is two steps:

wax weight = vessel volume × fill level × wax density

Worked through for a standard jar of 240 ml poured to 90% full, in soy: that comes to 194 g of wax.

What about the 0.86 rule?

You will see this everywhere: multiply your jar size in fluid ounces by 0.86 to get ounces of wax. It is a reasonable shortcut for soy in a standard jar, and it is wrong for everything else, because it hides two separate variables inside one number — a fill level and a density.

Beeswax is denser at about 0.958 g/ml, so it needs roughly 6% more by weight for the same jar. Pouring a tin nearly to the brim changes the fill level. In both cases the shortcut misses. Keeping the two variables apart costs nothing and stays correct.

Wax densities

Measuring a vessel nobody has specified

Suppliers do not always print a capacity, and vintage or reclaimed vessels never do. Use water:

1. Put the empty vessel on a scale and zero it.
2. Fill it with water to exactly where you intend to pour.
3. Read the weight in grams.

That number is your usable volume in millilitres, with the fill level already accounted for. Multiply by your wax density and you have the wax weight. This works for tapered glasses, teacups, tins with a rolled lip, and anything else geometry would struggle with.

Do not forget wastage

The figure above is what ends up in the candles. It is not what you need to melt. Wax clings to the pot, the jug and the spatula, and on a small batch that loss is proportionally worse. Melt about 5% more than the arithmetic says.