Vessel diameter, wax, and wick family in. A starting size and a test set out. No calculator can replace a burn test, so this one doesn't pretend to. It gets you to the right three wicks to try.
The middle size is your starting point, built from published manufacturer sizing for your wax and diameter, adjusted for dye and fragrance load. Buy it along with the size below and the size above. Pour three otherwise identical candles, burn each in a draft-free spot, and watch the melt pool: it should reach the glass within roughly one hour per inch of diameter. Wax left clinging to the walls after a full burn means size up. A deep pool, a dancing flame, soot, or a hot container means size down.
Every chart and calculator in this craft, including this one, is an estimate. Your specific wax batch, fragrance oil, dye load, and even the shape of your vessel move the answer. The suppliers know this, which is why their own charts carry the same warning in the fine print. The honest version of a wick calculator is one that hands you the right test set and gets out of the way.
Soy is the baseline here; it's viscous and usually needs the largest wick of the common container waxes. Paraffin flows easily and typically runs one size smaller. Coconut apricot blends burn hot with a wide pool, so they also start one size down from soy. Beeswax varies so much hive to hive that no chart is trustworthy: start from the soy suggestion and expect more rounds of testing. Vessels wider than 3.5" generally want two wicks; the calculator switches modes automatically and sizes each wick for its share of the pool.
When your wick is dialed in, run the numbers through the candle pricing calculator, and if you're scaling the recipe, the batch scaling calculator keeps your ratios true.
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