How much fragrance or essential oil should I add to melt and pour soap?

Fragrance oils are blended for soap: they last longer in the bar and come in scents nature can't give you, like chocolate, ocean or cotton candy. Essential oils come straight from plants. They're the choice when you want a plant-only scent, but they're more expensive and some, especially citrus, fade faster.

Choose either for the way it smells. Whichever you use, buy it from a soap supplier and make sure it's labelled skin-safe; candle fragrance isn't made for skin.

Melt the base under cling film in short bursts, then let it cool to 125°F (52°C) before you add scent. Hotter base drives off the lighter notes of the oil before the soap sets. Stir slowly for a few seconds so it spreads evenly, then pour.

Weigh your scent rather than counting drops when you can: drop size changes with the bottle and the oil, and a kitchen scale that reads to 0.1 oz is far more repeatable. Drops are fine for the small amounts in a single pound of base.

Every fragrance and essential oil has its own maximum for soap, set by the supplier. Most land around the amounts above, but some, like cinnamon or clove, are much lower. Check the label or the supplier's sheet, and never use more than it allows, whatever a recipe says.