What Is Castile Soap and How Do You Use It?

Look under most Australian kitchen sinks and you'll find the same story: a cluster of single-use plastic bottles, each doing exactly one job. A spray for the bench. A separate one for the bathroom. Another for mopping. A different brand again for dishes. The cabinet is full, the bin fills fast, and the ingredient labels read like a chemistry exam. There is a simpler way to clean a home, and it starts with one plant-based concentrate that has been doing the heavy lifting for centuries.

Castile soap is that concentrate. Made entirely from saponified plant oils, it can replace most of what fills the cabinet under the sink: hand wash, body wash, dishwashing liquid, all-purpose spray, floor cleaner, and laundry detergent. The market has matured well beyond the classic imported brands, too. Australian-made options, including By Me's lemon myrtle liquid castile, now offer a genuinely local, botanical alternative that is well worth knowing about.

This article covers four things: what castile soap actually is and how it's made, how it differs from conventional cleaners, how to use it correctly at home with real dilution numbers, and what to watch for when making the switch. By the end, you'll have everything you need to choose the right formula and start replacing the plastic lineup on your bench.

What castile soap is and how it's made

The Castile origin story

The name comes from the Castile region of Spain, where soap makers used the one ingredient they had in abundance: pure olive oil. Traditional Castile soap was 100% olive oil saponified with an alkali, resulting in a mild, conditioning bar with a distinctive soft lather. The name has since broadened to cover soaps made primarily from plant oils, though contemporary formulas vary considerably, most are plant-oil based, with some brands adding post-saponification preservatives or fragrances. The guiding principle across quality castile-style products remains the same: plant oils as the foundation, no animal fats. Many brands interpret this slightly differently, so reading an ingredient list remains worthwhile.

How saponification works

Saponification is the chemical process that turns oil into soap. Plant oils, olive, coconut, hemp, and sometimes jojoba, react with an alkali: potassium hydroxide for liquid soap, sodium hydroxide for bar soap. The reaction produces two things: soap molecules (specifically, potassium or sodium salts of fatty acids) and glycerin, a natural humectant. In a properly saponified formula, the alkali is fully consumed in the process and no active lye remains in the finished product. The result contains soap, glycerin, and water. Note that commercial liquid soaps may include additional preservatives, fragrances, or stabilisers added after saponification, check the label if your priority is a truly minimal-ingredient formula.

Modern formulas versus the traditional olive-oil base

Most quality liquid castile soaps today blend olive oil with coconut oil, for good reason. Coconut oil raises the lather; olive oil conditions the skin. Most international brands increase the coconut ratio to produce more foam. Knowing this helps you read an ingredient list with confidence and choose a formula that suits your skin and your cleaning goals.

What makes it different from conventional cleaners

Synthetic versus plant-based surfactants

Conventional supermarket cleaners rely on synthetic surfactants: sodium lauryl sulphate, artificial fragrance, preservatives, and stabilisers. These ingredients clean effectively, but some may leave residues on surfaces and skin, and certain synthetic surfactants can persist in waterways according to ecotoxicology research. A castile soap concentrate derives its cleaning power entirely from saponified plant oils. The difference matters not just at the ingredient level but in what goes down your drain, what your children and pets contact on freshly wiped surfaces, and what stays on your skin after rinsing.

One concentrate, many jobs

The single most practical advantage of liquid castile soap is its versatility at different dilutions. A household that switches can legitimately eliminate separate products for hand washing, dishwashing, surface spraying, mopping, and laundry, though with a few important caveats: it is not suitable for automatic dishwashers, performance can be affected by hard water, and facial use requires care (more on both below). Fewer products means less plastic waste, less storage clutter, and lower cumulative chemical exposure across the home. One bottle, used correctly and at the right ratios, does the work of an entire cleaning cabinet.

Castile soap uses and dilution: a room-by-room guide

Personal care from head to toe

For body wash, the concentrate works beautifully in a foaming pump dispenser at a ratio of one part soap to three parts water. That dilution produces a lather close to conventional foaming soap without waste or over-concentration. Unscented castile is always the safer choice for direct skin contact, particularly for those with sensitivity. An unscented castile formula is also worth considering for households with young children or anyone managing reactive skin.

For shampoo, a similar dilution applies, rinse thoroughly, and some people find their scalp benefits from a diluted apple cider vinegar rinse afterwards if dryness or tightness is an issue, though this is a personal preference rather than a universal recommendation. Facial use requires extra caution because of the soap's alkaline pH, which is covered in more detail below.

Household cleaning from kitchen to bathroom

For hand washing dishes, a small squirt of the concentrate directly onto a scrubbing brush cuts through grease cleanly. For an all-purpose surface spray, mix one part soap with ten parts water in a glass spray bottle and add a few drops of essential oil if you want fragrance. Bathroom tiles, bench tops, and basin surfaces all respond well to this solution. One important note: castile soap is not suitable for automatic dishwashers. Its saponification-based chemistry produces far too much foam for a closed machine and will cause overflow. Stick to hand dishwashing only.

Laundry without the plastic jug

Castile soap does not need pre-diluting for laundry. Add two to four tablespoons directly to a high-efficiency machine, or double that for a standard top-loader. The crucial addition for Australian conditions: half a cup of white vinegar in the rinse cycle. Many parts of Australia have moderately hard water, and the minerals in hard water bind with castile soap to form soap scum on fabric. The vinegar rinse dissolves this residue and keeps clothes soft. If you want to boost whitening or deodorising, washing soda is a more reliable option than baking soda in hard-water areas, where baking soda can worsen soap scum formation rather than help.

Getting dilution right for every room

Castile soap concentrate: dilution guide

The concentrate is highly concentrated, and the single most common mistake is using too much. Over-concentration leads to residue rather than cleaner results. Here are the ratios that work across every application in the home:

• Body wash: 1 part soap to 2, 3 parts water

• Hand wash in a foaming pump: 1 part soap to 10 parts water

• Dishwashing (by hand): 1 part soap to 10 parts water, or a small squirt on a wet brush

• All-purpose surface spray: 1 part soap to 10 parts water

• Carpet spot cleaner: 1 part soap to 20 parts water

• Laundry: 2, 4 tablespoons per load, no pre-dilution

Why the ratio matters more than the brand

Getting the dilution right is what makes castile soap both safe and effective, it is not a step to skip. Undiluted, it has a pH of approximately 9 to 10, compared to the skin's natural pH of 4.5 to 5.5. That gap is wide enough to temporarily disrupt the skin barrier, strip natural oils, and increase water loss through the skin. Studies on saponified soap formulas indicate that undiluted application can increase transepidermal water loss by up to 42% and cause visible dryness or flaking in a significant proportion of people with sensitive skin. Proper dilution reduces the hydroxide ion concentration significantly, bringing the lather into a range that most skin tolerates well. An under-concentrated solution, on the other hand, simply won't clean. The ratio is the skill that makes the soap outperform the alternatives.

By Me's lemon myrtle castile: an Australian botanical worth using

Australian ingredients in a plant-based formula

By Me's liquid castile soap is a Melbourne-made formula built around plant-based oils and scented with lemon myrtle essential oil, a native Australian botanical from the species Backhousia citriodora. Lemon myrtle is not simply a fragrance choice. Its primary bioactive compound, citral, disrupts microbial cell membranes and inhibits biofilm formation, making it a particularly effective natural antimicrobial. Research places its Rideal-Walker disinfectant coefficient at 16, compared to tea tree oil's 11, suggesting meaningfully stronger antimicrobial activity at comparable concentrations. The clean, bright scent is a bonus; the functional cleaning contribution is the real reason it belongs in a low-tox formula.

The refillable glass ecosystem

By Me's castile soap is designed to power an entire refillable system, not sit in a single-use bottle. The concentrate goes into amber glass foaming pump dispensers, available in a range of finishes to suit different bench aesthetics. One bottle diluted at the right ratios fills a hand wash dispenser, a body wash bottle, and an all-purpose spray bottle. The dispensers are designed to stay on the bench, functional and considered in equal measure. The complete swap comes down to one plant-based concentrate and one set of reusable glass dispensers, with no more single-use plastic cluttering the bathroom or kitchen.

Why choosing local matters

An Australian-made castile soap means a shorter supply chain, ingredients sourced closer to home, and a brand whose sustainability commitments are transparent and visible rather than printed on a label from overseas. By Me's approach to cruelty-free, plastic-free product design runs across the range, a philosophy that extends beyond the hero products. When you buy local, you also support a maker building the kind of low-tox, zero-waste ecosystem that makes sustainable cleaning genuinely practical, not just an intention.

Safety, skin sensitivity and what to watch for

Unscented castile for sensitive skin and vulnerable households

Castile soap's alkalinity (pH 9 to 10) is worth understanding clearly. The acid mantle, the skin's protective barrier, can take up to 90 minutes to recover after a single wash, and longer with repeated exposure. For everyday body and hand washing at proper dilution ratios, healthy skin handles the soap well. For the face, a dedicated pH-balanced cleanser remains the better choice, particularly for anyone managing eczema, rosacea, or acne-prone skin.

Standard castile soap is generally not recommended for infants, whose skin barrier is considerably more delicate and more vulnerable to alkaline disruption, paediatric guidance consistently favours specially formulated baby cleansers with a balanced pH. For pets, unscented versions are far safer than scented ones: essential oils such as eucalyptus and lavender can be harmful if licked or absorbed through the skin during grooming. Pregnant people with no existing skin sensitivities can generally use pure castile soap safely, though a patch test before full use is always sound advice, particularly given that hormonal changes can temporarily increase skin reactivity.

Making the switch: your starting point

Castile soap is one of the most genuinely versatile cleaning ingredients in the low-tox toolkit. At the right dilution for the right job, it replaces a significant portion of the conventional cleaner lineup without sacrificing performance. The key is understanding the ratios, respecting the pH, and pairing the concentrate with a refillable system that makes the routine stick.

For Australians ready to make the switch, By Me's lemon myrtle liquid castile gives you both the ingredient and the system to build a plastic-free cleaning routine from the ground up. One concentrate, multiple dispensers, and a bench that actually looks the way you want it to. That is a swap worth making.

Start with the foaming hand wash kit and build from there. Once you see how far a single bottle goes, the cabinet full of single-use plastic will feel like the thing that never made sense.