How to Make Laundry Detergent (With Calculator), According to Science

Short answer: grate one bar of laundry soap and stir it into 1 cup of washing soda and 1 cup of borax. Use 2 tablespoons per load in a standard washer or 1 tablespoon in an HE machine. One batch takes 15 minutes and covers about 24 standard loads or 48 HE loads.

I have made my own laundry powder for years, and the recipe that works is the one that matches your water. The soap does the cleaning. The washing soda stops the minerals in your tap water from wrecking the soap. Get that balance wrong and towels turn stiff and gray within a few months. Below are the recipe, a calculator that scales it to your loads and water hardness, and the chemistry behind every ingredient.

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Laundry detergent calculator

Pick your recipe, machine and water hardness, then enter how many loads the batch should cover. The calculator also shows how much hardness one dose can neutralize, which decides whether homemade detergent works in your house.

Most of the US Midwest, Southwest and Florida have hard or very hard water. The Pacific Northwest, New England and much of the Southeast have soft water. Your water utility lists its hardness in its annual water quality report.

Ingredients and tools

Tools

• A box grater, or a food processor with a grating disc
• A large mixing bowl and a spoon
• Measuring cups, or a kitchen scale for repeatable batches
• Rubber gloves. Washing soda is strongly alkaline and dries out skin.
• A jar with a tight lid for powder, or a 2 gallon bucket and jugs for liquid

How to make powder laundry detergent

1. Grate the soap finely. Use the smallest holes that do not clog. Fine flakes have more surface area and dissolve faster in the wash. If the bar is soft and gums up the grater, leave it unwrapped for a few days to dry out.
2. Measure the powders. Put on gloves and add the washing soda and borax (or baking soda) to the bowl. Use the amounts from the calculator.
3. Mix until even. Stir for a minute, breaking up clumps of soap. For the finest texture, pulse the batch in a food processor for 20 seconds and let the dust settle before you open the lid.
4. Store it dry. Pour the mix into a jar with a lid. Washing soda pulls moisture from the air and sets into hard lumps if left open. Keep a tablespoon in the jar.

How to make liquid laundry detergent

The liquid version uses the same ingredients dissolved in water. It takes more effort and dissolves instantly in cold washes, which is its main advantage. For each gallon of finished detergent:

1. Melt the soap. Heat 4 cups of water until steaming. Add 1 grated bar of soap and stir over low heat until no flakes remain, about 10 minutes. Keep it below a hard boil or it foams over.
2. Add the powders. Stir in 1 cup of washing soda (adjust for your water with the calculator) and ½ cup of borax until dissolved. The mix thickens as you stir.
3. Dilute. Pour 6 cups of hot tap water into a bucket, add the soap mix, then add cold water to reach 1 gallon (16 cups). Stir.
4. Let it set overnight. The liquid turns into a lumpy gel as the soap molecules pack into large structures while it cools. This is normal.
5. Break it up and bottle. Stir or use a stick blender to smooth it out, then pour it into jugs. Shake before each use.

How much to use per load

Machine	Powder	Liquid	Extra for heavy soil
Standard top-loader	2 tbsp	½ cup	+1 tbsp washing soda
HE front-loader or HE top-loader	1 tbsp	¼ cup	+1 tsp washing soda
Hand washing (1 gallon basin)	1 tsp, dissolved first	1 tbsp	Soak 30 minutes

For a cold wash, dissolve the powder in a cup of hot tap water first. Laundry soap made from tallow dissolves poorly below about 70 °F (21 °C), and undissolved flakes stick to dark fabric.

Test your water hardness at home

Water hardness is the amount of dissolved calcium and magnesium, reported as milligrams of calcium carbonate per liter (the same as parts per million). The US Geological Survey groups it like this:

Class	Hardness (mg/L as CaCO3)	Grains per gallon
Soft	0 to 60	0 to 3.5
Moderately hard	61 to 120	3.5 to 7
Hard	121 to 180	7 to 10.5
Very hard	Over 180	Over 10.5

Water hardness test strips give a number in seconds. With no strips, a soap jar test shows the same thing:

1. Fill two jars. One halfway with tap water, one with distilled water. The distilled jar is your control.
2. Add soap. The same small pinch of grated soap, or 10 drops of liquid castile soap, to each.
3. Shake. Close the lids and shake both hard for 10 seconds.
4. Compare. Soft water gives a tall, lasting foam over clear water, like the control. Hard water gives little foam and turns cloudy.

The cloudiness in the hard water jar is calcium soap, the gray scum that rings a bathtub. In a washing machine it settles into fabric instead. Add a pinch of washing soda to the hard water jar, shake again and the foam comes back. That is why washing soda is in the recipe.

The science: how homemade detergent cleans

Soap molecules grab oil and water at the same time

Soap is made by reacting fats or oils with lye (sodium hydroxide), a reaction called saponification. The product is a sodium salt of a fatty acid, such as sodium stearate. Each molecule has a hydrocarbon tail 12 to 18 carbons long that dissolves in oil, and a charged carboxylate head (COO⁻) that dissolves in water.

In the wash, the tails bury themselves in the film of skin oil and grease that holds dirt on fabric. The heads stay in the water. Agitation tears the oil into tiny droplets, each wrapped in a shell of soap molecules with their charged heads facing out. These structures are called micelles. Every micelle carries the same negative charge on its surface, so they repel each other and the fabric, and the oil stays suspended until the rinse carries it away.

Hard water turns soap into scum

Calcium and magnesium ions in tap water carry a charge of +2. Each one binds two soap molecules into an insoluble salt:

2 C₁₇H₃₅COO⁻ + Ca²⁺ → (C₁₇H₃₅COO)₂Ca (calcium stearate, soap scum)

Every soap molecule caught this way is lost for cleaning, and the waxy calcium stearate lodges between fibers. This is what makes towels stiff, whites gray and dark clothes dull after months of washing with plain soap in hard water.

Washing soda removes the hardness first

Sodium carbonate dissolves into carbonate ions that react with calcium faster and more completely than soap does. The calcium drops out as fine calcium carbonate, the same mineral as chalk and limescale, which rinses away:

Ca²⁺ + CO₃²⁻ → CaCO₃ (chalk)

The ratio is one to one: 106 g of sodium carbonate removes the equivalent of 100 g of calcium carbonate hardness. A 55 liter wash at 150 mg/L carries 8.3 g of hardness, which takes about 8.8 g of washing soda to clear. That is the calculation the calculator runs for your water.

Washing soda also raises the pH of the wash to about 11. At that pH, the fatty acids in body oil and food grease turn into soap themselves, so the dirt helps clean itself off. The high pH also swells cotton fibers slightly, loosening trapped soil.

Borax steadies the pH

In water, borax forms a mix of boric acid and borate ions that acts as a buffer near pH 9.2. A buffer resists change: acidic soils such as sweat would otherwise lower the pH, and soap cleans poorly once the pH falls below about 8. Borax also helps keep loosened dirt suspended.

Borax does less than its reputation. It does not turn water into hydrogen peroxide, a claim that comes from confusing it with sodium perborate, a different chemical used in older bleach powders. For whitening, add a tablespoon of sodium percarbonate (oxygen bleach) to the drum. It releases hydrogen peroxide when it dissolves and works best in water above 104 °F (40 °C).

Why some soaps fail in cold water

A soap dissolves only above a temperature called its Krafft point. Soaps made from saturated fats with long chains, such as tallow (mostly stearate and palmitate), have Krafft points well above room temperature when pure. Real tallow soaps are mixtures and dissolve slowly in a cold wash, leaving flakes behind. Soaps from coconut oil (laurate, 12 carbons) and olive oil (oleate, with a kink in the chain) dissolve in cold water. If you wash cold, choose a coconut-based laundry bar or castile soap, or switch to the liquid version.

Homemade vs store-bought detergent

Commercial laundry detergent is not soap. Its main cleaners are synthetic surfactants such as linear alkylbenzene sulfonate. Their calcium salts stay far more soluble than calcium soap, and store detergents add builders that tie up the rest of the hardness, so they keep working in hard water. That is why detergents replaced soap for laundry after World War II.

Feature	Homemade (soap based)	Store-bought detergent
Main cleaner	Soap (fatty acid salts)	Synthetic surfactants
Hard water	Needs washing soda to work	Tolerates it
Cold water	Depends on the soap; tallow soaps struggle	Formulated for cold
Enzymes for protein, starch and grease stains	None	Usually several
Optical brighteners	None	Common
Fragrance and dyes	Only what you add	Common, with free-and-clear options
Residue over time	Possible in hard water	Low
Cost per load	Lower	Higher

The missing enzymes are the biggest practical gap. Proteases digest blood, egg and grass stains, amylases digest starch and lipases break down grease. A homemade mix cannot match them on those stains. Pretreat them with a paste of water and washing soda, or keep a small bottle of enzyme stain remover for the tough ones.

Troubleshooting

Problem	Cause	Fix
Towels stiff or scratchy	Calcium soap building up in the fibers	Raise the washing soda. Add ½ cup white vinegar to the rinse.
Whites turning gray	Soap scum and redeposited dirt	Raise the washing soda. Add oxygen bleach to white loads.
White flecks on dark clothes	Undissolved soap	Grate finer, dissolve in hot water first or switch to liquid.
Sour smell after drying	Too much soap left in fabric	Cut the dose by a third. Run one hot wash with no detergent.
Powder is a solid lump	Washing soda absorbed moisture	Break it up and store in a sealed jar.
Liquid separated into water and gel	Normal settling	Shake or stir before each use.
Musty smell in an HE washer	Soap residue in the door seal	Run a hot cleaning cycle with 1 cup washing soda once a month. Leave the door open.

To strip months of buildup from towels, soak them for 4 hours in a bathtub of hot water with ¼ cup each of washing soda and borax, then wash with no detergent. The gray water that comes out is old soap and minerals.

Safety

Homemade soap detergent is safe for septic systems at normal doses. Soap is biodegradable, and a load adds a few grams of carbonate and borate. For cloth diapers, follow the diaper maker’s guidance, since soap residue reduces absorbency.

The same hard water chemistry decides whether soap works at the kitchen sink. Our guide to how to make dish soap has a calculator that checks your water.

Frequently asked questions

Sources

• US Geological Survey. Hardness of water
• American Cleaning Institute. An introduction to the science of how things get clean
• PubChem. Sodium carbonate
• PubChem. Borax
• European Chemicals Agency. Disodium tetraborate decahydrate substance information
• American Cleaning Institute. Soaps and detergents history
• American Cleaning Institute. Soaps and Detergents (book)