Model Paint Thinning Ratio Calculator

A ratio is parts, not volumes, so the first thing the calculator needs is the paint actually sitting in the cup. Everything after that is proportion.

1. Paint in the cup. Your amount is converted to millilitres, because that is the unit the arithmetic runs in.
2. One part. The paint is divided by the paint parts of your ratio. A 2:1 mix means each part is half the paint volume.
3. Thinner to add. One part is multiplied by the thinner parts: half the paint for 2:1, the same as the paint for 1:1, twice the paint for 1:2. The result is given in millilitres, fluid ounces and drops at your own dropper rate.
4. Total in the cup. Paint plus thinner is what the cup has to hold, which is worth checking on a 3 ml detail cup.

If you are trying an unfamiliar ratio, an optional test is to stir in about half of the thinner, check a scrap pass at your intended setting, then decide whether to add the rest. It is a workflow choice, not a requirement, and the paint product's own instructions take precedence.

Parts paint : parts thinner

The order this calculator uses, and the order the mix log on this site records. 2:1 means two parts paint to one part thinner. Some labels print the ratio thinner-first, so read before you pour — if yours does, swap the two numbers.

Millilitres

The working unit for the arithmetic, and the unit most airbrush cups are marked in.

Fluid ounces

US fluid ounces, for dropper bottles and cup markings that use them. One US fluid ounce is about 29.57 ml.

Drops

A drop is a volume, not a fixed one: it depends on the dropper, the liquid and how hard you squeeze. About 20 drops per millilitre is the usual figure for water-thin liquids, which is why the drop rate is a field you can set to your own count.

Mix character

A plain description of how much thinner is in the mix relative to paint — light, moderate, heavy or thin enough to read as a wash. It describes the proportion you entered, not a recommended recipe.

Every figure in the result is arithmetic on your own two numbers. There is no brand recipe hidden in the tool, because this site's thinning guides do not hand one out: they name the setup and the numbers rather than one universal recipe, and the mix you can repeat is the only ratio that fits your brush. The preset ratios in the dropdown are common starting points to try and log, not a recommendation for any particular paint — and every one of them is overridable.

The method around the arithmetic comes from the same place: read your paint's label for the thinner it names for that exact line, stir the bottle rather than shaking it, decant only what you need into the cup, add thinner in small increments, retest on scrap, change one variable per test, and write the mix down as parts paint and parts thinner. That last step is what makes a lucky pass repeatable, and it is why the copy button hands you a log-ready line.

• It does not tell you which ratio to use. That comes from your paint's own instructions and your logged mixes, not from a calculator.
• It does not name a thinner for your paint line. Use the one your label names for that exact product.
• It does not judge the mix. Consistency in the cup and the pass on scrap are the real test; the numbers only get you to a repeatable starting point.
• It assumes parts are measured accurately — a syringe, pipette or graduated cup. Two eyeballed parts are not a 2:1 mix.

What ratio should I thin model paint to for an airbrush?

There is no single ratio that fits every paint, needle or pressure, and this site deliberately does not hand one out. Work from the ratio your paint's own instructions name for that exact line, thin only the amount in the cup, then adjust from your own logged mixes. The calculator is there to turn whichever ratio you settle on into real volumes and drops for the cup in front of you.

Is a thinning ratio written paint to thinner or thinner to paint?

It depends on who wrote it, so read the label before you pour. This calculator writes every ratio as paint : thinner, so 2:1 means two parts paint to one part thinner — that is the order the site's mix log records parts paint and parts thinner in. If your paint's instructions quote the ratio the other way round, swap the two numbers before you enter them.

How many drops are in a millilitre of thinner?

About 20 drops per millilitre is the usual figure for a water-thin liquid, where one drop is treated as 0.05 ml. Real droppers differ: a fine-tipped one may take 30 or more. Count yours once — drip into a 1 ml syringe or a marked measuring spoon — and set the drop rate field to your own number so the drops figure matches what you are actually holding.

Can I thin the whole bottle instead of the cup?

No. Thinning the bottle commits every future session to one ratio, shortens the paint's life and cannot be undone. Decant the working amount into the cup, thin that, and keep the rest of the bottle as it came.

Why does the calculator need the amount of paint in the cup?

A ratio is parts, not volumes: 2:1 tells you the proportion but not how much thinner to pour. Multiplying your cup amount by the ratio turns the proportion into millilitres, fluid ounces and drops, so the same ratio scales from a 3 ml detail cup to a 30 ml batch of one colour.

The mix is one variable. Pressure, nozzle size and technique are the others, and they are worth setting before you blame the ratio for a bad pass.

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