Mixing two paint colours sounds easy: pour a bit of one, a bit of the other, and stir. But the colour you get depends entirely on the proportion, and eyeballing it means the second batch never matches the first. A ratio fixes this. If your mix is 3 parts of colour A to 2 parts of colour B, then every batch with that ratio gives the same colour, whether you mix 50 millilitres or 5 litres. This calculator turns any ratio and any target total into exact millilitres of each colour.
The maths is a simple share of the total. Add the parts together, then give each colour its share. With 3 parts A and 2 parts B, the total is 5 parts, so A gets three fifths and B gets two fifths. For a 500 millilitre target, that is 300 millilitres of A and 200 millilitres of B. The tool also simplifies the ratio: 4 parts to 2 parts is the same mix as 2 parts to 1 part, and the simpler form is easier to measure and to remember.
Here is a worked example with a less tidy ratio. Suppose your test mix used 7 parts of white and 3 parts of blue, and you now need 1,000 millilitres. The parts add to 10, so white gets seven tenths, which is 700 millilitres, and blue gets three tenths, which is 300 millilitres. The percentages are 70 and 30, which the tool also shows. Percentages are handy when you measure by weight on a kitchen scale, while parts are handy when you measure with a cup or a syringe.
Measuring well matters more than the maths. Use the same measuring tool for both colours, measure at eye level, and stir fully before judging the colour: unstirred paint looks streaky and darker in places, which tempts people to add more of one colour and ruin the ratio. Pour the lighter colour first and add the darker colour slowly, since a little dark paint changes the mix far more than a little light paint. Write the ratio on the can the moment you mix it.
Always test before you commit. Mix a small batch, paint a swatch on the same surface you will paint, and let it dry completely before you judge it. Wet paint looks lighter than dry paint, and the colour changes again under room lighting versus daylight. If the test swatch is right, mix the full amount in one batch: two batches mixed separately will differ slightly even with the same ratio, because measuring is never perfect, and a difference across a wall shows as a band.
Keep a record of the exact mix for touch-ups. Note the paint brands and product names, the ratio, the total volume, and the date, and keep a small sealed jar of the mixed paint if you can. A year later, that jar is worth more than any note, because paint ranges change and your memory of the ratio will fade. Label the jar with the room and the wall it belongs to, not just the colour name.
Remember that this is a planning estimate, not a professional quote and not a colour-matching service. A ratio controls proportions only: it cannot predict how two specific paints will interact, how the colour will look on a different surface, or how it will age in sunlight. For a large or visible job, ask a paint shop to mix the colour and keep the formula on file, and use this tool to scale that formula up or down. Test the mix under the room lights you actually use, since shop lighting can flatter a colour badly.
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