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Wash Volume Calculator — Watercolour Mix Amounts
Calculate paint-to-water ratio and volume for large watercolour washes.
How We Calculate This
This calculator estimates the volume of diluted watercolour needed for washes based on the area to cover, paper absorbency and desired paint strength.
Volume formula
Base water = (Area ÷ 100) × 1.5ml × Absorbency factor × Wash-type factor
Total water = Base water × Coats × (1 + Buffer %)
Paint amount = Total water × (Strength ratio ÷ 100)
Absorbency & wash-type factors
- Absorbency: 300gsm+ ×1.2, 200–299gsm ×1.0, under 200gsm ×0.8
- Wash type: flat ×1.0, graded ×1.15, variegated ×1.20 (multi-colour and graded washes need a little more mix)
Strength ratios
- Very light: 0.5% paint (1:200)
- Light: 1% paint (1:100)
- Medium: 3% paint (1:33)
- Strong: 6% paint (1:17)
These figures are a rule-of-thumb estimate, not a fixed standard. Watercolour is mixed by eye — artists judge consistency by feel (the classic “weak tea” for a light wash, up to “single cream” for a strong one) rather than by millilitres. Actual volume varies with pigment strength, brush size and load, paper sizing and how fast you work, so treat the result as a starting quantity. The built-in buffer reflects the universal studio rule to mix more wash than you think you need: once a flat wash is underway you cannot stop to mix more without leaving a hard edge.
Frequently Asked Questions
The most common reason is not mixing enough wash before starting. Once you begin a flat wash, you cannot stop to mix more without creating a hard edge. Always mix more than you think you need — at least 30% extra as a buffer. Leftover wash can be saved in a sealed jar.
A medium-strength wash uses approximately 3% paint to water by volume. For a light wash, use about 1%. For a strong, saturated wash, use up to 6%. These are approximate — the exact ratio depends on the pigment concentration of your particular paint brand.
Yes. Heavier paper (300gsm+) absorbs more water and needs a larger volume of wash. Lighter paper (190gsm) absorbs less but is more prone to cockling. This calculator adjusts the volume based on your paper weight to account for absorbency differences.
Work at a slight angle (about 15 degrees) so gravity pulls the bead of paint downward. Load your brush fully for each stroke, overlap slightly with the previous stroke, and work steadily without going back into drying areas. Pre-wet the paper for the smoothest results.
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Last updated: March 2026
All calculations are estimates. Always verify quantities before purchasing materials.