Why do I run out of wash mid-way?

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.

What is the paint-to-water ratio for a medium wash?

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.

Does paper weight affect wash volume?

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.

How do I keep a flat wash even?

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.

Wash Volume Calculator — Watercolour Mix Amounts — Free Watercolour Tool

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

Total water = Base water × Coats × (1 + Buffer %)

Paint amount = Total water × (Strength ratio ÷ 100)

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

Related Calculators

Wash Size Calculator

Calculate the volume of diluted watercolour paint needed for flat, graded and variegated washes. Never run out mid-wash again.

Watercolour Dilution Calculator

Calculate watercolour paint-to-water ratios for different wash strengths. Covers wet-on-dry, wet-on-wet, glazing and dry brush.

Paper Stretch Calculator

Calculate gummed tape length, soaking time and drying time for stretching watercolour paper. Covers different paper weights and sizes.

Masking Fluid Calculator

Calculate masking fluid needed for watercolour painting based on area to mask, coverage percentage and number of coats applied.

Colour Lift Calculator

Check which watercolour pigments are liftable, staining or non-staining and plan your painting layers for easier corrections.

Last updated: March 2026

All calculations are estimates. Always verify quantities before purchasing materials.