Paint Thinning Calculator — Thinner Volume & Ratio for Model Paints

This calculator takes a typical starting thinning ratio for each consistency (base coat, wash, glaze, dry brush) and nudges it for paint chemistry, current viscosity and bench conditions. The base ratios reflect widely-used modelling guidance — roughly 1:0.5 to 1:1 paint:thinner for airbrushed base coats and around 4:1 thinner:paint for panel-line washes. They are starting points, not exact specifications: actual thinner needed varies with paint age, batch, colour pigment load, room temperature and humidity, so always do a test spray or flow-test and adjust.

The formula

Thinner volume = Paint volume × Base ratio × Paint-type modifier × Viscosity modifier × Evaporation factor × Humidity factor

• Base ratio — a typical starting thinning ratio for each consistency (e.g. ~4:1 thinner:paint for washes, ~1:0.5 paint:thinner for base coats)
• Paint-type modifier — a general adjustment: lacquers tend to be sprayed thinner than oils, with acrylics in between. These are rule-of-thumb factors, not manufacturer figures
• Viscosity modifier — thicker paint straight from the pot needs proportionally more thinner to reach the same target
• Evaporation rate — faster-evaporating solvents (or a warm room) call for slightly more thinner to offset tip-dry
• Humidity factor — affects water-based paint open time and flow

The modifier and working-time figures are approximate bench guidelines drawn from common modelling practice rather than published datasheet values; treat all outputs as a starting recipe to refine on a test piece.

Working figures for this are collected in our Paint Thinning Ratios reference table.