Oil Paint Medium Ratio Calculator

Fat-over-lean means each successive layer of oil paint should contain more oil (fat) than the layer below it. Layers with more oil dry more slowly and remain more flexible. Painting a slow-drying (fat) layer over a fast-drying (lean) layer causes the top layer to crack as it dries faster than the layer below can flex.

A common progression for three layers: underpainting (1 part oil : 3 parts solvent = lean), middle layer (1:1 oil to solvent), final layer (2:1 oil to solvent or straight paint). The exact ratios are flexible; the key principle is that each layer must have more oil than the previous one.

Common drying oils include linseed oil (most common), walnut oil (yellows less), and poppy oil (slowest drying). Common solvents include turpentine and odourless mineral spirits (OMS). Alkyd mediums (like Liquin) speed drying and follow the same fat-over-lean principle.

Use a pipette, dropper bottle, or small measuring cup. For small amounts, count drops (approximately 20 drops per ml). For larger studio batches, use millilitres. The key is consistency between layers, not absolute precision. Record your ratios so you can reproduce them.

Yes, and it causes the dreaded 'cracking' failure in oil paintings. The most common way this happens is using a fast-drying medium (alkyd or lots of solvent) on top of a slow-drying fat layer. Always ensure later layers have more oil than earlier layers, and allow each layer to dry before applying the next.