Measure the paint area
Multiply width by height for each flat face you plan to paint. Add the sides if they will show. For round ornaments, estimate generously because curves use more brushwork.
Plan for coats
Light colors over dark surfaces often need two or three coats. Dark colors over primed or pale surfaces may cover in one or two coats.
Surface matters
Raw wood, cardboard, and clay absorb more paint than sealed surfaces. A quick primer coat can save color paint and give a smoother finish.
Batch tip
Paint one sample piece first and note how much paint is gone. Multiply from that real sample when making ornaments, favors, or classroom projects.