Free Primer Quantity calculator with clear step-by-step results.
Primer is bought in whole cans but specified as coverage per litre, so the calculation is area ÷ coverage, uplifted for how thirsty the substrate is, then multiplied by the number of coats. Stated coverage is measured on a sealed, smooth panel; bare plaster, new render, MDF edges and sanded timber can absorb 20-50% more than the label suggests.
Primer per coat
Litres per coat = area ÷ coverage × (1 + porosity allowance)
Cans to buy
Cans = ceil(litres per coat × coats ÷ can size)
Follow the primer manufacturer's data sheet for substrate preparation, coverage, thinning, recoat times and ventilation. Some primers require personal protective equipment and specific disposal.
Label figures assume a smooth, non-absorbent surface and even application. Porous, textured or previously unpainted substrates absorb more, and roller application puts down more than a sprayer for the same visible result.
Usually one is enough on sound surfaces. Use two on bare plaster, over strong stains, or when priming a very dark colour before a light topcoat.
Only within the manufacturer's stated limits. Over-thinning reduces film build and defeats the sealing and adhesion the primer exists to provide.