Lime Plaster Calculator — How Much Lime Plaster Do I Need?

This calculator estimates the quantity of lime and sharp sand needed for lime plastering based on the area to be covered, the type of lime, the coat being applied, and the thickness per coat.

The formula

Lime (kg) = Coverage rate (kg/m²/mm) × Area (m²) × Thickness (mm) × Number of coats × Wastage factor

Sand (kg) = Lime (kg) × Sand ratio

Coverage rates (binder only)

• Lime putty (CL90): ~1.35 kg per m² per mm thickness (binder only; sand is added separately)
• NHL 3.5: ~1.7 kg per m² per mm thickness
• NHL 5: ~1.8 kg per m² per mm thickness

Values are the binder fraction of a typical 1:3 sand:lime mix, derived from Building Limes Forum, Anglia Lime, and Mike Wye manufacturer technical datasheets.

Sand-to-lime ratios

The mix ratio varies by coat. Scratch coats use a 3:1 sand-to-lime ratio (by volume) for added strength, while float and finish coats use a 2.5:1 ratio for a finer, more workable mix. These ratios are approximate weight equivalents based on typical bulk densities.

Standards

Lime binders should conform to BS EN 459-1 (Building lime — definitions, specifications and conformity criteria). External rendering guidance is BS EN 13914-1; internal plastering is BS EN 13914-2 (BS 5262 was withdrawn and replaced by these EN standards). For listed buildings, always consult with your conservation officer regarding appropriate lime types and finishes.