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 method

Lime mortar is mixed by volume (one part lime to a set number of parts sand), and the lime binder fills the voids between the sand grains — so the finished mortar volume is roughly equal to the sand volume. The calculator works from the mortar volume you need, then splits it into lime and sand:

Mortar volume (m³) = Area (m²) × Thickness (m) × Number of coats × Wastage factor

Lime (kg) = Mortar volume × Binder content (kg/m³)

Sand (kg) = Mortar volume × Sand bulk density (~1500 kg/m³)

Binder content per m³ of mortar

For natural hydraulic lime the calculator uses the Hanson (Heidelberg Materials) Natural Hydraulic Lime data sheet, whose “standard mixes per 25 kg bag” table gives the lime content of the finished mortar. NHL 3.5 (most common for internal plastering):

• 1:3 mix (leaner — scratch coats): ~205 kg NHL per m³ of mortar
• 1:2.5 mix (float coats): ~245 kg NHL per m³
• 1:2 mix (richer — finish coats): ~275 kg NHL per m³

NHL 5 is a harder, more hydraulic grade, but the binder dosage in a given lime-to-sand volume mix is set by the mix proportions, not the grade, so it uses the same ~205 / 245 / 275 kg/m³ figures (NHL 5 powder is only marginally denser — around 700 g/litre versus 650 g/litre for NHL 3.5 per Saint-Astier, a difference well within the 10% wastage allowance). Choose NHL 5 for strength and exposure, not for a different quantity.

Lime putty (CL90 fat lime) is supplied wet and is denser per m³, so the calculator uses a higher figure (~450 kg wet putty per m³ at 1:3). These are planning estimates — always batch by volume and follow your supplier’s mix instructions.

Lime-to-sand ratios

The mix ratio varies by coat (all by volume). Scratch coats use a leaner 1:3 lime-to-sand mix to provide a strong key, the float coat is typically 1:2.5, and the finish coat is richer in lime at around 1:2 with a finer sand for a smooth surface. You can override any of these in the advanced options.

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 External rendering guidance is BS EN 13914-1; internal plastering is BS EN 13914-2 (BS 5262 was withdrawn and replaced by these EN standards).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.