Lime Plaster Calculator
Estimate hydrated lime, sand, and water for a lime plaster from wall area, plaster thickness, and lime-to-sand ratio. Defaults to a 1:3 mix at 12 mm thickness.
How this works
Wet plaster volume bulks ~30% when dry- batched, then is split per the ratio:
wetVol = wallArea × plasterThickness
dryVol = wetVol × 1.30
limeVol = dryVol × limeParts / (limeParts + sandParts)
sandVol = dryVol × sandParts / (limeParts + sandParts)
water = limeVol × 600 (litres; lime needs lots of water)Common ratios: 1:2 for fine internal finishing, 1:3 for general-purpose plaster (the default), 1:4 for backing coats over lime mortar masonry.
Worked example
A wall of 100 m², 12 mm lime plaster at 1:3:
- Wet vol = 100 × 0.012 = 1.20 m³
- Dry vol = 1.20 × 1.30 = 1.56 m³
- Lime (1/4) = 0.39 m³
- Sand (3/4) = 1.17 m³
- Water = 0.39 × 600 ≈ 234 L
Sources
- Traditional lime plaster proportioning (cited in IS 712 / NBC 109 lime mortar guidelines)
FAQ
Why use lime plaster instead of cement plaster?
Lime plaster is breathable, self-healing of fine cracks, and gentle on natural-wall substrates like stone, brick-with-mud-mortar, cob, adobe, and rammed earth. Cement plaster, by contrast, is rigid, vapour-tight, and tends to trap moisture behind it — which is the wrong surface treatment for vernacular masonry. Lime is also lower in embodied carbon than cement.
Why does the dry volume bulk by 30 percent over the wet volume?
Dry-batched lime and sand have inter-particle voids that disappear when water is added and the mix is consolidated against the wall. The 1.30 factor — sometimes quoted as 1.27 to 1.33 in textbooks — covers this volumetric loss. If you only have wet volume figures from a finished sample, divide by 1.30 to back out the dry-batch order quantity.
What does '1:3' mean for a lime plaster?
It means 1 part hydrated lime (or lime putty) to 3 parts well-graded sharp sand by volume. The default 1:3 is the workhorse mix for general-purpose internal and external plaster on traditional masonry. Use 1:2 for fine smooth-finish skim coats; use 1:4 only when applying over heavy lime-mortar substrates that need a leaner backing coat.
Why does lime need 600 litres of water per m³?
Lime is hygroscopic — it absorbs and holds water during the long carbonation cure. A typical mix needs about 600 L of water per m³ of lime, well above the water demand of cement plaster. The water is consumed slowly over weeks as the lime reacts with atmospheric CO₂, so do not let the wall dry out prematurely; mist it for a week after application.
How long does lime plaster take to cure?
Initial set is in hours, but full carbonation can take 6 to 12 months. Avoid painting or papering for at least 6 to 8 weeks. In dry weather, mist the surface daily for the first week to prevent rapid water loss and surface chalking. Cement-modified lime plaster (e.g. lime + 10% white cement) sets faster but loses some of the breathability benefit.
Can I substitute hydraulic lime (NHL) for hydrated lime?
Yes — Natural Hydraulic Lime (NHL 2, 3.5, or 5) sets faster, sets harder, and works in damp conditions where pure hydrated (air) lime stalls. NHL is the right choice for external plaster, chimneys, and damp substrates. Pure hydrated lime is gentler and more breathable, better for internal walls and historic restoration. The 1:3 ratio applies equally to both.