Workspace
The calculator's own fields, action and results arrive with the verified pack when you load it. Nothing is computed in this page.
Plastering · multi-coat volume
Plaster multi-coat system volume
Work out the wet plaster volume a two-coat system needs: a scratch coat and a finish coat, each at its own thickness, summed into the build over the wall.
Verified engine
What the engine returns
Two figures come back: the combined coat thickness and the wet volume the system demands. Read the volume against how the material will be bought — it is the figure a site-mix batching or a bagged product’s yield-per-bag converts into an order.
Inputs the result is built from
Wall area to plaster
Scratch coat thickness
Finish coat thickness
How the result is derived
MethodCombined thickness as the sum of the scratch and finish coat thicknesses; wet plaster volume as the wall area times that combined thickness.
StandardTwo-coat plastering practice (Chudley and Greeno, Building Construction Handbook)
GuardA combined build beyond thirty millimetres is refused, and the pack ships the refusal as a declared test vector: past that, the work is no longer a two-coat system but dubbing-out — additional backing passes, each applied and left to stiffen before the next, each deserving its own line.
How the plaster volume moves with the scratch-coat thickness
Scratch and set — how two coats build one thickness and one wet volume
The scratch coat is the structural half of the build. It is laid on generously, ruled roughly straight, and then deliberately scored — scratched — while still green, so that the next pass has a mechanical key to grip. Almost all of the system’s volume lives in this coat, because absorbing the background’s unevenness is its whole job.
The finish, or setting, coat is a skin. A few millimetres trowelled and polished to the surface the decorator will see, it contributes little to the volume and everything to the appearance — which is why its thickness input is small and why erring on it moves the total far less than the same error on the scratch coat.
The combined thickness is reported beside the volume because it is the figure the rest of the job speaks in: specifications quote it, the guard judges it, and the batching take-off downstream multiplies the same area by it.
The answer is deliberately a wet volume and nothing more. Whether the system is site-mixed sand and cement or a bagged gypsum undercoat, the volume is what either must fill — hand it to the ratio-batching page for site materials, or to a bagged product’s stated yield per bag.
Coats are a sequence, so the model extends naturally: a three-coat system on lath or a dubbed-out background is this same sum with another pass, run as a second take-off at the extra coat’s thickness over its own area.
Combined thickness as the sum of the scratch and finish coat thicknesses; wet plaster volume as the wall area times that combined thickness.
When this calculation is used
- Pricing the plaster volume of a specified two-coat system before choosing between site-mixed and bagged material.
- Turning a surveyed wall area and a specification’s coat thicknesses into the combined build the batching take-off needs.
- Checking whether a proposed build fits a single system or has grown into dubbing-out territory.
- Comparing the volume cost of a thicker scratch coat against straightening the background first.
Worked example
The pack’s declared vector: fifty square metres of wall under a ten-millimetre scratch coat and a three-millimetre finish coat.
Two figures come back: the combined coat thickness and the wet volume the system demands. Read the volume against how the material will be bought — it is the figure a site-mix batching or a bagged product’s yield-per-bag converts into an order.
Every figure in this example is produced by the certified engine when the calculator loads, checked against the signed pack’s declared test vectors; nothing on this page stores an answer. Grow the scratch coat and the finish coat by the same amount and watch the total move identically — the coats differ in purpose, not in arithmetic.
What each input represents
Wall area to plaster
The surface both coats will cover, in square metres, with openings deducted — coats share one area even though they differ in everything else.
Scratch coat thickness
The base pass that straightens the background and carries the build. Nearly all the volume — and nearly all the sensitivity of the answer — lives here.
Finish coat thickness
The thin setting pass trowelled over the keyed scratch coat; a surface, not a filler, and the smaller number by design.
Assumptions and limits
- The system is two coats over one shared area; a third pass on lath or heavy dubbing-out is the same sum run again for the extra coat.
- Each thickness is an average over the area; a background so uneven that averages mislead should be split into zones and taken off separately.
- The result is wet volume in place, upstream of material choice — bulking, ratios, densities and bag yields belong to the batching pages downstream.
- Beads, stops, scrim at junctions and the labour of keying between coats are not volume and are not counted.
What the guards protect against
- A combined build beyond thirty millimetres is refused, and the pack ships the refusal as a declared test vector: past that, the work is no longer a two-coat system but dubbing-out — additional backing passes, each applied and left to stiffen before the next, each deserving its own line.
- The wall area and both coat thicknesses must be strictly positive — a system with a zero-thickness coat is a different system, and the engine refuses to price one pretending to be the other.
Provenance
Two-coat plastering practice (Chudley and Greeno, Building Construction Handbook)
Combined thickness as the sum of the scratch and finish coat thicknesses; wet plaster volume as the wall area times that combined thickness.
A system-volume convention to be verified against the project specification; coat counts and materials for a particular background are never determined here. The signed pack carries its own citation, and the page reports the verification state of the release it mounted.