PROFESSIONAL COATING SYSTEM
Build a coat-by-coat order for primer, base coat, mesh and finish render.
BUILD MEASURE HELPER
Calculation summary
Planning estimate only. Verify product specifications, site conditions and purchase quantities before ordering.
DIY complexity
Achieving a smooth, even plaster or render finish by hand is one of the hardest trade skills to self-teach — most first attempts show visible trowel marks and unevenness that only sanding or a skim coat by a professional can fully correct.
This calculator estimates the bags of base-coat and finish-coat plaster or render, primer and reinforcing mesh needed to coat a wall, ceiling or facade. It works from the net coated area you enter, with door and window openings subtracted, and calculates each layer separately: a base coat that builds thickness and levels the substrate, and a finish coat that gives the final smooth or textured surface. Base and finish coats are never mixed into one figure because they use completely different products with different consumption rates and bag sizes — a base coat might use 5 kg per m² while a skim finish uses under 2 kg per m². The tool also sizes a primer coat where included, measured in litres rather than kilograms, and adds reinforcing mesh at joints and corners with an allowance for the overlap where strips are lapped together. Results are shown in metric and imperial units, and every material — bags, primer cans, mesh rolls — is rounded up to whole purchase units after your chosen waste allowance. It cannot judge substrate compatibility, verify that a given render system is rated for the local weather exposure, or replace the cure-time and layer-thickness limits stated on the manufacturer's data sheet.
Quick answer
For a wall of 8 × 2.7 m minus 4 m² of openings (net 17.6 m²), with a base coat at 5 kg/m² in 25 kg bags and a finish coat at 1.2 kg/m² in 20 kg bags and 8% waste, the base coat needs ceil(17.6 × 5 × 1.08 ÷ 25) = 4 bags and the finish coat needs ceil(17.6 × 1.2 × 1.08 ÷ 20) = 2 bags — calculated and rounded independently, never combined into one bag total.
net_area = gross_area − (doors × door_area) − (windows × window_area)The gross wall, ceiling or facade area is not what gets plastered — door and window openings are cut out of it, because those surfaces are not coated. This net figure is the single input every layer's material quantity is derived from.
Rounding rule: Net area itself is not rounded; rounding only happens once each layer's quantity is converted into whole bags or containers.
Limitation: Uses one average opening size across the whole area; a wall with openings of very different sizes should have them entered individually or grouped by similar size for an accurate net area.
bags_layer = ceil(net_area × rate_layer × (1 + waste) ÷ bag_size_layer)Each layer — base coat, finish coat, primer — is calculated as its own independent formula using that layer's own consumption rate, waste allowance and bag or container size. The formula never mixes the base-coat rate with the finish-coat bag size, or vice versa.
Rounding rule: Each layer is rounded up (ceiling) independently — a base coat needing 3.2 bags still requires 4 whole bags, regardless of how many finish-coat bags are also needed.
Limitation: Consumption rates depend heavily on substrate flatness and required build thickness — an uneven wall needing a thicker base coat in places can consume noticeably more than the flat-wall rate on the product's data sheet, so treat the entered rate as an editable planning figure, not a fixed constant.
mesh_length = joint_and_corner_length × (1 + mesh_lap)Reinforcing mesh is only needed at joints between different substrates, at corners and around openings, not across the whole coated area, so its length is calculated from those specific junction lengths rather than from area. The lap percentage accounts for the overlap where adjoining mesh strips must cross each other.
Rounding rule: Rounded up to whole mesh rolls based on the roll length sold locally; a small leftover from the last roll is normal and expected.
Limitation: Assumes mesh is only required at the junctions you specify; some render systems require full-area mesh embedding across the whole facade for crack control, which is a different quantity entirely and should be checked against the system's technical approval.
An 8 m long, 2.7 m high interior wall (21.6 m² gross) with 4 m² of door and window openings, net 17.6 m². Base coat at 5 kg/m² in 25 kg bags, finish coat at 1.2 kg/m² in 20 kg bags, primer at 0.15 L/m² in 10 L cans, lapped reinforcing mesh at the two internal corners (5 m of junction length, 15% lap), 8% waste on the coats.
Net area = 21.6 − 4 = 17.6 m². Base coat = ceil(17.6 × 5 × 1.08 ÷ 25) = ceil(3.80) = 4 bags. Finish coat = ceil(17.6 × 1.2 × 1.08 ÷ 20) = ceil(1.14) = 2 bags. Primer = ceil(17.6 × 0.15 ÷ 10) = ceil(0.264) = 1 can. Mesh = ceil(5 × 1.15) = ceil(5.75) = 6 m, rounded to 1 roll if sold in 10 m rolls.
4 base-coat bags and 2 finish-coat bags are two entirely separate purchase lines, not a combined total of 6 identical bags — they are different products at different bag sizes. The primer, at just over a quarter of one 10 L can theoretically, still rounds up to a whole can, and the mesh need of 5.75 m still requires 1 whole 10 m roll.
⚠ The 5 kg/m² base-coat and 1.2 kg/m² finish-coat rates are editable planning examples, not universal constants — always use the coverage stated on the specific product's data sheet, since real rates depend on substrate flatness and required thickness.
A 32 m long, 2.8 m high facade (89.6 m² gross) with 9.6 m² of window and door openings, net 80 m². Base coat rate 5 kg/m² in 25 kg bags, finish coat rate 2.5 kg/m² (a heavier textured render finish) in 25 kg bags, primer rate 0.18 L/m² in 10 L cans, mesh lap 15% over a 24 m total junction and corner length, 8% waste on the coats.
Net area = 89.6 − 9.6 = 80 m². Base coat = ceil(80 × 5 × 1.08 ÷ 25) = ceil(17.28) = 18 bags. Finish coat = ceil(80 × 2.5 × 1.08 ÷ 25) = ceil(8.64) = 9 bags. Primer = ceil(80 × 0.18 ÷ 10) = ceil(1.44) = 2 cans. Mesh = ceil(24 × 1.15) = ceil(27.6) = 28 m, rounded to 3 rolls of 10 m.
Despite using the same bag size for both coats here, 18 base-coat bags and 9 finish-coat bags stay as separate line items because they are different products with different rates — combining them into 27 identical bags would misrepresent what actually needs to be ordered. The mesh length is driven entirely by junction and corner length, not by the 80 m² facade area, which is why a smaller facade with more corners could need more mesh than this one.
⚠ Exterior render systems are weather- and cure-time sensitive — do not apply below the manufacturer's minimum temperature, and respect the stated interval between base and finish coats; rushing the cure can cause cracking regardless of how accurately the quantities were calculated.
A 10 m² bathroom ceiling in sound condition needing a skim finish coat only (no base coat, existing plaster is flat and sound), finish rate 1.5 kg/m² in 20 kg bags, no primer, no mesh, 5% waste, with an example local price per bag.
Finish coat only = ceil(10 × 1.5 × 1.05 ÷ 20) = ceil(0.7875) = 1 bag. At an example price of 22 per bag: 1 × 22 = 22 total material cost.
Only 0.79 bags are theoretically needed, but plaster is only sold in whole bags, so 1 full bag is purchased with roughly a quarter left over — normal surplus, not a sign the calculation over-provisioned. Because base coat and mesh were switched off for this sound-substrate skim job, they contribute nothing to either the material list or the cost.
⚠ Changing the selected currency only changes the display symbol — it does not convert the price you entered. Enter a current local supplier price in the currency you have selected before relying on the cost total.
The result panel lists base coat, finish coat, primer and mesh as separate line items, each rounded to its own whole purchase unit — they are never summed into one combined bag count, because they are different products bought from different price points. A skim-only or base-only job simply shows zero for the layer that was switched off, rather than an estimated figure. Where a price is entered, each layer's cost is calculated from its own rounded quantity and then totalled, so the breakdown shows exactly which layer drives the budget.
Plaster and render consumption rates, bag sizes and finish types (gypsum, lime, cement-based, silicone, mineral or acrylic exterior renders) vary significantly by manufacturer, country and climate — always take the rate and bag size from the selected product's own technical data sheet rather than a generic default. Silicone and acrylic renders typically have different flexibility, breathability and colour-fastness properties than mineral renders, and are rarely interchangeable within the same system. Metric markets size bags in kilograms and rates in kg per m²; some imperial markets measure coverage per bag differently, so confirm units match your supplier's listing. Selecting a different currency changes only the displayed symbol — enter a current local supplier price for an accurate cost estimate, and confirm sales tax separately since it is not built into the calculator's cost output.
This calculator produces a planning estimate, not a plasterer's quotation or a guarantee of finish quality. It cannot verify substrate compatibility, confirm that a render system is rated for the local weather exposure, or replace the cure-time, layer-thickness and application-temperature limits stated on the manufacturer's data sheet. Complex substrates — mixed materials on one wall, damp or unstable backgrounds, heritage lime plaster repairs — should be assessed on site before finalising quantities. Always confirm final consumption rates, bag sizes and mesh specification against the selected product's current technical data sheet before ordering or starting work.
Last reviewed: 2026-08-25
Because they are different products with different consumption rates and usually different bag sizes; adding their kilograms together before dividing by a bag size would produce a number that matches neither product's actual purchase unit.
No. A refresh over sound, flat existing plaster might use a finish-only skim coat, while a scratch coat on new blockwork might use base coat only with no finish yet applied. Select only the layers your job actually needs.
Primer is important on highly absorbent substrates such as lightweight block or very dry backgrounds, where it evens out suction so the base coat cures consistently instead of drying too fast in patches.
Mesh is only needed at specific junctions — where substrates change, at corners, and around openings — not across the whole surface, so its quantity follows the length of those junctions plus a lap allowance for overlaps.
Plaster and render waste covers mixing losses, trowel waste and the extra material needed to fill an uneven substrate, which can vary more than simple cutting waste on a flat sheet material.
No. Exterior render systems are typically applied thicker and use different rates for weather resistance; always use the rate stated for the specific product and application you have selected.
No. Changing currency only changes the display symbol and formatting; enter a current supplier price in the currency you select.
No. It is a planning estimate. Confirm substrate compatibility, minimum application temperature, cure time between coats and maximum layer thickness against the selected product's technical data sheet before starting work.