Render and plaster are sold by the bag but consumed by the millimetre: double the layer thickness and you double the material, whatever the wall’s area. This mortar calculator makes that product explicit — wall area × layer thickness × a published consumption rate — and returns the dry mortar weight and the number of 25 or 40 kg bags to buy. The rate is stated, not hidden: pre-mixed cement render and plaster mortars are published by their manufacturers at roughly 1.4–1.6 kg per square metre per millimetre of thickness, and the calculator defaults to the middle of that range, 1.5, with the rate left editable for the product in your hand.
The consumption rate, and why it is a range
A square metre of render one millimetre thick is exactly one litre of material — that part is pure geometry (the calculator shows it: volume in litres = area × thickness in mm). What varies is how much dry powder makes that litre: light plasters and bonding coats sit near 1.2–1.3 kg per litre, standard cement renders and pre-mixed plaster mortars at 1.4–1.6, dense repair mortars higher. Bag labels print their own figure (“consumption: approx. 1.5 kg/m² per mm” or a coverage per bag at a stated thickness) — when the bag disagrees with the default here, type the bag’s number into the rate field. That single honesty is the difference between an estimate and a guess.
Worked example
A wall of 20 m² rendered at 15 mm — a normal two-coat build-up over blockwork — at the default rate and no waste: 20 × 15 × 1.5 = 450 kg of dry mortar, occupying 0.30 m³ (300 litres) wet. In bags that is 450 ÷ 25 = 18 bags of 25 kg, or ceil(450 ÷ 40) = 12 bags of 40 kg. Add a 10% waste allowance for droppings, hawk losses and the last uneven corner — realistic on site — and the weight becomes 495 kg, which is 20 bags of 25 kg. Notice the bags round harder than the kilos: a small margin in weight can cost a whole bag, which is exactly why experienced plasterers quote in bags, not kilos.
Thickness is the decision, area is just the measure
Measure thickness from the substrate’s high spots, not its average: a wall that waves between 8 and 20 mm needs the 20 mm figure (or a dubbing-out coat first, costed separately at its own thickness). Scratch coats of 5–8 mm, float coats of 8–12 mm and finish coats of 2–3 mm add; enter the sum for one number, or run the calculator per coat if different products are involved. Ceilings take the same maths with worse waste — raise the allowance to 15% for overhead work.
Scope and limits
This is a consumption estimator for bagged dry mortar: it does not design the mix for site-batched mortar (cement:sand ratios and water are a different calculation), it does not judge whether a substrate needs a bonding agent, and it cannot see your wall — suction, key and straightness all move real consumption by more than the rate’s range. What it settles is the order quantity for a stated thickness at a stated rate, which is the number the merchant needs and the number most under-orders get wrong.
Reading a bag label like a plasterer
Manufacturers print consumption two ways, and confusing them halves or doubles an order. “Coverage: 1.6 m² per bag at 10 mm” is a per-bag figure at one stated thickness — convert it before comparing: at 10 mm, one square metre at 10 mm thickness is 10 litres, so 1.6 m² per 25 kg bag means the bag yields 16 litres, a rate of about 1.56 kg per m² per mm. “Consumption: 1.4 kg/m²/mm” is already the rate this calculator wants. When a label gives a range, plan from the top of it: render over high-suction blocks drinks more, and the rate assumes the stated thickness everywhere, while your wall has hollows. The calculator’s editable rate exists so the bag in your hand — not a generic table — sets the number you order from.
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