Render Coverage Calculator
Gross area is the perimeter times the height. Each door removes 1.9 square metres and each window removes 1.2 square metres, the same openings used for exterior paint. Coverage starts at 1.8 m² per bag, the low end QUIKRETE prints for an 80 lb Scratch and Brown bag at 3/8 inch. The same sheet changes that figure with thickness, so edit it when you are not placing 3/8 inch.
Last reviewed: 2026-10-28 · How we calculate
How we calculate
Gross area = perimeter × height. Net area = gross area − 20 sq ft (1.9 m²) per door − 13 sq ft (1.2 m²) per window, and never below zero. Area with waste = net area × (1 + waste %). Bags = area with waste ÷ square feet per bag (square feet per bag on the metric toggle), rounded up, only when coverage is greater than zero.
Worked example
A 10 m perimeter at 2.5 m, with no openings and no waste, is 25 m². At the starting 1.8 m² per bag that is 14 bags. At 5 m² per bag it is 5 bags. The area stays either way.
Base coat on brick versus on board
Render over brick and render over a boarded wall do not share a thickness. A scratch and brown base coat at 3/8 inch is the coverage this page starts from: 1.8 square feet per 80 lb bag, the low end of the QUIKRETE Scratch and Brown sheet. A finish coat is a different product and a different coverage. Measure the walls you are actually coating, take off doors and windows, and change the coverage when the specification changes the thickness.
Measuring a render wall
Render is sold by the bag, and the bag is honest about coverage only at a stated thickness. This calculator refuses to invent that coverage. You measure the wall, you subtract ordinary openings, and you type the square metres one bag claims. Gross area is the simple rectangle around the house or around the elevation you are coating: perimeter times height. A 20 m perimeter at 2.4 m is 48 square metres before openings. One door and two windows remove 1.9 plus 2.4 square metres, leaving 43.7. Ten percent waste makes that about 48.1 square metres to cover. Until coverage per bag is typed, bags stay 0. That zero is the correct output, not a broken field.
How render area becomes bags
The opening allowances are shared with exterior paint so the two pages do not disagree about a normal door or a normal window. They are allowances, not a survey. A 2.4 m wide garage door is closer to 5 square metres than to 1.9. Either drop the door count to zero and subtract the real door from the area yourself, or shorten the perimeter so the missing wall is not in the gross area. Gables, chimneys, and returns around a window reveal are extra area. Add a triangle as base times rise divided by two. Reveals are a narrow strip: opening perimeter times the reveal depth. They are small until the wall is thick. Include them when the specification says the reveal is rendered.
Ordering base-coat stucco
Waste is not a second coat. If the bag says 6 square metres at 10 mm for one coat and you will apply two coats, the honest move is to use the two-coat coverage from the bag, or to run the area twice and add the bags. Adding 10% waste does not stand in for a second coat, and it does not stand in for a scratch coat on a rough block wall that drinks the first pass. Rough backgrounds are why the waste field exists, and you should raise it when the wall is open-textured rather than pretending the bag's smooth-wall number still holds. The cinder block calculator counts blocks. It does not know how thirsty the face is.
Checking a render order
Order bags only after the coverage field matches the product. If the area with waste is 48 square metres and the bag says 8 square metres, you need 6 bags. If the bag says 7.5, you need 7 because the count rounds up. Buy the same batch when the colour is mixed in, and keep one bag back for patches if the supplier will take the rest back. Do not convert the bag count into money on this page. Exterior paint, if the render will be painted, belongs on the exterior paint calculator after the render is flat, using the same net wall as a check. Siding is the alternative cladding, not a coat of render. Limewash and masonry paint are different products with their own coverage lines.
Render fields and starting values
Wall perimeter starts at 36 m. Wall perimeter in metres. Switch to ft / in for feet. Wall height starts at 2.7 m. Wall height in metres. Switch to ft / in for feet. Doors starts at 1. Each door removes 20 sq ft (1.9 m²). A garage door is larger. Reduce the perimeter or accept a high area and measure the door yourself. Windows starts at 2. Each window removes 13 sq ft (1.2 m²). Large shop windows need a real measurement instead of this allowance. Waste allowance starts at 10 %. For droppings, a rough background, and a slightly thicker dubbing coat. It is not a second coat. Coats belong in the coverage figure on the bag. Coverage per bag starts at 1.8 m². 1.8 m² is the low end QUIKRETE prints for an 80 lb Scratch and Brown bag at 3/8 in (1.8–2.2 m²), checked 27 October 2026. Use 1.4 m² at 1/2 in or 0.93 m² at 3/4 in if that is the thickness.
Base-coat stucco bags at 3/8 inch
Coverage is 1.8 m² per 80 lb bag, the low end QUIKRETE prints for Scratch and Brown Base Coat Stucco at 3/8 in. One door, two windows, and 10% waste.
| Wall | Net area | 80 lb bags |
|---|---|---|
| 20×3 m | 55.7 m² | 35 |
| 30×3 m | 85.7 m² | 53 |
| 40×2.7 m | 103.7 m² | 64 |
The same sheet lists 1.4–1.7 m² at 1/2 in and 0.93–1.1 m² at 3/4 in. Change the coverage when the thickness changes.
Common mistakes
- Typing a guessed coverage so the bag line is not zero. Zero means the bag has not been read yet.
- Using paint coverage, in square feet per gallon (square metres per litre on the metric toggle), as if it were render coverage per bag.
- Forgetting that a second coat is a different coverage, not something the waste percentage adds.
- Leaving a garage door in the wall as if it were a 20 sq ft (1.9 m²) house door.
- Rendering only the front and still entering the whole house perimeter.
- Treating the bag count as a price.
Frequently asked questions
How much render do I need for a wall?
Perimeter times height, minus openings, then divide by the square metres printed on the bag. Without that bag figure the area is the result and the bags stay at 0.
How do I convert render coverage to bags?
Divide the area with waste by the coverage of one bag. A 25 m² wall at 5 m² per bag is 5 bags. Round up only at the end.
Does thickness change the bag count?
Only through the coverage you type. A thicker coat covers fewer square feet per bag (square feet per bag on the metric toggle). Read the line on the bag that matches the thickness.
Are doors and windows deducted?
Yes, at 20 sq ft (1.9 m²) and 13 sq ft (1.2 m²), matching the exterior paint calculator. Measure anything that is obviously different.
Is render the same as mortar?
No. The mortar calculator divides an area by a bag coverage you also have to enter, but it is aimed at bedding masonry. Use the yield for the product in your hand.
Can this price the render?
No. Take the area or the bag count to the supplier.