Estimate how much primer you need for a paintable area, based on your coverage rate, number of coats, and a wastage allowance.
Enter coverage according to your primer product.
How to Use the Primer Calculator
Enter your paintable area — use the Paint Quantity Calculator or Wall Area Calculator to work this out first if needed.
Enter your primer's coverage rate, the number of coats, and a wastage percentage.
Click "Calculate" to see the required primer, before and after wastage.
Worked Example With the Default Values
The form starts with 1,000 sq.ft of paintable area, a coverage of 120 sq.ft per litre, 1 coat and 10% wastage:
Required primer = 1,000 × 1 ÷ 120 = 8.33 L
With wastage = 8.33 × 1.10 = 9.17 L
Primer is usually sold in fixed pack sizes, so round up to what is available, for example a single 10 L pack rather than several small tins. Check whether the coverage on the label is for the primer as supplied or after it has been thinned with water as the maker recommends; mixing the two will throw your estimate off.
How Coverage and Coats Change the Quantity
For the same 1,000 sq.ft and 10% wastage, here is how the answer moves:
Coverage (sq.ft/L)
Coats
Required Primer
With Wastage
160
1
6.25 L
6.88 L
120
1
8.33 L
9.17 L
80
1
12.50 L
13.75 L
120
2
16.67 L
18.33 L
Real coverage depends on the product and the surface. Fresh cement plaster and rough exterior walls soak up much more primer than a smooth, previously painted wall, so use the lower end of the range printed on the pack or the manufacturer's technical data sheet for porous surfaces.
Choosing the Right Primer
Interior wall primer: water-based, used on plaster, putty and previously painted interior walls under emulsion paint.
Exterior primer: formulated to resist moisture and alkali on outside walls, especially important during and after the monsoon.
Metal primer, such as red oxide or zinc-based primers: for grills, gates and railings before enamel paint, to slow rusting.
Wood primer: seals timber on doors and windows so the top coat does not soak in unevenly.
On new interior walls, many painters apply primer, then wall putty, then a second thin coat of primer before the finish paint. If your job follows that sequence, enter 2 coats. Calculate wall and metal areas separately because they need different products.
Frequently Asked Questions
Required Primer (L) = Paintable Area × Coats ÷ Coverage. A wastage percentage is then added on top to account for spillage and surface absorption.
It's generally recommended, especially on new or bare walls, repairs, or when changing to a very different color — it improves paint adhesion and finish. Check your paint manufacturer's guidance for your specific surface.
The default of 10% suits most brush and roller work on walls. Increase it for spray application, very rough or porous surfaces, or jobs with many small areas such as grills, where more material is lost.
It is not a substitute. Primer is designed to seal the surface and help the top coat bond, while paint is designed for colour and durability. Using paint as a base coat on bare plaster often leads to patchy absorption and needs extra coats.
No. It is the total area of the walls and ceilings you will prime, which is much larger than the floor area. Measure each wall as length × height, add the ceiling, and subtract large openings such as doors and windows.