Primer Calculator
Enter your room size, then choose a surface preset or add the coverage from your primer label.
Your measurements
See how we calculated this
Check your order
Your painting order
- Record the coating, colour, finish, coverage per coat and number of coats.
- Round the calculated litres or gallons to the actual tins or cans. Estimate primer separately.
- Check roller fabric and nap against the coating and surface; reuse suitable tools you already own.
Tools for this job
Primer still needs the right applicator: check the coating label and surface texture before choosing a roller or brush.
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- Cover the open wall areas
Roller kit
Wooster Painter's Choice roller kit
A roller kit to consider for painting wall areas. Match the cover and nap to your paint and surface texture before choosing a kit.
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Angled brush
Wooster Shortcut 2-inch angled brush
An angled brush for the edges and trim a roller cannot reach. Check brush compatibility with the coating you plan to use.
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Already have suitable tools? Reuse them. These are optional product suggestions, not a complete equipment list or a hands-on comparison. Amazon may redirect Amazon.com links to local products or search results. Amazon.nl links search for alternatives, which may differ in brand, size and specification. Check suitability, seller, delivery and any import costs on your chosen store before ordering.
How much primer do I need?
Primer quantity is area × coats ÷ coverage, with the chosen allowance added. Use a substrate preset for a planning estimate, or enter the actual one-coat rate in Product coverage. A positive value overrides the preset; 0 restores it. Coverage used in the result confirms the active rate. Match the field’s m²/L or sq ft/US gal unit.
Choose the primer and coat count from its instructions for the actual substrate. One coat is a calculation default, not a universal specification for timber, stains or colour changes.
The formula
Worked example
A 5 m x 4 m room with 2.5 m walls has 45 m² of wall. Deduct two average openings at 1.7 m² each, leaving 41.6 m² to prime.
For one mist coat on new plaster at 8 m²/L, 41.6 ÷ 8 = 5.2 litres. Add the 5% allowance and you need 5.46 litres. The metric suggestion is one 2.5 L tin and three 1 L tins: 5.5 L purchased, leaving 0.3 L beyond the estimated coat coverage, including the allowance. The suggestion minimises surplus volume across the listed sizes; compare actual pack prices before buying.
Imperial mode starts from the same unrounded requirement of about 1.44 US gallons and suggests one 1-gallon can plus two 1-quart cans: 1.5 US gallons in total. The purchased volume and leftover follow this order. Metric sizes are 10, 5, 2.5 and 1 L; US sizes are 5 gallons, 1 gallon and 1 quart. Check the actual product sizes and any dilution instructions before ordering.
Which primer for which surface
| Substrate | Primer type | Coverage (m²/L) |
|---|---|---|
| New plaster | Thinned emulsion mist coat, or dedicated plaster primer | 8 to 10 |
| Bare or patched drywall | Acrylic drywall primer / sealer | 6 to 8 |
| Bare softwood, MDF edges | Acrylic wood primer-undercoat | 8 to 10 |
| Knots in new timber | Shellac knotting solution (spot treatment) | Spot only |
| Ferrous metal, radiators | Metal / rust-inhibiting primer | 10 to 12 |
| Bare masonry, render, concrete | Stabilising solution or masonry primer | 4 to 6 |
| Water, smoke or nicotine stains | Shellac or solvent stain block | 5 to 7 |
| Glossy or tiled surfaces | Adhesion / bonding primer | 9 to 11 |
The mist coat rule for new plaster
Use the coating manufacturer’s preparation instructions for new plaster, including the required drying condition and whether a dedicated primer or mist coat is suitable. Only dilute a product when its instructions specify a ratio.
The New plaster setting is an 8 m²/L planning preset that you can replace with Product coverage. Added water does not establish an equal increase in coverage, and the calculator does not model dilution. Use the published coverage basis and mixing instructions for a specified diluted coat.
When you can skip primer
- Repainting a sound, matt, previously painted wall in a similar colour: go straight to two top coats.
- Using a self-priming paint on a sealed surface, where the first of two coats does the sealing.
- You still need primer if there are filler patches larger than a coin, any bare substrate, any stain, or any glossy surface.
Sources and assumptions
The estimate divides the surface area by a spreading rate for each coat. Product coverage (optional) overrides the substrate preset when positive; 0 keeps the preset. Check Coverage used and use the primer’s own one-coat rate, not the topcoat rate.
- Zinsser Bulls Eye 1-2-3: product specification: Zinsser publishes 10 m²/L for this primer and lists its intended uses. Other primers and preparation systems have different rates and instructions.
Before using this estimate
- Room mode deducts 1.7 m² per opening. Measured net area is more reliable for large doors or windows. Product coverage uses m²/L in Metric mode and sq ft/US gal in Imperial mode.
- Substrate presets are illustrative rates. Select the primer and any dilution from the chosen coating system instructions. A generic mist-coat or stain-blocking rate does not establish compatibility or cure an underlying damp problem.
Source notes reviewed . See how we check calculations and handle corrections.
Planning your order
Check the inputs before buying: measure the surfaces you need, then apply an allowance and round to whole packs. The guides include worked examples you can follow alongside this calculator.
Planning several rooms? Keep primer separate when combining a multi-room paint order. Record its own product coverage, coat count and compatible surfaces before pooling quantities.
Published by OrderEnough. This is a planning estimate: check the product specification and the assumptions above before ordering. Last updated . Read how we source our figures on the methodology page.