How to estimate paint for a job

Getting paint quantities wrong costs money both ways. Order too little and you lose half a morning driving to the supplier, and you may end up with a slightly different batch on the same wall. Order too much and you’re paying for tins that sit in the van. A good estimate takes ten minutes with a tape measure and a calculator, and it also gives you the basis for pricing the labour.

Step 1: Measure every surface

Work room by room and write the measurements down as you go.

• Walls: measure the length of each wall and the ceiling height. Add the wall lengths together to get the room’s perimeter, then multiply by the height. That gives you the total wall area in square metres.
• Ceilings: multiply the length of the room by the width. For an L-shaped room, split it into two rectangles and add them together.
• Trims: measure skirting, architraves and scotia in linear metres, and count doors and window frames separately.
• Anything unusual: stairwells, raked ceilings, feature walls in a different colour, or areas that need a different product.

Measure each room on its own rather than estimating the whole house in one go. Rooms often have different colours, finishes and ceiling heights.

Step 2: Decide whether to subtract openings

Doors and windows don’t get wall paint, so you can take them off the wall area. Some painters subtract every opening. Others ignore small ones and only subtract large areas like ranch sliders, big windows or built-in wardrobes.

Leaving small openings in gives you a little extra paint, which helps cover touch-ups and waste. Subtracting everything gives a tighter number. Either works, as long as you’re consistent and know which method you used when you check how the job went.

Step 3: Read the coverage rate on the tin

Every paint has a spreading rate, usually given on the label or the product data sheet as square metres per litre (m²/L). It varies a lot between products. As examples, Resene’s own paint calculator uses 11 m²/L for one of its interior wall paints, and some Dulux interior wall paints are labelled at 16 m²/L. Always use the figure for the product you’re actually using.

That figure is a theoretical rate, measured on a smooth, sealed surface under good conditions. Real coverage on site is usually lower, especially on:

• New plasterboard, bare timber or fresh plaster, which soak up paint
• Rough or textured surfaces, such as bagged brick, stipple or weatherboards
• Corrugated and profiled surfaces, which have more surface area than their flat measurement
• Big colour changes, which may need an extra coat to cover

Many painters knock a margin off the label rate for this. Your own records from past jobs are the best guide to how much.

Step 4: Count the coats

Multiply the area by the number of coats. Two finish coats is common on walls and ceilings, but check the data sheet for the product. A dark colour over a light one, or the other way round, may need three.

Step 5: Don’t forget primer and sealer

New or bare surfaces usually need a primer, sealer or undercoat before the finish coats. It’s a separate product with its own spreading rate, so work it out as a separate line. Previously painted surfaces in good condition may not need it, but repairs and filled patches usually need spot priming.

Step 6: Work out trims and doors

Trims and doors are usually done in a different product, often an enamel, so estimate them separately. You can work out the area of each door (height times width, times two for both sides) and add a rough area for frames and skirting. On a small job, many painters simply allow one litre of trim paint per room or two, then adjust from experience.

Step 7: Round up to tin sizes

Paint comes in set sizes, and the sizes vary by brand and product. For example, one Resene interior wall paint comes in 1, 2, 4 and 10 litre tins. Once you have the litres needed, pick the combination of tins that covers it with a small amount spare. Keep a little left over for touch-ups after handover. Clients appreciate a labelled tin left behind for future repairs.

Buy all the paint for one colour at the same time where you can, so it’s tinted from the same batch.

A worked example

Here’s an example for one bedroom, using made-up round figures. Your own measurements and product rates will be different.

The room is 4.0 m by 3.5 m with a 2.4 m ceiling. The perimeter is 15.0 m, so the wall area is 15.0 x 2.4 = 36.0 m². There’s one door (0.8 m x 2.0 m = 1.6 m²) and one window (1.5 m x 1.2 m = 1.8 m²), so the net wall area is 36.0 minus 3.4 = 32.6 m². The ceiling is 4.0 x 3.5 = 14.0 m².

The tin says 14 m²/L, but the walls are a bit porous, so this example uses 12 m²/L.

Surface	Area	Coats	Area to cover	Litres at 12 m²/L	Buy
Walls	32.6 m²	2	65.2 m²	5.4 L	4 L + 2 L
Ceiling	14.0 m²	2	28.0 m²	2.3 L	2 L + 1 L
Door, frame and skirting	about 5 m²	2	10 m²	0.8 L	1 L

That leaves around 0.6 litres of wall paint and 0.7 litres of ceiling paint spare for touch-ups. If the walls were new plasterboard, you would add a separate line for a sealer coat at its own rate.

Estimating the labour

The materials are usually the smaller part of a painting price. The labour is where you make or lose money, and preparation is often the biggest part of it.

Break the labour down the same way you broke down the paint:

1. Protection: moving furniture, drop sheets, masking
2. Preparation: washing, sanding, filling, caulking, spot priming
3. Cutting in and rolling each coat on walls and ceilings
4. Trims and doors
5. Drying time between coats, and how it affects return visits
6. Clean-up, de-masking and touch-ups

The best way to put hours against each step is your own records. If you know from past jobs roughly how many square metres an hour you roll on a ceiling, or how long a door takes both sides, you can price from that. Track your hours on every job for a few weeks and the numbers will come quickly.

A room with lots of repairs, high ceilings or awkward access will take longer than the same area in an empty new house. Look at the condition of the surfaces on the site visit and price the prep you’ll actually do.

If you’d like a quick check on your sums, the Painting Calculator works out areas and litres from your measurements.

Common mistakes

• Using the label coverage rate on porous or rough surfaces
• Forgetting primer or sealer on new work
• Lumping trims in with wall paint
• Not rounding to real tin sizes
• Buying a top-up tin mid-job from a different batch
• Pricing the paint carefully but guessing the labour

Measure properly, use honest coverage rates and track your hours, and your painting prices will get more accurate on every job.