This paint calculator compares the cans you would buy for topcoat and primer separately. Enter each product’s measured net area, verified coverage per coat, number of coats, allowance and final can prices in Australian dollars (AUD). It does not decide whether your surface needs primer.
Keep primer and topcoat purchases separate: their cans, coverage and prices may differ. Compare the cash needed at checkout with the surplus above each product’s planning target, without treating the two products as interchangeable paint.
Separate paint and primer can costs
Enter confirmed net areas, per coat rates and final AUD prices. Fixed sizes: 1/4/10/15L. Price 0 means unavailable, not free. No existing stock subtraction.
Unknown basis blocks results. Already buffered coverage requires zero extra allowance. Primer off disables and ignores its fields. Load example uses invented prices.
This tool runs in your browser. Your entries are not sent or saved. All amounts are in Australian dollars (AUD).
Load example fills hypothetical figures. They are not current rates or recommendations.
Use confirmed values; up to two decimals, whole coats. See article limits.
Loading the calculator.
What must you confirm before entering the numbers?
Measure the net area that each product will actually cover. Deduct unpainted openings yourself using your own measurements. This tool does not infer standard doors, windows, room shapes or which surfaces belong in a primer calculation. Enter the resulting area in square metres.
Read the particular product’s instructions or obtain confirmation of its coverage and coat requirement. Keep a note of the product, base, sheen and colour outside this tool. Each positive-priced can within one group must refer to that same confirmed product, base, sheen and colour, not merely the same brand.
Use a verified coverage rate for one coat. A headline saying a can covers an entire job might already describe several coats. If the basis is unknown, stop. The whole-job option stops the calculation: obtain the per-coat rate rather than multiplying an uncertain claim again.
The tool accepts two decimal places for area, coverage, allowance and prices, and whole-number coats. Each active area and rate must be positive. The tool accepts 1 to 10 whole-number coats as an input range, not a recommendation; enter only the number confirmed for your selected product and surface. Enter zero for an unavailable fixed can size and a positive final price for each available size. At least one size per active product must have a positive price. Blank active inputs remain unknown, and actual free paint is not supported.
Why should primer have its own purchase calculation?
Confirm whether primer is required for your selected product and surface; the calculator cannot make that decision. If primer is not required, choose No primer. Its fields are disabled and ignored, even when blank; do not enter zeros just to satisfy the form. Switching primer back on makes its coverage, area, coats, allowance and price entries subject to validation again.
When primer is required, enter its own area, coverage, coats and prices. A can of topcoat cannot satisfy the primer line merely because the combined litres fit inside its capacity. Keep different products, bases, sheens or colours in separate purchase calculations.
How does the coverage calculation avoid counting twice?
Dulux’s quantity-planning guide explains the stable starting point: surface area divided by coverage per litre, multiplied by coats. Bunnings’ paint calculator is another planning reference. Before buying, confirm the selected product’s coverage per coat and required coats for the surface from its instructions or supplier; do not treat an example as your product’s rate.
For an unbuffered per-coat rate, the model is net area × coats ÷ coverage × (1 + extra allowance ÷ 100). An allowance is your explicit planning input, not a measured industry-wide loss rate. Five per cent in the example is invented for calculation practice.
If your confirmed per-coat planning coverage already incorporates an allowance, select that basis and enter zero extra allowance. The tool blocks a positive extra percentage in this mode. It cannot inspect a label or establish whether a supplier has already included a buffer.
What does the two-product hypothetical example show?
For this hypothetical example only, assume primer is required and each product covers a measured 48 m². Assume a hypothetical topcoat rate of 16 m²/L per coat, two coats and an additional 5 per cent allowance. The calculation is 48 × 2 ÷ 16 × 1.05 = 6.300 L.
For the separate primer, assume an example rate of 14 m²/L per coat, one coat and 5 per cent extra. Then 48 × 1 ÷ 14 × 1.05 = 3.600 L. These rates are example inputs, not universal rates for Australian paint or a recommendation for your wall.
| Product | 1 L | 4 L | 10 L | 15 L |
|---|---|---|---|---|
| Topcoat | $30 | $60 | $70 | $120 |
| Primer | $18 | $45 | $95 | $140 |
At these invented topcoat prices, minimum spend is one 10 L can for AUD 70. It leaves 3.700 L above the planning target. The least-surplus purchase is one 4 L can plus three 1 L cans, costing AUD 150 and leaving 0.700 L.
Primer prices are different. One 4 L primer can costs AUD 45 and leaves 0.400 L. It is both the lowest-spend and least-surplus choice under these inputs. Use the separate primer prices rather than applying the topcoat’s price table to both products.
Combined minimum spend is AUD 115, with 4.100 L of product-specific surplus across both groups. Combined least-surplus cost is AUD 195, with 1.100 L total surplus. The AUD 80 difference buys a different quantity mix; it does not prove that either choice suits your project.
What do minimum spend and least surplus mean?
Minimum spend finds the cheapest whole-can combination that meets each product’s rounded planning target. If combinations tie on price, it chooses the smaller purchased volume. Least surplus first minimises purchased volume, then chooses the cheaper combination at that volume. Both results apply only to the available fixed sizes and prices you entered, not the cheapest paint across the whole market.
Because these are independent products with independent prices, adding their individual minimum costs gives the combined minimum cost. Likewise, minimising each product’s surplus minimises the sum. This would not remain sufficient with shared discounts, bundled prices or a delivery charge depending on the combined order.
Surplus means bought volume minus the rounded planning target. It is not a prediction of unusable waste, stored paint life or the amount actually left after painting. Existing stock is not subtracted. Product suitability, coating quality and future touch-up use remain outside the calculation.
For the wider cash plan, put this one-off purchase alongside your regular costs using our Australian household budget review. Keep the cash paid for cans separate from the calculated surplus: surplus litres do not return cash to this month’s budget.
How are precision and the search bound handled?
Inputs are converted to integers at their stated precision. Money uses cents. The area, rate, coats and allowance form an integer fraction; required volume is explicitly rounded upwards to the next millilitre, never downwards.
The can capacities are 1000, 4000, 10000 and 15000 mL. Only sizes with positive prices enter the search. Their greatest common divisor sets its capacity unit. With only 4 L available, that unit is 4000 mL, so 6300 required millilitres needs two units: two 4 L cans. The target litres displayed remain distinct from purchasable litres.
Dynamic programming finds the cheapest combination at each exact capacity. It searches through target units + largest can units − 1. Any combination at or above target + largest can remains sufficient after removing a can; with strictly positive prices, removing it reduces both cost and surplus. Such a combination cannot improve either objective.
The calculation caps the largest capacity index at 10000, plus the zero state. If the bound exceeds that limit, it stops with an error rather than truncating the search or changing your input. Inputs beyond stated precision also stop; they are not silently rounded to fit.
When does this paint purchase comparison not fit?
Do not use it for unverified coverage, whole-job coverage claims, uncertain coat counts or a decision about whether primer is needed. Rates inferred from a broad brand average are not a substitute for the chosen product’s instructions and the actual surface.
It also does not fit limited quantities, shared promotions, mixed colours, spraying adjustments or comparing different paint qualities. It supports only the fixed 1, 4, 10 and 15 L slots; a product sold only in custom sizes such as 2, 2.5 or 5 L cannot be calculated here. Mark unavailable fixed sizes with zero and confirm that every positively priced size is available. Actual free paint is not supported. Fees outside those prices and delivery arrangements need a separate check.
If required volume changes, calculate again. When the surface or expected allowance is uncertain, try separately labelled scenarios, but do not present one as certain. Neither objective establishes that the paint will cover evenly, that a refund is available or that a purchase saves money in practice.
Quick decision checklist
- Confirm separate products, colours and measured net areas for primer and topcoat.
- Verify per-coat coverage and coat requirements; stop if their basis is unclear.
- Count an allowance once; enter zero for unavailable fixed sizes and confirm each priced can and its final AUD price.
- Compare minimum spend with least surplus without pooling incompatible products or subtracting existing stock.
- Check suitability, availability and excluded fees before making a purchase.
Sources and notes
- Dulux paint calculator: the area, per-coat coverage and coats planning principle; check the selected product’s spread rate.
- Bunnings paint calculator: a quantity-planning reference. Confirm coverage and coats for your selected product before purchase.
Source pages were checked on 14 October 2026 UTC. The worked example’s coverage rates, coat counts, prices and allowances are hypothetical, not verified product specifications or current market quotes. This guide was prepared with AI assistance and source checking; it has not received professional financial or legal review.
