Paint calculator

DIY complexity

Beginner

Painting is one of the most beginner-friendly home projects — rollers and brushes are inexpensive, mistakes are easy to fix, and there is no structural or safety risk beyond basic ladder care.

Introduction

This calculator estimates how many litres of paint — and how many tins — you need to decorate a room, from a single accent wall to a full repaint of walls and ceiling. It starts from the net wall and ceiling area you enter, with doors and windows subtracted automatically, then multiplies by the number of coats you plan to apply and divides by the coverage of the paint you have chosen, expressed as square metres per litre. Coverage varies hugely from one product to another and even from one wall to the next, because porous new plaster, textured surfaces and dark colours all soak up far more paint than a smooth, previously painted wall. The tool also flags when a primer coat is worth adding — new plaster, a big colour change, or a stain that needs blocking — and when switching to a much darker or lighter colour may need an extra coat to hide the old one evenly. Litres are rounded up to whole tins based on the tin sizes you select, since paint is only sold in fixed containers. It cannot judge how porous your particular wall is, guarantee colour consistency, or replace a manufacturer's technical data sheet — coverage, drying time and recoat intervals should always be confirmed against the tin you actually buy, and batch or tint numbers should be checked so that all tins used on one wall match.

Quick answer

For a 12 m² bedroom wall area with 2 coats, primer, and coverage of 10 m² per litre, the paint need is (12 × 2) ÷ 10 = 2.4 L for the top coats plus roughly 1.2 L for the primer coat, about 3.6 L in total — rounded up to whole tins, for example two 2 L tins, this typically lands around 7-9 L once real-world coverage and edging losses are included.

Calculation methodology

01Measure the room by length, width and height, or enter wall and ceiling area directly, and choose whether the ceiling is included.
02Subtract the area of doors and windows from the gross wall area, using their count and average size, to get the net paintable area.
03Multiply the net area by the number of coats you plan to apply — most jobs use two coats for even coverage and colour consistency.
04Divide the coated area by the coverage per litre stated for the chosen paint, which depends heavily on surface porosity, colour and application method.
05Add a separate primer allowance, calculated as one coat over the same net area, if new plaster, a major colour change or stain blocking makes priming worthwhile.
06Round the total litres up to whole tins based on the tin size selected, since paint can only be purchased in fixed container sizes.

Formulas

net_area = (wall_area + ceiling_area) − (doors × door_area) − (windows × window_area)

The gross wall and ceiling area is not what you paint — doors and windows are cut out of it, because those surfaces are not painted with the same wall coat. Ceiling area is added separately only when the ceiling toggle is switched on.

Variables

wall_area = gross wall surface from room dimensions (m² or sq ft)
ceiling_area = ceiling surface, included only if selected (m² or sq ft)
doors, windows = count of each opening
door_area, window_area = average area per opening (m² or sq ft)
net_area = actual surface to be painted (m² or sq ft)

Rounding rule: Net area itself is not rounded; rounding only happens later once litres are converted to whole tins.

Limitation: Uses an average door and window size; a room with unusually large glazing or an oversized double door should have its opening areas entered individually rather than relying on the default average.

litres = ceil_to_tins( (net_area × coats ÷ coverage) + (primer ? net_area ÷ primer_coverage : 0) )

The net area is multiplied by the number of coats, because each coat needs its own full quantity of paint, then divided by how many square metres one litre actually covers. A separate primer quantity is added for a single coat over the same area when priming is switched on, then the combined total is rounded up to whole tins.

Variables

coats = number of top-coat applications, typically 2
coverage = m² covered per litre for the selected paint and surface
primer = whether a primer coat is included
primer_coverage = m² covered per litre of primer, usually similar to or lower than top-coat coverage
litres = total volume rounded up to whole tins

Rounding rule: Litres are always rounded up to the nearest combination of whole tins in the sizes you selected — you cannot buy a fraction of a tin, and any leftover paint is a normal and expected surplus.

Limitation: Coverage per litre is a product- and surface-specific figure, not a universal constant — a dark, previously unpainted or heavily textured wall can need well under half the coverage stated for a smooth, light-coloured repaint, so always start from the figure on the tin's label rather than a generic default.

Inputs that matter

Number of coatsDirectly multiplies the paint quantity; two coats is standard for durability and even colour, but a strong colour change may need a third.
Coverage per litreVaries hugely by surface porosity, texture and colour — using the manufacturer's stated coverage instead of a generic default is the single biggest accuracy factor.
Primer toggleAdds a separate coat sized to the same net area; worth including for new plaster, bare surfaces, big colour changes or stains that need blocking.
Surface / colour changeSwitching to a very different colour, especially light-to-dark or dark-to-light, often needs an extra coat to fully hide the old colour evenly.
Doors and windowsTheir count and average size are subtracted from the gross area before litres are calculated — undercounting them overstates paint need.
Tin sizeDetermines how the total litres round up into whole containers, and mixing tin sizes can reduce leftover surplus compared to buying only one size.

Worked examples

Bedroom walls, 12 m² with primer, 2 coats

A small bedroom with 12 m² of net wall area after subtracting one door and one window, freshly plastered so a primer coat is included, 2 top coats, coverage of 10 m² per litre for both primer and top coat, 3 L tins.

Top coats = (12 × 2) ÷ 10 = 2.4 L. Primer = 12 ÷ 10 = 1.2 L. Total = 2.4 + 1.2 = 3.6 L. tins = ceil(3.6 ÷ 3) = 2 tins of 3 L, giving 6 L purchased against a 3.6 L theoretical need.

3.6 L is the theoretical combined need for primer and two top coats; 6 L is the final purchase quantity because paint is only sold in whole 3 L tins here. The 2.4 L surplus is normal and useful as touch-up reserve, not a sign of overestimation.

⚠ 10 m² per litre is an editable example coverage — always replace it with the figure printed on the specific tin you plan to buy, since it can vary by a factor of two between products.

Open-plan room, walls and ceiling, colour change, 30 m²

A 30 m² open-plan living area, walls and ceiling both included, 2 doors and 3 windows averaging 1.8 m² and 1.2 m² respectively, a significant colour change from a dark to a light tone (an extra coat applied), coverage of 9 m² per litre for this colour, no primer, 5 L tins.

Openings = 2 × 1.8 + 3 × 1.2 = 3.6 + 3.6 = 7.2 m². Net area = 30 − 7.2 = 22.8 m². Coats used = 3 (2 standard + 1 for the colour change). Litres = (22.8 × 3) ÷ 9 = 7.6 L. tins = ceil(7.6 ÷ 5) = 2 tins of 5 L, giving 10 L purchased.

22.8 m² is the net paintable area including the ceiling; 7.6 L is the theoretical need across all three coats; 10 L is the final purchase quantity in whole 5 L tins. The third coat exists specifically to hide the strong colour contrast evenly, not as a general safety margin.

⚠ All tins bought for one job should share the same batch or tint number where the retailer offers it — machine-mixed colours can vary slightly between batches, and a visible seam can appear where two batches meet on the same wall.

20 m² ceiling repaint with cost estimate

A 20 m² ceiling only, no doors or windows to subtract, 2 coats, coverage of 10 m² per litre with a 10% allowance built into the coverage figure for roller loss and edging, no primer, sold in 2 L and 5 L tins at example local prices.

Theoretical litres = (20 × 2) ÷ 10 = 4.0 L. With the 10% allowance already folded into the coverage figure, effective need = 4.0 × 1.10 = 4.4 L. tins = one 5 L tin (ceil against the nearest available size), or one 2 L plus one 2 L plus a small top-up depending on sizes offered — using a single 5 L tin here at an example price of 45 gives a total material cost of 45 for 4.4 L needed.

4.4 L already includes an application-loss allowance folded into the coverage rate, which is different from the waste percentage applied to material area in other trades — check whether your chosen coverage figure already includes a loss margin before adding another one on top.

⚠ Changing the selected currency only changes the displayed symbol — it does not convert the price you entered. Enter a current local supplier price in the currency you have selected before relying on the cost total.

Reading the results

The result panel separates the net paintable area, the theoretical litres needed for the chosen coats and any primer, and the final whole-tin purchase quantity — each figure is larger than the last for a distinct reason, not a rounding inconsistency. Primer, where included, is shown as its own line because it uses a separate coverage rate and often a different product entirely from the top coat. Where a price is entered, cost is calculated from the rounded tin quantity actually purchased, not the theoretical litre figure, so the total reflects what you will really pay at the till.

Common mistakes

Using the physical wall dimensions without subtracting doors and windows, which overstates the paint needed.
Applying a generic coverage figure instead of the one printed on the specific tin, ignoring that dark colours and porous new plaster reduce real coverage substantially.
Forgetting a primer coat on new plaster or bare drywall, which leads to patchy, uneven top-coat absorption and a poor final finish.
Assuming a colour change needs the same number of coats as a like-for-like repaint, when a strong light-to-dark or dark-to-light change often needs an extra coat.
Buying tins in a single size without checking whether a mixed combination of sizes wastes less paint and money.
Mixing tins from different batch or tint numbers on the same wall, risking a visible colour seam even within the same product line.

Adapting to your country

Paint coverage, tin sizes and terminology vary by country and manufacturer — a litre of the same nominal product can cover very different areas depending on formulation, so always take the coverage figure from the selected product's own label or data sheet rather than a generic default. Tin sizes commonly sold range from small sample pots up to 5 L or larger trade sizes, and some markets sell by the US gallon instead of the litre — check which unit the calculator and your local supplier both use. Primer, top-coat and specialist products (stain-blocking, masonry, trim) each have their own coverage rate and are rarely interchangeable. Selecting a different currency changes only the displayed symbol — enter a current local supplier price for an accurate cost estimate, and confirm sales tax separately since it is not built into the calculator's cost output.

Limitations and verification

This calculator produces a planning estimate, not a decorator's quotation or a guarantee of colour matching. It cannot judge the actual porosity or condition of your specific wall, verify compatibility between primer and top-coat systems, or account for application method differences between roller, brush and spray, which all consume paint at different rates. Complex surfaces — heavy texture, water damage, multiple existing colours showing through — should be assessed in person before finalising quantities. Always confirm coverage, drying and recoat times, and batch or tint matching against the actual tins purchased before starting work.

Last reviewed: 2026-08-25

Frequently asked questions

Why does the calculator subtract doors and windows from the wall area?

Because those surfaces are not painted with the wall coat, so including them would overstate how much paint you actually need for the walls themselves.

How much does coverage per litre really vary between products?

It can vary by a factor of two or more — a smooth, previously painted wall in a light colour might need only 8-10 m² per litre while a porous new-plaster wall in a dark colour can need well under that. Always use the figure on the tin, not a generic assumption.

When should I include a primer coat?

Primer is worth adding on new or bare plaster, over stains that need blocking, and when making a major colour change, since it seals the surface and helps the top coat cover more evenly with fewer coats.

Does a big colour change really need an extra coat?

Often yes. A strong light-to-dark or dark-to-light change can leave patches of the old colour showing through two coats, so a third coat is commonly needed for even, uniform coverage.

Why does the result round up to more paint than the raw calculation shows?

Paint is only sold in fixed tin sizes, so the theoretical litre figure is rounded up to the nearest combination of whole tins — leftover paint is a normal and useful reserve for touch-ups.

Should ceiling paint be calculated the same way as wall paint?

The area and coats formula is the same, but ceiling paint is often a different, flatter-finish product with its own coverage rate, so keep the ceiling litres as a separate line rather than merging it with wall paint.

Does changing currency convert the material price?

No. Changing currency only changes the display symbol and formatting; enter a current supplier price in the currency you select.

Is the result accurate enough to order paint directly without checking anything else?

It is a planning estimate. Confirm the coverage, recoat interval and batch or tint numbers on the actual tins you plan to buy, and check that all tins for one wall share a matching batch.

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