Paint calculator guide
The calculator above works out how much paint you need, the cheapest tins to buy to cover it, and what the job will cost — for one wall or a whole house, in litres or US gallons, all in your browser.
The guide below explains how to estimate paint for walls, ceilings, doors and trim, what paint coverage really means, how many coats and how much primer to allow, and worked examples — including the classic 12×12 room — that you can reproduce by hand.
How much paint do I need?
To work out how much paint you need, measure the surface area to paint, subtract doors and windows, multiply by the number of coats, then divide by the coverage rate of the paint. A small allowance for waste turns that into the amount to actually buy.
In short: paint (litres) = painted area × coats ÷ coverage, plus a waste margin. The calculator does every step for you and then rounds the result up to whole tins, but the maths is simple enough to check by hand.
As a rule of thumb, one litre of emulsion covers about 12 m² in one coat (one US gallon covers roughly 400 ft²). So a typical 4 m × 5 m bedroom with 2.4 m walls needs around 7 litres for two coats — and the calculator turns that into the exact tins to buy and the cost.
How this calculator works
You describe the job; the calculator handles the arithmetic. Add a simple area when you already know the square metres or feet, or add a room to enter walls, a ceiling and trim with their doors and windows. As you type it returns the net paintable area, the paint volume, the recommended tins, leftover paint and the cost.
Coats, coverage, a waste percentage, your tin sizes and prices, and an optional primer all live in the assumptions panel, so you can match the exact paint and pack sizes you are buying. Switch between metric (litres, m², m²/L) and imperial (US gallons, ft², ft²/gal) at any time and every value converts.
Results can be exported to CSV for a spreadsheet or printed as a clean shopping list. Everything is calculated on your device — nothing you enter is uploaded, and your project is saved only in your own browser.
Estimate your paint in seven steps
- Choose units and currencyPick metric (litres, m²) or imperial (US gallons, ft²) and the currency you buy paint in.
- Add an area or a roomAdd a simple area if you know the square metres or feet, or add a room to enter walls, a ceiling and trim.
- Enter dimensions and openingsType each surface's size and list the doors and windows so they are subtracted from the wall area.
- Set coats, coverage and wasteSet how many coats, the paint's coverage rate, and a waste margin in the assumptions panel.
- Add primer if neededTurn on primer for bare, patched or porous surfaces and set its own coverage and coats.
- Enter tin sizes and pricesList the pack sizes you can buy and their prices so the calculator finds the cheapest combination.
- Read the results, export or printCheck the volume, tins, leftover and cost, then export to CSV or print the shopping list.
Coverage and the formulas behind the numbers
Paint coverage (also called the spreading rate) is the area one litre or one gallon covers in a single coat. It is the figure printed on the tin, and it is the one number that most changes the estimate: a smooth sealed wall stretches paint further than bare plaster or a textured surface.
Four formulas drive the result: net area, paint volume, the amount to buy after waste, and cost. The calculator applies them per item and then totals the project.
The numbers in the results come from these formulas.
Net paintable area
net area = (wall width × height − doors & windows) + ceiling + trim length × profile width- Openings are subtracted from each wall; a ceiling is entered as an area; trim is a length times a narrow profile width.
- The total net area is what you actually paint, before coats.
Paint volume (before waste)
volume = net area × coats ÷ coverage- Coverage is the per-coat spreading rate (m²/L or ft²/gal).
- More coats or a lower coverage rate both raise the volume in direct proportion.
Volume to buy (after waste)
volume to buy = volume × (1 + waste %)- Waste covers roller take-up, trays and touch-ups — typically 5–10%.
- The calculator then rounds this up to whole tins.
Cost and tins
cost = Σ (tins bought × price); cost per area = cost ÷ net area- With prices, the calculator buys the cheapest set of tins that meets the volume; without prices it buys the fewest tins.
- Leftover = total tin volume − volume to buy.
Typical paint coverage rates
Coverage depends on the paint and the surface, so always use the spreading rate on your tin where you have it. The table gives typical per-coat ranges to estimate with when you don't, in both metric and imperial. The calculator defaults to 12 m² / 400 ft² per litre/gallon, which suits smooth interior walls; change it in the assumptions panel for primer, trim or rough surfaces.
| Surface / paint | Coverage (m² per litre) | Coverage (ft² per US gallon) |
|---|---|---|
| Smooth interior walls — emulsion / latex | 11–14 m²/L | 350–450 ft²/gal |
| Bare or new plaster — mist (first) coat | 7–10 m²/L | 250–350 ft²/gal |
| Primer / undercoat | 10–12 m²/L | 300–350 ft²/gal |
| Textured or rough walls | 6–9 m²/L | 200–300 ft²/gal |
| Wood & metal trim — gloss / satin | 12–16 m²/L | 350–450 ft²/gal |
| Exterior masonry paint | 6–10 m²/L | 200–300 ft²/gal |
Primer and undercoat
Primer seals and evens out the surface so the topcoat covers properly and lasts. You generally need it on bare or new plaster, fresh filler and patches, bare wood and metal, and when making a big colour change or painting over a glossy or stained surface. Over sound, previously painted walls in a similar colour you can usually skip it.
Turn primer on in the assumptions panel and the calculator estimates it as a separate line with its own coverage, coats and tin sizes — primer is usually applied as a single coat and spreads a little less than topcoat. A common system is one coat of primer or undercoat followed by two topcoats.
Primer volume uses the same formula as topcoat: area × primer coats ÷ primer coverage, plus waste. Its cost is added to the project total and shown separately on the shopping list.
Walls, ceilings, doors and trim
Walls: enter each wall's width and height, then list the doors and windows on it. The calculator subtracts the openings from the gross area so you only pay for the wall you actually paint. A standard door is roughly 1.7 m² (about 20 ft²) and an average window about 1.2 m² (about 15 ft²), but you set the real sizes.
Ceilings: a ceiling has no openings, so you can enter its area directly — usually the room's floor area (length × width). Ceilings often take flat paint and may need an extra coat for even coverage.
Trim, skirting and doors: trim is measured as a length, not an area, so the calculator multiplies the run by a profile width to get the painted area. Skirting, architrave, window frames and door casings are all entered this way; door faces can be added as a simple area at roughly 1.8–2 m² per side.
Should you subtract doors and windows? For an accurate estimate, yes — on a small room the openings can be 10–15% of the wall area. The calculator never lets the openings exceed the wall, and flags it if you enter an opening larger than the wall it sits on.
Worked examples
Each example reproduces what the calculator returns, so you can check the method by hand. Coverage is per coat; figures use the calculator's defaults (12 m²/L or 400 ft²/gal, 8% waste) unless stated.
Each example reproduces the calculator's output for the given inputs.
Paint for a 4 m × 5 m bedroom (walls)
Metric · 2 coats- Given
- Room 4 m × 5 m, walls 2.4 m high
- One door 0.84 × 1.98 m, one window 1.0 × 1.2 m
- Coverage 12 m²/L, 2 coats, 8% waste, tins 2.5 L and 5 L
- Formula
volume = (wall area − openings) × coats ÷ coverage × (1 + waste)- Steps
- Wall area: perimeter 18 m × 2.4 m = 43.2 m²
- Openings: 1.66 + 1.20 = 2.86 m² → net walls 40.34 m²
- Volume: 40.34 × 2 ÷ 12 = 6.72 L, × 1.08 = 7.26 L
- Cheapest tins: one 5 L + one 2.5 L = 7.5 L
- Result
- Buy 7.5 L (5 L + 2.5 L), about 0.24 L leftover
Paint for a 12 × 12 ft room (walls)
Imperial · 2 coats- Given
- Room 12 ft × 12 ft, walls 8 ft high
- One door 3 × 7 ft, one window 4 × 3 ft
- Coverage 400 ft²/gal, 2 coats, 8% waste, 1-gallon cans
- Formula
gallons = (wall area − openings) × coats ÷ coverage × (1 + waste)- Steps
- Wall area: perimeter 48 ft × 8 ft = 384 ft²
- Openings: 21 + 12 = 33 ft² → net walls 351 ft²
- Gallons: 351 × 2 ÷ 400 = 1.76 gal, × 1.08 = 1.90 gal
- Round up to whole cans: 2 × 1-gallon
- Result
- Buy 2 US gallons, about 0.1 gal leftover
Primer for bare plaster walls
Metric · 1 coat- Given
- Net wall area 40 m²
- Primer coverage 11 m²/L, 1 coat, 8% waste
- Tin sizes 1, 2.5, 5 L
- Formula
primer = area × coats ÷ primer coverage × (1 + waste)- Steps
- Volume: 40 × 1 ÷ 11 = 3.64 L
- After waste: 3.64 × 1.08 = 3.93 L
- Round up to tins: one 5 L tin (or 2.5 L + 2.5 L)
- Result
- About 3.9 L of primer — one 5 L tin
Cost of painting 40 m² of walls
Cost · cheapest tins- Given
- Net wall area 40 m², 2 coats, coverage 12 m²/L, 8% waste
- Tins: 5 L at 30, 2.5 L at 18, 1 L at 9
- Formula
cost = Σ (tins × price); cost per m² = cost ÷ area- Steps
- Volume to buy: 40 × 2 ÷ 12 × 1.08 = 7.2 L
- Cheapest tins for ≥ 7.2 L: 5 L + 2.5 L = 7.5 L
- Cost: 30 + 18 = 48; per m²: 48 ÷ 40 = 1.20
- Result
- 48 for the paint, about 1.20 per m²
Paint cost and which tins to buy
Knowing the litres is only half the job — paint is sold in fixed tin sizes, so the real question is which tins to buy. Enter your tin sizes and their prices and the calculator picks the combination that covers the volume for the lowest cost, falling back to the fewest tins when you leave prices blank.
It then shows the leftover paint, the cost per item, the project total and the cost per square metre or foot, in your chosen currency. That makes it easy to compare a few big tins against several small ones, or trade paint against retail, before you buy.
A little leftover is normal and worth keeping for touch-ups. If you would rather not overbuy, add a smaller tin size to the set so the optimiser can get closer to the exact volume.
Common mistakes to avoid
Forgetting the number of coats. Most jobs need two coats; estimating for one is the most common reason people run out of paint halfway.
Trusting the coverage rate on smooth and rough surfaces alike. Bare plaster, textured walls and masonry drink far more paint than a sealed wall — lower the coverage for them.
Not subtracting doors and windows on small rooms, or conversely subtracting them on a ceiling that has none.
Ignoring waste. Roller take-up, trays, spills and second-coat touch-ups add up; a waste margin of around 5–10% keeps you from a second trip to the shop.
Skipping primer on bare or patched surfaces, then needing a third topcoat to hide it — usually more expensive than priming once.
Measuring trim as an area. Skirting and architrave are a length × a narrow profile width, not a big rectangle — entering them as area hugely overestimates the paint.
Frequently asked questions
Short answers to the questions people most often ask about estimating paint and using this calculator.
How much paint do I need?
Measure the area to paint, subtract doors and windows, multiply by the number of coats, then divide by the paint's coverage rate, and add a small waste margin. In short: paint = area × coats ÷ coverage × (1 + waste). The calculator does this as you type and rounds it up to whole tins.
How much paint do I need for a room?
A typical 12 × 12 ft room with 8 ft walls has about 350 ft² of wall after one door and window, so it needs roughly 2 US gallons for two coats. A similar 4 m × 5 m room with 2.4 m walls is about 40 m² and needs around 7 litres for two coats. Enter your exact sizes for a precise figure.
How much does one litre of paint cover?
One litre of interior emulsion typically covers about 11–14 m² in a single coat, and the calculator defaults to 12 m²/L. Bare plaster, textured walls and masonry cover less (around 6–10 m²/L), so lower the coverage rate for them and always check the tin.
How many square feet does a gallon of paint cover?
One US gallon usually covers about 350–450 ft² in one coat on smooth walls; the calculator defaults to 400 ft²/gal. Because most jobs need two coats, plan for roughly 175–200 ft² of finished wall per gallon. Primer and rough surfaces cover less.
How many coats of paint do I need?
Two coats is standard for an even, durable finish. You may get away with one when repainting a similar colour over a sound surface, and need three when going from dark to light. Set the number of coats per surface in the assumptions panel — it scales the paint directly.
Do I need primer?
Use primer on bare or new plaster, fresh filler and patches, bare wood or metal, and for big colour changes or painting over glossy or stained surfaces. Over sound, previously painted walls in a similar colour you can usually skip it and go straight to topcoats.
How much primer do I need?
Primer uses the same formula as topcoat — area × coats ÷ primer coverage, plus waste — but it is usually one coat and covers a little less (around 10–12 m²/L). Turn primer on in the assumptions panel and the calculator estimates it as a separate line and adds it to the cost.
How do I calculate paint for walls?
Add the area of each wall (width × height), subtract the doors and windows, multiply by the number of coats and divide by the coverage rate. The room mode does this automatically: enter each wall and its openings and it returns the net area, litres or gallons, and tins.
Should I subtract doors and windows?
Yes, for an accurate estimate. On a small room, doors and windows can be 10–15% of the wall area. Enter each opening's size and count and the calculator subtracts it; it also warns you if an opening is larger than the wall it sits on.
How do I work out paint for a ceiling?
A ceiling has no openings, so enter its area directly — usually the room's floor area (length × width). Add it as a ceiling surface in room mode, set the coats, and it is included in the project total. Ceilings often need an extra coat for even coverage.
How do I estimate paint for trim, skirting and doors?
Trim is measured as a length, not an area. Enter the total run of skirting, architrave or frame and the calculator multiplies it by a narrow profile width to get the painted area. Door faces can be added as a simple area at roughly 1.8–2 m² per side.
Do textured or rough walls need more paint?
Yes. Textured walls, bare plaster, render and masonry have more surface and soak up more paint, so coverage drops to roughly 6–10 m²/L. Lower the coverage rate for these surfaces and consider a higher waste margin — often 15–20% more paint than a smooth wall.
Does the calculator work for exterior paint?
Yes — enter the wall areas and use an exterior coverage rate, which is usually lower (about 6–10 m²/L, 200–300 ft²/gal) because masonry is porous and textured. The method is identical; only the coverage and number of coats change.
How much will the paint cost?
Enter your tin sizes and their prices and the calculator finds the cheapest combination that covers the volume, then shows the project total, the cost per item and the cost per square metre or foot in your currency. Leave prices blank to just get volumes and tins.
Which tin or can sizes should I buy?
List the pack sizes available to you (for example 1, 2.5, 5 and 10 L, or quart, 1-gallon and 5-gallon cans) and the calculator buys the cheapest set that covers the volume — or the fewest tins if you leave prices blank. Adding a smaller size lets it get closer to the exact amount with less leftover.
Can I export or print the result?
Yes. Export CSV downloads the breakdown for Excel, Google Sheets or Numbers, and Print / PDF produces a clean shopping list with the areas, volumes, tins and cost — handy to take to the paint shop.
Is the calculator private?
Yes. Every calculation runs in your browser — nothing you enter is uploaded to a server. Your project is saved only in your own browser's local storage, so you can close the tab and return to it later.