Paint Calculator
Paint, primer and putty for any room or wall, with the tins to buy and the cost.
What are you painting?
Doors and windows to leave out
Paint
Primer
Wall putty
Extra and labour
Shopping list
| Item | Needed | Buy | Cost |
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About the Paint Calculator
Work out how much paint, primer and wall putty to buy for a room, a single wall, a ceiling or a whole exterior. Add each room or wall, list the doors and windows to leave out, type the coverage printed on the tin and the number of coats, and get the litres (or US gallons) and kilos you need — plus the cheapest or least-wasteful mix of pack sizes and the cost if you add prices.
Coverage is never assumed: it changes from one product, sheen and surface to the next, so the calculator uses the spreading rate from your label or technical data sheet. Labels give it either per coat or for two coats; choose which and the arithmetic adjusts.
How to use it
- Pick Metric (m, litres) or US (ft, gallons). Add your surfaces: a room (length, width and height — tick Paint the ceiling too if you want it), a single wall, a ceiling or a typed area for an exterior. Feet and inches such as
12' 6"work. - Add the doors and windows you will not paint. They are taken off the wall area.
- For paint, type the coverage from the tin, say whether it is per coat or for 2 coats, and set the coats. Tick primer and wall putty if you need them and fill in their label figures too.
- Check the pack sizes you can buy and, if you like, their prices. With prices the tool finds the cheapest mix; without them, the one that leaves the least over.
- Read the litres or kilos and the packs to buy, then copy the list or download it as CSV.
Examples
Door 0.9 × 2.1 m, two windows 1.2 × 1.5 m · coverage 10 m²/L per coat · 5% extra
42 m² of walls − 5.49 m² of openings + 12 m² of ceiling = 48.51 m² → 48.51 × 2 ÷ 10 × 1.05 = 10.19 L → buy 1 × 10 L + 1 × 1 L
48.51 ÷ 10 × 1.05 = 5.09 L → least left over: 1 × 4 L + 2 × 1 L; fewer packs: 2 × 4 L
36.51 m² = 393 sq ft → 393 ÷ 10 × 1.05 = 41.26 kg → 1 × 40 kg + 2 × 1 kg (or 1 × 40 kg + 1 × 5 kg)
416 − 21 − 12 = 383 sq ft → 383 × 2 ÷ 400 × 1.05 = 2.01 gal → 2 gallons + 1 quart
How it is calculated
Wall area of a room = 2 × (length + width) × height; a single wall is length × height and a ceiling length × width. Doors and windows (width × height × number) are taken off the walls, never off the ceiling.
Quantity = area × coats ÷ spreading rate per coat × (1 + extra %). A label figure for 2 coats is turned into a per-coat rate by doubling it — 5 m² per litre for two coats is 10 m² per litre per coat.
The pack plan tries every combination of the pack sizes you entered and keeps the best: the cheapest when prices are given (packs without a price are left out of that plan), otherwise the one with the least left over. When a plan with fewer packs leaves less than your second-smallest pack spare, it is shown as well.
Reading the coverage on the label
- Use the figure for the surface you have. Labels often give a range; the lower number suits rough, porous or textured walls, the higher one smooth, sealed ones.
- New plaster, bare masonry and a big colour change take more paint than repainting a sound wall in a similar colour — raise the coats or use the lower coverage.
- Primer and putty have their own figures; putty bags state coverage per kg, sometimes for two coats.
- US cans quote square feet per gallon; Indian and European tins usually quote m² or square feet per litre. Choose the unit that matches your label.
Units
A US gallon is exactly 3.785411784 litres and a quart a quarter of that (NIST SP 811). A square foot is exactly 0.09290304 m². Imperial (UK) gallons are not used — UK and European paint is sold in litres.
Limitations
- Coverage is only as good as the figure you type: texture, porosity, the roller or sprayer and the painter all change it.
- Doors, windows, trim and skirting boards are not added as painted surfaces; for gloss on woodwork, add their area as a separate typed area with that product’s coverage.
- Sloping ceilings, gable ends and stairwells are not rectangles; measure them as typed areas.
- Pack sizes and prices are whatever you enter — check what your shop actually stocks.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
How much paint do I need for a 10 × 12 ft room?
With 8 ft walls, the walls are 2 × (10 + 12) × 8 = 352 sq ft before you take off doors and windows; the ceiling adds 120 sq ft. Multiply by the coats and divide by the coverage on your tin — at 400 sq ft per gallon per coat, two coats on the walls alone are 1.76 gallons, so 2 gallons.
Should I deduct doors and windows?
Yes, list them and the calculator takes them off the walls. Only leave a window in if you are painting over it (for example a boarded-up opening).
What does "coverage for 2 coats" mean?
Some labels say how much area a litre covers when you apply two coats. Per coat it covers twice that. Choose for 2 coats next to the coverage and the calculator does the conversion.
How much primer and wall putty will I need?
Work them out the same way, with their own label coverage and coats — usually one coat of primer and two of putty. Putty normally goes on walls only; choose Walls only if you will not putty the ceiling.
Why does the plan sometimes buy more than I need?
Paint only comes in whole packs, so the total is rounded up to a combination of the sizes you can buy. The plan shows how much will be left over; keep it for touch-ups.
Does it work for exterior walls?
Yes. Add the elevations as single walls (length × height) or type the total area, take off the openings, and use the exterior paint’s own coverage, which is usually lower on rough render.