Paint Calculator
Area and coats to gallons and buckets.
Also known as paint quantity · litres of paint · paint coverage
With the default inputs: Painting area 800 ft² · Number of coats 2 coats · Coverage per gallon 350 ft²/gal etc.Paint required 4.6 gal
Use your own dimensions ↓Inputs
Result
| Coats incl. primer | 2coats |
| Whole gallons | 5gal |
| 5 gal pails | 1cans |
| 1 gal cans | 0cans |
| Litres | 17.3L |
| Coverage | 350ft²/gal |
How this is worked out
Gallons = area x coats ÷ coverage. Coverage is the variable that bites: interior flat latex is labelled 350-400 ft²/gal, but textured walls, dark-over-light and spraying all come in below label, so 350 is the safer figure and exterior rough siding takes 250-300. Primer pays for itself: new drywall and patched areas are very absorbent, and without a primer coat the topcoat needs three passes, so total paint goes up and the colour goes patchy. The answer is rounded up to whole gallons because paint is not mixed in half gallons - leftover keeps for a year or two sealed and cool, and is worth keeping for touch-ups.
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Formula
| Coats = topcoats + primer if selected | |
| Gallons = area × coats ÷ coverage | |
| Whole gallons = round up | no half-gallon mixing |
| Buckets = 5 gal pails + 1 gal cans |
How to work it out
- Work out the net paintable area: walls less openings, ceilings in full, summed across rooms.
- Set the coat count: two for a same-colour repaint, plus a primer on new work, dark-to-light or strong colours.
- Pick coverage: 400 ft²/gal on smooth, 300 on textured, 250 on rough exterior.
- Gallons = area x coats ÷ coverage, rounded up to whole gallons.
- Consider a 5 gal pail for one colour over a large area, and 1 gal cans where rooms differ.
Where it goes wrong
- Buying to the label figure of 400 ft²/gal - real application almost never reaches it and you run half a gallon short in the last room.
- Two topcoats on new drywall with no primer - uneven absorption, patchy colour, and a third coat anyway, so total paint goes up.
- Adding all the rooms into one figure - colours cannot be pooled, so each has to be rounded up separately.
Material and cost
Interior latex runs 25-50 a gallon mid-range and 60-90 premium, with 5 gal pails at 100-200. Primer is 20-35 a gallon. Painting labour including protection, filling and two coats commonly runs USD 2-4 per ft² of wall, and a whole house interior typically lands at 3000-8000.
On site
- Buy a quart extra per colour for touch-ups - the same colour code from a later batch can still differ.
- Store leftovers sealed and upside down for a year or two, labelled with room and date.
- Wipe walls with a damp cloth before painting; dust stops the film bonding and edges peel.
Basis
- MPI Master Painters Institute Approved Products List (spreading rates)
- ASTM D4958 Standard Test Method for Comparison of the Brush Drag of Latex Paints
- BS 6150:2019 (coverage and coats — imperial conversion reference)
FAQ
How many square feet does a gallon cover?
Is primer really necessary?
Is a 5 gallon pail worth it?
Cite this page
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Building Calculator. Paint Calculator. 2026-10-25. https://jianzhujisuan.com/en/paint-gallons