Running out of paint two-thirds of the way up a wall is one of the most common and most avoidable DIY frustrations. So is the opposite problem: stacking four half-used cans in the garage because you guessed high. The fix is a five-minute measurement and a little arithmetic the calculator above does for you. This guide explains exactly where those numbers come from so you can trust the result and adapt it to your own walls.
The formula behind a paint estimate
Every paint estimate reduces to one idea: area to cover, divided by how far a unit of paint goes. The area is the wall surface, and the coverage is printed on the can. The calculator works through these steps:
Paintable area = wall area − doors − windows (+ ceiling if painted)
Total to cover = paintable area × number of coats
Paint to buy = (total ÷ coverage) × 1.10, rounded up
The perimeter of the room is 2 × (length + width). Multiply that by the ceiling height and you have the gross wall area. From there you subtract the surfaces you will not paint — doors and windows — then multiply by the number of coats, divide by the coverage rate, and add a waste allowance.
How to measure your room correctly
You need three measurements: the two floor dimensions (length and width) and the ceiling height. A laser measure is fastest, but a tape works fine. A few tips that keep the estimate honest:
- Measure to the nearest half-foot (or 10 cm). Paint is sold in whole cans, so extreme precision is wasted.
- Use the tallest wall height if your ceiling is sloped, then trust the waste allowance to absorb the difference.
- Count every door and window that breaks up the wall. The calculator subtracts about 21 sq ft per standard door and 15 sq ft per window.
- Decide on the ceiling separately. Ceilings often need a different paint (flat white) and a different can, so tick the box only if you are using the same product.
What coverage rate should you use?
Coverage is the variable that changes the answer most, and it is not the same for every wall. The 350 sq ft per gallon default assumes a smooth, already-primed surface. Real surfaces vary:
| Surface | Coverage (sq ft/gal) | Coverage (m²/L) |
|---|---|---|
| Smooth, primed drywall | 350–400 | 9–10 |
| Previously painted, same colour | 400 | 10 |
| Lightly textured wall | 300–350 | 7.5–9 |
| Bare drywall or plaster | 250–300 | 6–7.5 |
| Rough masonry / stucco | 200–250 | 5–6 |
If you are painting a porous or unpainted surface, either lower the coverage figure in the calculator or — better — prime first. Primer seals the surface so your finish coats cover at the higher rate, which usually saves a can of the more expensive colour paint.
A worked example
Take a 15 × 12 ft bedroom with 9 ft ceilings, two windows and one door, painted in two coats and leaving the ceiling alone:
- Perimeter = 2 × (15 + 12) = 54 ft
- Gross wall area = 54 × 9 = 486 sq ft
- Subtract openings = 486 − 21 (door) − 30 (two windows) = 435 sq ft
- Two coats = 435 × 2 = 870 sq ft to cover
- At 350 sq ft/gal = 2.49 gallons → ×1.10 waste = 2.74 → buy 3 gallons
Notice the rounding matters: 2.49 gallons rounds to 3 once you add the waste factor, and that third gallon is what saves you a second trip to the store mid-project. Buying the extra also gives you a sealed touch-up can in the exact batch colour.
Common mistakes that throw the estimate off
- Forgetting the second coat. The single biggest cause of under-buying. Always plan two unless you are refreshing the identical colour.
- Ignoring the primer. Bare or patched drywall needs primer; skipping it means more finish coats and more paint.
- Trusting one-coat marketing claims. “One-coat” paints still need careful application and rarely cover dramatic colour changes in a single pass.
- Mixing batches late. Two cans bought weeks apart can differ slightly in shade. Buy enough up front and “box” (combine) your cans for a uniform colour.
Estimating cost from the gallons
Once you know the gallons, cost is straightforward. Mid-grade interior wall paint runs roughly $30–$60 per gallon, premium lines $60–$90. The three-gallon bedroom above lands around $120–$180 in paint, before rollers, trays, tape and drop cloths — budget another $40–$60 for those if you are starting from scratch. If you are comparing a contractor's quote, remember labour is usually 60–70% of a professional painting bill, so the paint itself is the smaller line item.
When your numbers are ready, the same measuring approach feeds straight into related jobs — priming, wallpapering or tiling the same room. The related tools below pick up where this one leaves off.
Frequently asked questions
How much paint do I need for a 12x12 room?
A 12×12 ft room with 8 ft ceilings has about 384 sq ft of wall. After subtracting one door and one window you are near 350 sq ft, which is roughly 1 gallon per coat. For the typical two coats, buy 2 gallons. Add a third gallon if you are also painting the ceiling.
How much does a gallon of paint cover?
One US gallon covers about 350–400 sq ft in a single coat on a smooth, primed surface. Rough, porous or previously unpainted surfaces drink more paint, dropping coverage closer to 250–300 sq ft per gallon.
Do I need to subtract doors and windows?
For a quick estimate you can ignore them, but for accuracy subtract about 21 sq ft per standard door and 15 sq ft per average window. Our calculator does this automatically when you enter the counts.
How many coats of paint should I plan for?
Plan for two coats in almost every case. One coat rarely hides the previous colour evenly. Going from a dark colour to a light one, or painting bare drywall, may need a primer coat plus two finish coats.
Should I buy extra paint?
Yes — keep a 10% waste allowance (our default) for touch-ups, uneven rolling and future repairs. Paint from the same batch is the only reliable colour match later on.