How to Calculate How Much Paint You Need
I built this paint calculator to help you buy the right amount — not too much, not too little. Running out of paint mid-wall and having to rush back to the store is frustrating, but buying excess from a custom-tinted batch is wasteful because most stores won't accept returns on custom colours. The goal is to hit the right number of gallons on the first purchase.
The calculation starts with paintable wall area: add up the area of all walls (perimeter × ceiling height), subtract doors (each roughly 20 sq ft) and windows (roughly 15 sq ft each), then multiply by the number of coats. Most paints cover 350–400 square feet per gallon. Divide your total square footage by the coverage rate to get gallons needed. Always round up to the nearest quart or gallon.
How Many Coats Do You Actually Need?
The number of coats depends on the colour change you're making and the surface condition. Here are the real-world guidelines:
- White or light colour over light colour: 2 coats is standard for a professional finish.
- Dark colour over white or light colour: 2 coats minimum; deep saturated colours (red, navy, orange) often need 3 coats.
- Light colour over dark colour: almost always requires primer plus 2 finish coats, or 3 finish coats without primer.
- New drywall or fresh plaster: must be primed first; skip primer and the topcoat will look flat and absorb unevenly.
Choosing the Right Paint Finish
Paint finish (sheen level) affects both durability and appearance, and using the wrong finish in the wrong room is one of the most common DIY painting mistakes. Flat/matte finish hides surface imperfections well and is ideal for ceilings and low-traffic bedroom walls, but it scuffs easily and is hard to clean. Eggshell and satin finishes offer a slight sheen, are easy to wipe down, and are the right choice for living rooms, hallways, and children's rooms.
Semi-gloss and gloss finishes are durable, moisture-resistant, and easy to clean — they're the right choice for kitchens, bathrooms, trim, and doors. The downside is that higher sheen shows surface imperfections more prominently, so prep work (filling nail holes, sanding) matters more with gloss paints. I use eggshell on walls throughout the house and semi-gloss on all trim, doors, and window casings.
Frequently Asked Questions
Do I need to prime before painting?
Primer is required in three situations: painting over new drywall or plaster (which is highly porous), making a dramatic colour change from dark to light, and painting over a stained or glossy surface. For most repaints — same colour family, good condition walls — a quality paint-and-primer-in-one product eliminates the separate primer step and saves time and money. If you're painting over a stain (water damage, smoke, marker), use a stain-blocking primer specifically designed for that purpose.
How long should I wait between coats?
Most latex paints are dry enough to recoat in 2–4 hours at room temperature (68–77°F) with normal humidity. However, "dry to touch" is not the same as "ready for a second coat" — applying a second coat too soon can cause the first coat to peel or leave roller marks. Wait the full recoat time on the label. In humid conditions or below 60°F, double the stated drying time. Oil-based paints require 24 hours between coats.
What is the best order to paint a room?
The professional sequence is: ceiling first, then walls, then trim last. Painting the ceiling first lets you be less careful about getting ceiling paint on the walls — you'll paint over it. Cutting in (brushing the edges) before rolling is the standard approach on each surface. Paint the trim last because it's easiest to tape off trim from freshly painted walls than to cut a clean line against trim while painting walls.
