Properly estimating the amount of paint you'll need is crucial for any painting project, whether you're refreshing a single room or tackling an entire house. Overestimating can lead to wasted money and unused product, while underestimating means an inconvenient trip back to the store mid-project, potentially with an exact color match issue. This paint calculator helps you make an informed estimate based on the dimensions of your space and the properties of the paint you plan to use.
How the Calculation Works
The core of the calculation involves determining the total surface area that needs to be painted and then dividing that by the paint's coverage rate. Here's a breakdown of the steps involved:
Calculate Wall Area: For a rectangular room, we calculate the perimeter (sum of all wall lengths) and multiply it by the room's height.
Perimeter = 2 * (Room Length + Room Width) Total Wall Area = Perimeter * Room Height
Calculate Ceiling Area: The ceiling area is simply the length of the room multiplied by its width.
Ceiling Area = Room Length * Room Width
Subtract Areas for Openings: Doors and windows don't require paint. We subtract a standard area for each. A typical door is assumed to be 20 sq ft, and a standard window is assumed to be 15 sq ft. These are approximations and can be adjusted if you have unusually sized openings.
Total Door Area to Subtract = Number of Doors * 20 sq ft Total Window Area to Subtract = Number of Windows * 15 sq ft Total Area to Subtract = Total Door Area to Subtract + Total Window Area to Subtract
Calculate Net Paintable Area: This is the total surface area (walls + ceiling) minus the area of doors and windows.
Net Paintable Area = (Total Wall Area + Ceiling Area) – Total Area to Subtract
Account for Coats: Most painting jobs require at least two coats for even coverage and durability. The calculator multiplies the net paintable area by the number of coats needed.
Total Square Footage to Paint = Net Paintable Area * Number of Coats Needed
Determine Gallons Needed: Finally, divide the total square footage to paint by the paint's coverage rate (square feet per gallon). We then round up to the nearest whole gallon, as you can't buy fractions of a gallon and it's always better to have a little extra.
Gallons Needed = Total Square Footage to Paint / Paint Coverage (sq ft per gallon) Rounded Gallons Needed = Ceiling(Gallons Needed)
When to Use This Calculator
Planning interior wall and ceiling painting projects.
Estimating paint needs for a single room or multiple rooms.
Comparing different paint coverage rates.
Budgeting for home renovation or DIY painting jobs.
Remember that these are estimates. Factors like paint texture, surface porosity (e.g., new drywall vs. previously painted surfaces), and application method (roller, brush, sprayer) can affect actual paint consumption. It's generally recommended to buy slightly more than calculated to ensure you have enough for touch-ups later.
function calculatePaint() {
var roomLength = parseFloat(document.getElementById("roomLength").value);
var roomWidth = parseFloat(document.getElementById("roomWidth").value);
var roomHeight = parseFloat(document.getElementById("roomHeight").value);
var doorCount = parseInt(document.getElementById("doorCount").value);
var windowCount = parseInt(document.getElementById("windowCount").value);
var paintCoverage = parseFloat(document.getElementById("paintCoverage").value);
var coatsNeeded = parseInt(document.getElementById("coatsNeeded").value);
var gallonsResultSpan = document.getElementById("gallonsResult");
var sqftResultSpan = document.getElementById("sqftResult");
// Clear previous results
gallonsResultSpan.textContent = "-";
sqftResultSpan.textContent = "-";
// Validate inputs
if (isNaN(roomLength) || roomLength <= 0 ||
isNaN(roomWidth) || roomWidth <= 0 ||
isNaN(roomHeight) || roomHeight <= 0 ||
isNaN(doorCount) || doorCount < 0 ||
isNaN(windowCount) || windowCount < 0 ||
isNaN(paintCoverage) || paintCoverage <= 0 ||
isNaN(coatsNeeded) || coatsNeeded totalAreaToPaint) {
areaToSubtract = totalAreaToPaint;
}
var netPaintableArea = totalAreaToPaint – areaToSubtract;
var totalSquareFootageToPaint = netPaintableArea * coatsNeeded;
// Ensure total square footage is not negative
if (totalSquareFootageToPaint < 0) {
totalSquareFootageToPaint = 0;
}
var gallonsNeeded = totalSquareFootageToPaint / paintCoverage;
var roundedGallonsNeeded = Math.ceil(gallonsNeeded);
// Display results
sqftResultSpan.textContent = totalSquareFootageToPaint.toFixed(2);
gallonsResultSpan.textContent = roundedGallonsNeeded;
}