Before starting any DIY home improvement project, knowing exactly how much paint to buy is crucial to avoid mid-project trips to the hardware store or wasting money on excess supplies. This square footage paint calculator simplifies the process by accounting for wall dimensions and subtracting non-paintable areas like doors and windows.
The Basic Formula
To calculate the paint needed, follow these steps:
Step 1: Multiply the total width of all walls by the wall height to get the gross square footage.
Step 2: Subtract the area of doors (standard doors are approx. 20 sq. ft.) and windows (standard windows are approx. 15 sq. ft.).
Step 3: Multiply the net square footage by the number of coats desired.
Step 4: Divide the total square footage by the coverage rate of your paint (usually 350-400 sq. ft. per gallon).
Example Calculation
If you have a room with 40 feet of total wall length and 9-foot ceilings:
Gross Area: 40 * 9 = 360 sq. ft.
Subtract 1 Door (20 sq. ft.) and 2 Windows (30 sq. ft.): 360 – 50 = 310 sq. ft.
For 2 coats: 310 * 2 = 620 sq. ft. total coverage needed.
Always consider the surface texture. Highly textured walls (like stucco or popcorn finishes) can require up to 20% more paint than smooth surfaces. Additionally, if you are changing colors drastically—such as painting light gray over a dark navy—a primer coat or a third coat of paint may be necessary for full opacity.
function calculatePaint() {
var width = parseFloat(document.getElementById("wallWidth").value);
var height = parseFloat(document.getElementById("wallHeight").value);
var doors = parseFloat(document.getElementById("numDoors").value) || 0;
var windows = parseFloat(document.getElementById("numWindows").value) || 0;
var coats = parseInt(document.getElementById("numCoats").value);
var coverage = parseFloat(document.getElementById("paintCoverage").value);
if (isNaN(width) || isNaN(height) || width <= 0 || height <= 0) {
alert("Please enter valid positive numbers for width and height.");
return;
}
if (isNaN(coverage) || coverage <= 0) {
alert("Please enter a valid coverage rate.");
return;
}
// Standard deductions
var doorArea = doors * 20;
var windowArea = windows * 15;
var totalGrossArea = width * height;
var netAreaPerCoat = totalGrossArea – (doorArea + windowArea);
if (netAreaPerCoat < 0) netAreaPerCoat = 0;
var totalPaintableArea = netAreaPerCoat * coats;
var gallonsNeeded = totalPaintableArea / coverage;
var quartsNeeded = gallonsNeeded * 4;
document.getElementById("resTotalArea").innerText = totalGrossArea.toLocaleString();
document.getElementById("resNetArea").innerText = totalPaintableArea.toLocaleString();
document.getElementById("resGallons").innerText = gallonsNeeded.toFixed(2);
document.getElementById("resQuarts").innerText = Math.ceil(quartsNeeded);
document.getElementById("paint-results").style.display = "block";
}