Calculating measurements in the imperial system can be tricky because it is not base-10. There are 12 inches in a foot, which means you cannot simply add the numbers as you would with decimals.
Steps to calculate manually:
Convert both measurements entirely into inches (Feet × 12 + Inches).
Perform the addition or subtraction on the total inch values.
Divide the resulting total by 12 to find the number of feet.
The remainder is the remaining inches.
Practical Example
Suppose you are a carpenter and need to add two boards together. Board A is 5 feet 8 inches and Board B is 3 feet 10 inches.
Board A: (5 × 12) + 8 = 68 inches
Board B: (3 × 12) + 10 = 46 inches
Total: 68 + 46 = 114 inches
Conversion: 114 ÷ 12 = 9 with a remainder of 6.
Result: 9 feet 6 inches.
Common Conversions Table
Fractional Inch
Decimal Inch
Notes
1/4″
0.25″
Common wood thickness
1/2″
0.5″
Standard drywall thickness
3/4″
0.75″
Standard nominal lumber
12″
1.0′
Exactly 1 foot
Why Use a Feet and Inches Calculator?
This tool is essential for construction, woodworking, and interior design projects where measurements are provided in the US Customary System. It eliminates rounding errors and prevents the "off-by-one" mistakes that occur when forgetting to carry over the 12-inch mark to the feet column.
function calculateFeetInches() {
var f1 = parseFloat(document.getElementById('feet1').value) || 0;
var i1 = parseFloat(document.getElementById('inches1').value) || 0;
var f2 = parseFloat(document.getElementById('feet2').value) || 0;
var i2 = parseFloat(document.getElementById('inches2').value) || 0;
var op = document.getElementById('operation').value;
var totalInches1 = (f1 * 12) + i1;
var totalInches2 = (f2 * 12) + i2;
var resultInches = 0;
if (op === 'add') {
resultInches = totalInches1 + totalInches2;
} else {
resultInches = totalInches1 – totalInches2;
}
var isNegative = resultInches < 0;
var absInches = Math.abs(resultInches);
var finalFeet = Math.floor(absInches / 12);
var remainingInches = (absInches % 12).toFixed(2);
// Clean up .00 from inches
if (remainingInches.endsWith('.00')) {
remainingInches = Math.floor(remainingInches);
}
var resultString = (isNegative ? "-" : "") + finalFeet + " ft " + remainingInches + " in";
var decimalString = "Total in Decimal: " + (resultInches / 12).toFixed(3) + " ft (" + resultInches.toFixed(2) + " inches)";
document.getElementById('finalResult').innerHTML = resultString;
document.getElementById('decimalResult').innerHTML = decimalString;
document.getElementById('resultArea').style.display = 'block';
}