Quick answer
Add or subtract lengths by converting to inches (12 × feet + inches), operating, then converting back. 5 ft 8 in + 3 ft 7 in = 68 + 43 = 111 in = 9 ft 3 in, or 281.94 cm.
Enter two lengths in feet and inches and choose an operation. The calculator returns the result in feet and inches, total inches and centimetres.
Related tools: Length Converter: Convert between metric and imperial lengths. Scale Conversion Calculator: Convert between model and actual lengths at a 1:N scale. SAE to Metric Converter: Fractional or decimal inches to millimeters.
How to use the Feet and Inches Calculator
- Enter first length: feet (0 or more).
- Enter first length: inches (0 or more).
- Enter second length: feet (0 or more).
- Enter second length: inches (0 or more).
- Choose operation: Add and Subtract.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| First length: feet | A number | ≥ 0 |
| First length: inches | A number | ≥ 0 |
| Second length: feet | A number | ≥ 0 |
| Second length: inches | A number | ≥ 0 |
| Operation | Choose one: Add; Subtract | Starts at Add |
Formula and method
Formula
total inches = 12 × feet + inches
IEEE-754 double precision; shown to the decimals indicated.
1 ft = 12 in and 1 in = 2.54 cm exactly.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: First length: feet = 5; First length: inches = 8; Second length: feet = 3; Second length: inches = 7; Operation = Add.
| Result | Value |
|---|---|
| Result | 9 ft 3 in |
| Total inches | 111 |
| In centimeters | 281.94 |
Example 2
Inputs: First length: feet = 10; First length: inches = 2; Second length: feet = 4; Second length: inches = 9; Operation = Subtract.
| Result | Value |
|---|---|
| Result | 5 ft 5 in |
| Total inches | 65 |
| In centimeters | 165.1 |
Example 3
Inputs: First length: feet = 2; First length: inches = 11; Second length: feet = 2; Second length: inches = 11; Operation = Add.
| Result | Value |
|---|---|
| Result | 5 ft 10 in |
| Total inches | 70 |
| In centimeters | 177.8 |
The first line is the headline result (result). Below it the feet and Inches Calculator lists total inches and in centimeters, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Working by hand
Add feet and inches separately: 5 + 3 = 8 ft and 8 + 7 = 15 in. Since 15 in is 1 ft 3 in, carry it: 9 ft 3 in. When subtracting, borrow 12 inches from the feet if needed.
Negative results
A negative result is shown with a minus sign, for example when you subtract a longer length from a shorter one. Multiplication and division operate on the total inches.
How the result changes with first length: feet
Holding every other input at the first worked example, this table changes only first length: feet. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| First length: feet | Result | Total inches | In centimeters |
|---|---|---|---|
| 2.5 | 6 ft 9 in | 81 | 205.74 |
| 5 | 9 ft 3 in | 111 | 281.94 |
| 10 | 14 ft 3 in | 171 | 434.34 |
| 20 | 24 ft 3 in | 291 | 739.14 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- First length: feet: entering "-1" shows "First length: feet must be at least 0." beside that field and no result.
- First length: inches: left empty shows "Enter first length: inches." beside that field and no result.
Choosing between the Feet and Inches Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Length Converter](/converters/length-converter/) | value, from and to | Mile (mi) → Kilometre (km) |
| [Scale Conversion Calculator](/converters/scale-conversion/) | scale denominator N, convert and length | Converted length |
| [SAE to Metric Converter](/converters/sae-to-metric-calculator/) | size in inches | Millimeters |
| [Linear Feet to Square Feet Calculator](/converters/lf-to-sf/) | linear feet and width | Area |
| [Square Feet to Cubic Yards Calculator](/converters/ft2-yd3/) | area and depth | Volume |
Background: unit conversion
Many customary units are defined exactly in terms of metric ones: since the 1959 International Yard and Pound Agreement an inch is exactly 2.54 cm and a pound is exactly 0.45359237 kg (NIST SP 811 lists these). So most length and mass conversions are exact apart from the rounding of the displayed result.
To convert, multiply the value by the source unit's factor to reach the base unit, then divide by the target unit's factor; the unit table on this page lists every factor used. Temperature and fuel economy are the exceptions, because they are not simple multiples.
Assumptions and limitations
- A negative result is shown with a minus sign
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for first length: feet and first length: inches to confirm this.
Frequently asked questions
How many inches in 5 ft 8 in?
5 ft 8 in is 68 inches.
How do I convert to centimetres?
Multiply inches by 2.54.
Can I use fractions of an inch?
Enter decimals such as 0.5 for half an inch.
What does the Feet and Inches Calculator calculate?
Add or subtract two lengths given in feet and inches. It returns Result, Total inches and In centimeters.
What formula does the Feet and Inches Calculator use?
total inches = 12 × feet + inches. 1 ft = 12 in and 1 in = 2.54 cm exactly.
What do I need to enter in the feet and Inches Calculator?
You enter First length: feet, First length: inches, Second length: feet, Second length: inches and Operation.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. A negative result is shown with a minus sign.
What does the “Operation” option change?
It sets how the feet and Inches is worked out. The choices are Add and Subtract.
What is an example of a feet and Inches result?
With first length: feet 5, first length: inches 8, second length: feet 3, second length: inches 7 and operation Add, the result is 9 ft 3 in.
Where does the method come from?
The method follows: International Yard and Pound Agreement (1959).
Related calculators
- Length Converter — Convert between metric and imperial lengths.
- Scale Conversion Calculator — Convert between model and actual lengths at a 1:N scale.
- SAE to Metric Converter — Fractional or decimal inches to millimeters.
- Linear Feet to Square Feet Calculator — Square feet covered by a length of material of a given width.
- Square Feet to Cubic Yards Calculator — Cubic yards of material covering an area at a depth.
- Volume Converter — Convert litres, gallons, cups, spoons and more.
Sources
- International Yard and Pound Agreement (1959)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.