Feet and Inches to Millimeters Calculator

This feet-and-inches-to-millimeters calculator converts a length entered in feet and inches into millimeters (mm). Enter the feet and the remaining inches separately; the result is one metric length, calculated without rounding the input first.

Formula

First combine the two customary units into total inches, then convert inches to millimeters:

Millimeters = (feet × 12 + inches) × 25.4

• Feet (ft) is the number of whole feet entered.
• Remaining inches (in) is the additional length, from 0 up to but not including 12 inches.
• 12 is the number of inches in one foot.
• 25.4 mm is exactly one inch.

The calculator also reports centimeters and meters: centimeters equal millimeters divided by 10, and meters equal millimeters divided by 1,000. All outputs describe the same length.

Worked Example

Using the defaults—5 feet and 8 inches—the conversion is:

1. Convert the feet to inches: 5 × 12 = 60 inches.
2. Add the remaining inches: 60 + 8 = 68 inches.
3. Convert total inches to millimeters: 68 × 25.4 = 1,727.20 mm.
4. Convert the result to other metric units: 1,727.20 ÷ 10 = 172.72 cm, and 1,727.20 ÷ 1,000 = 1.727 m.

The displayed millimeter result is rounded to two decimal places. In this example, the conversion happens to produce an exact value at that precision.

How the Conversion Works

A foot and an inch are both units of length in the customary measurement system. Because one foot contains 12 inches, a mixed measurement such as 5 feet 8 inches must first be rewritten as a single number of inches. Multiplying that total by 25.4 gives millimeters.

The relationship between inches and millimeters is exact: 1 inch = 25.4 mm. This is a defined conversion, not an approximation based on measuring an object. Any rounding in the calculator’s answer is only for display; it does not change the underlying conversion.

For a measurement written as feet and inches, the inches field is the remainder after the whole feet. For instance, 6 feet 3 inches is entered as 6 and 3, not as 6 and 39. If a measurement is already given as total inches, enter the whole-foot portion and remainder instead: 75 inches is 6 feet and 3 inches.

When to Use This Calculator

This conversion is useful when a length from a tape measure, drawing, product specification, or other customary-unit source needs to be expressed in metric units. Millimeters are common for small dimensions and detailed specifications; centimeters or meters may be easier to interpret for larger lengths. The calculator displays all three metric forms so you can choose the most readable one.

• Use millimeters for a compact, precise-looking dimension, such as 1,727.20 mm.
• Use centimeters when a value in millimeters would have many digits, such as 172.72 cm.
• Use meters for room-scale or other relatively long measurements, such as 1.727 m.

Choose the output unit that matches the context and the precision of the original measurement. A calculator can convert a value exactly, but it cannot make an imprecise measurement more precise.

Common Entry and Rounding Mistakes

The most frequent error is treating the inches field as a decimal fraction of a foot. Feet and inches are not decimal notation: 5 feet 8 inches means 5 feet plus 8 inches, not 5.8 feet. Enter the two parts in their separate fields, or convert to total inches before using a calculator that accepts only inches.

Another common mistake is entering 12 or more in the remaining-inches field. Twelve inches make one additional foot, so 5 feet 12 inches should instead be entered as 6 feet 0 inches. The calculator limits the inches field to values below 12 to keep the mixed-unit entry in its standard form.

Do not round the total inches or an intermediate conversion if you need the most accurate result. For example, convert 5 feet 8 inches as 68 × 25.4, not by rounding 25.4 to 25. The final display uses two decimal places for millimeters and centimeters and three for meters; those display choices are not a claim that the original measurement was known to that precision.

Feet and Inches Compared With Metric Units

Millimeters, centimeters, and meters are related by powers of ten: 10 mm = 1 cm and 100 cm = 1 m. Feet and inches use a different relationship, with 12 inches to a foot. The conversion factor of 25.4 mm per inch links the systems directly and makes it possible to convert a combined feet-and-inches measurement in one calculation.

When comparing measurements, keep the unit attached to each number. A bare value such as “1727” could mean millimeters, centimeters, or something else; “1,727 mm” is unambiguous. If you need to compare two lengths, convert both to the same unit before subtracting or deciding which is greater.

Frequently Asked Questions

How many millimeters are in one foot?
One foot is 12 inches, and each inch is 25.4 mm. Therefore, one foot equals 12 × 25.4 = 304.8 mm.

How many millimeters are in one inch?
One inch is exactly 25.4 millimeters.

How do I convert 5 feet 8 inches to millimeters?
Convert the feet to inches and add the remainder: 5 × 12 + 8 = 68 inches. Then multiply by 25.4 to get 1,727.2 mm.

Is 5.8 feet the same as 5 feet 8 inches?
No. A decimal 5.8 feet means 5 feet plus 0.8 of a foot; 0.8 × 12 = 9.6 inches. Five feet 8 inches is 5 + 8/12 = about 5.667 feet.

Should I round the converted measurement?
Keep the unrounded value for calculations when possible, then round the displayed result to suit the precision needed. Rounding cannot improve the precision of the measurement you started with.

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in ft | Feet | ft | 5 | 0
in inch | Remaining inches | in | 8 | 0 | 11.999999
out mm | Length | mm | (ft * 12 + inch) * 25.4 | 2
out cm | Length | cm | mm / 10 | 2
out m | Length | m | mm / 1000 | 3