Every length conversion is a single multiplication by a fixed ratio. This calculator stores the standard factors and applies them for you.

1. Enter the value: Type the number you want to convert. If your measurement is a fraction, turn it into a decimal first (see the worked example below).
2. Choose the source unit: Pick the unit you are starting from — inches, feet, millimeters, or centimeters.
3. Choose the target unit: Pick the unit you want the answer in. The dropdown only shows units valid for your source.
4. Read the result: The tool multiplies your value by the stored factor.
   Converted = Value × (Target units per Source unit)

The anchor factor is 1 inch = 25.4 mm exactly. Everything else is derived from it and from 12 inches per foot, so the math stays consistent in both directions.

Example 1: Metric plan to an imperial drill.
A shelf-pin layout calls for 32 mm centers. Convert 32 mm to inches: 32 × (1 ÷ 25.4) ≈ 1.26 inches. Mark 1.26″ between holes instead of guessing 1.25″, and your jig still lines up at the tenth hole.

Example 2: Fractional inch to millimeters for a metric blade guard.
A 5⅝″ tenon must clear a guard. First decimalize: 5 + (5 ÷ 8) = 5.625″. Then convert: 5.625 × 25.4 = 142.875 mm. Enter 142.9 mm as the cutting height.

Example 3: Feet to centimeters for sheet goods.
A 6-foot offcut: 6 × 30.48 = 182.88 cm, useful when a European hardware listing is sized in centimeters.

• Converting a fraction directly: Entering “5 5/8” as 5.58 instead of 5.625. Decimalize the fraction first, then convert.
• Rounding 25.4 to 25: A 1.6% error that compounds across many parts. Always use the exact 25.4 mm-per-inch factor.
• Swapping the direction: Multiplying millimeters by 25.4 to get inches. Going metric→imperial means dividing by 25.4, not multiplying.
• Trusting tool labels blindly: A “1/4-inch” bit is nominal; measure the shank in millimeters before you match it to a 6 mm collet.

"Pick one system as your shop's native unit and convert only at the boundary — reading a metric plan, buying a metric fastener. When everything on the bench is inches or everything is millimeters, you make fewer transcription errors than if you mix them mid-project." — Bench Builder

"Nominal sizes lie; measured sizes don't. A '2×4' and a '1/4-inch dowel' are labels. When a fit matters, measure the real part and convert that number, not the name on the box."

• Foreign plans: Turn metric dimensions from European or Asian free plans into the inches your tape reads.
• Hardware matching: Confirm a metric bolt, nut, or router-bit shank against its imperial nickname before you buy.
• CNC and CAD import: Reconcile a file drawn in millimeters with stock ordered in feet and inches.
• Quick shop math: Check a rough figure by hand when you don't want to trust a mental conversion.

• Imperial System: A system of measurement using units like inches, feet, pounds, etc. Primarily used in the United States.
• Metric System (SI): The international standard system of measurement using units like meters, grams, etc. Used by most of the world.
• Conversion Factor: The exact number you multiply by to change one unit into another, such as 25.4 mm per inch.
• Nominal Size: A rounded trade name for a part, like “1/4 inch,” that may differ slightly from its true measured size.