Basic Conversions

The Basic Conversions category covers the physical quantities you encounter most often in daily life. Almost every one of these conversions is anchored by an exact, definitionally-fixed relationship — not a measured value — which is why M-Convert can display results to many significant figures without ambiguity.

Anchor relationships

• 1 inch = 2.54 cm exactly (1959 international yard and pound agreement)
• 1 foot = 0.3048 m exactly
• 1 international mile = 1.609344 km exactly
• 1 pound = 0.45359237 kg exactly
• 1 US liquid gallon = 3.785411784 L exactly
• 1 metric ton = 1000 kg exactly
• 1 knot = 1 nautical mile per hour = 1852/3600 m/s exactly
• 1 standard atmosphere = 101,325 Pa exactly
• 1 thermochemical calorie = 4.184 J exactly
• 1 mechanical horsepower = 550 ft·lb/s = 745.6998715822... W

Where a factor is definitionally exact, the calculator's precision is limited only by double-precision floating-point representation. Where a factor is derived (like the knots-to-m/s conversion above), the derivation is noted in the page's accuracy section.

Temperature is special

Celsius, Fahrenheit, Kelvin, and Rankine are not interchangeable via a simple scale factor — they have offsets as well as scale ratios. M-Convert handles temperature through explicit forward and reverse functions, never through multiplication. This prevents a common class of errors where a naive implementation multiplies by 1.8 and forgets the +32 offset.

Precision and significant figures

Every converter page displays results with an automatic significant-figure cap that scales with the input magnitude. Small inputs (like 0.5 m) get more decimal places; large inputs (like 10,000 m) get fewer, keeping the display readable and honest about precision. See the Significant Figures & Rounding guide for details.