Unit Conversions

Inches to Centimeters - Forward Factor

2.54

Inches to Centimeters - Reverse Factor

0.3937

Inches to Centimeters - Exactness

Exact (Defined)

Inches to Centimeters - Category

Length

Inches to Centimeters - Formula

L{cm} = L{in} \times 2.54

Inches to Centimeters - Origin

In 1959, the international yard was defined as exactly 0.9144 meters, making the inch exactly 2.54 cm.

Inches to Centimeters - Mental Math Tip

Multiply by 2.5 (or divide by 4 and multiply by 10).

Feet to Meters - Forward Factor

0.3048

Feet to Meters - Reverse Factor

3.2808

Feet to Meters - Exactness

Exact (Defined)

Feet to Meters - Category

Length

Feet to Meters - Formula

L{m} = L{ft} \times 0.3048

Feet to Meters - Origin

Based on the 1959 agreement; 12 inches × 2.54 cm.

Feet to Meters - Mental Math Tip

A meter is roughly 3 feet and 3 inches. Divide feet by 3 for a rough estimate.

Miles to Kilometers - Forward Factor

1.60934

Miles to Kilometers - Reverse Factor

0.62137

Miles to Kilometers - Exactness

Exact (Defined)

Miles to Kilometers - Category

Length

Miles to Kilometers - Formula

L{km} = L{mi} \times 1.609344

Miles to Kilometers - Origin

One mile is 5,280 feet.

Miles to Kilometers - Mental Math Tip

Use the Fibonacci sequence: 3 mi ≈ 5 km, 5 mi ≈ 8 km, 8 mi ≈ 13 km. Or multiply by 1.6.

Pounds to Kilograms - Forward Factor

0.45359

Pounds to Kilograms - Reverse Factor

2.2046

Pounds to Kilograms - Exactness

Exact (Defined)

Pounds to Kilograms - Category

Mass

Pounds to Kilograms - Formula

M{kg} = M{lb} \times 0.45359237

Pounds to Kilograms - Origin

Defined by the international avoirdupois pound.

Pounds to Kilograms - Mental Math Tip

A kg is roughly 2.2 lbs. To get kg, take weight, divide by 2, then subtract 10%.

Ounces to Grams - Forward Factor

28.3495

Ounces to Grams - Reverse Factor

0.03527

Ounces to Grams - Exactness

Approximate (to 4 decimal places)

Ounces to Grams - Category

Mass

Ounces to Grams - Formula

M{g} = M{oz} \times 28.3495

Ounces to Grams - Origin

1/16th of a pound.

Ounces to Grams - Mental Math Tip

Think 30g (slightly less). A standard paperclip is 1g.

Gallons (US) to Liters - Forward Factor

3.7854

Gallons (US) to Liters - Reverse Factor

0.26417

Gallons (US) to Liters - Exactness

Exact (Defined)

Gallons (US) to Liters - Category

Volume

Gallons (US) to Liters - Formula

V{L} = V{gal} \times 3.785411784

Gallons (US) to Liters - Origin

Based on the wine gallon (231 cubic inches).

Gallons (US) to Liters - Mental Math Tip

A gallon is roughly 4 liters (just under).

Fluid Ounces (US) to Milliliters - Forward Factor

29.5735

Fluid Ounces (US) to Milliliters - Reverse Factor

0.03381

Fluid Ounces (US) to Milliliters - Exactness

Approximate

Fluid Ounces (US) to Milliliters - Category

Volume

Fluid Ounces (US) to Milliliters - Formula

V{ml} = V{fl oz} \times 29.5735

Fluid Ounces (US) to Milliliters - Origin

1/128th of a US Gallon.

Fluid Ounces (US) to Milliliters - Mental Math Tip

Think 30ml per ounce. A standard shot glass is 1.5 oz (approx 45ml).

Celsius to Fahrenheit - Forward Factor

N/A (Uses offset)

Celsius to Fahrenheit - Reverse Factor

N/A (Uses offset)

Celsius to Fahrenheit - Exactness

Exact

Celsius to Fahrenheit - Category

Temperature

Celsius to Fahrenheit - Formula

T{F} = (T{C} \times 1.8) + 32

Celsius to Fahrenheit - Origin

Fahrenheit set 0 at lowest temp of salt/ice mix; Celsius set 0 at water freezing.

Celsius to Fahrenheit - Mental Math Tip

Double C, subtract 10%, add 32. (e.g., 20C -> 40 -> 36 -> 68F).

Celsius to Kelvin - Forward Factor

N/A (Offset only)

Celsius to Kelvin - Reverse Factor

N/A (Offset only)

Celsius to Kelvin - Exactness

Exact

Celsius to Kelvin - Category

Temperature

Celsius to Kelvin - Formula

T{K} = T{C} + 273.15

Celsius to Kelvin - Origin

Kelvin is the absolute scale starting at absolute zero.

Celsius to Kelvin - Mental Math Tip

Just add 273. 0C is 273K. Room temp (20C) is 293K.

Acres to Hectares - Forward Factor

0.40468

Acres to Hectares - Reverse Factor

2.471

Acres to Hectares - Exactness

Approximate

Acres to Hectares - Category

Area

Acres to Hectares - Formula

A{ha} = A{ac} \times 0.404686

Acres to Hectares - Origin

Acre = amount of land a yoke of oxen could plough in one day. Hectare = 10,000 m².

Acres to Hectares - Mental Math Tip

A hectare is roughly 2.5 acres.

Calories (thermochemical) to Joules - Forward Factor

4.184

Calories (thermochemical) to Joules - Reverse Factor

0.2390

Calories (thermochemical) to Joules - Exactness

Exact (Defined)

Calories (thermochemical) to Joules - Category

Energy

Calories (thermochemical) to Joules - Formula

E_{J} = E_{cal} \times 4.184

Calories (thermochemical) to Joules - Origin

Defined by the specific heat of water (energy needed to raise 1g of water by 1°C).

Calories (thermochemical) to Joules - Mental Math Tip

Multiply calories by 4.2. (Note: Food "Calories" are actually kilocalories, so multiply by 4200).

Kilowatt-hours to Joules - Forward Factor

3,600,000

Kilowatt-hours to Joules - Reverse Factor

2.778 × 10^-7

Kilowatt-hours to Joules - Exactness

Exact

Kilowatt-hours to Joules - Category

Energy

Kilowatt-hours to Joules - Formula

E_{J} = E_{kWh} \times 3.6 \times 10^6

Kilowatt-hours to Joules - Origin

1 Watt = 1 Joule/second. 1 hour = 3600 seconds.

Kilowatt-hours to Joules - Mental Math Tip

1 kWh is 3.6 Megajoules. It's a huge amount of energy compared to a single Joule.

Electronvolts to Joules - Forward Factor

1.602 × 10^-19

Electronvolts to Joules - Reverse Factor

6.242 × 10^18

Electronvolts to Joules - Exactness

Approximate (Dependent on elementary charge)

Electronvolts to Joules - Category

Energy (Atomic)

Electronvolts to Joules - Formula

E_{J} = E_{eV} \times 1.602 \times 10^{-19}

Electronvolts to Joules - Origin

The energy gained by an electron accelerating through a potential difference of 1 volt.

Electronvolts to Joules - Mental Math Tip

Memorize the exponent (-19). It's the same number as the elementary charge.

Horsepower (Mechanical) to Watts - Forward Factor

745.7

Horsepower (Mechanical) to Watts - Reverse Factor

0.001341

Horsepower (Mechanical) to Watts - Exactness

Defined

Horsepower (Mechanical) to Watts - Category

Power

Horsepower (Mechanical) to Watts - Formula

P_{W} = P_{hp} \times 745.7

Horsepower (Mechanical) to Watts - Origin

James Watt defined it to compare steam engines to draft horses.

Horsepower (Mechanical) to Watts - Mental Math Tip

Think 3/4 of a kilowatt (750W). A 100hp car is roughly 75kW.

Atmospheres to Pascals - Forward Factor

101,325

Atmospheres to Pascals - Reverse Factor

9.869 × 10^-6