The three basic metric units are the meter(length), the gram(weight),and the liter(volume).

Weight

1 kilometer = 0.62137 mi
1 meter =3.2808 ft
1 centimeter = 0.3937 in
1 mile =1.6093 km
1 foot = 0.3048 m
1 inch = 2.54 cm
 
 Volume 

1 hectoliter = 100 l = 0.1 m³ = 26.4172 gal
1 liter = 100 cl = 1 dm³ = 1.0567 qt
1 gallon (U.S.) = 3.7853 l    
1 cubic inch = 16.3871 cm³    
 
  Area

1 square kilometer = 1 million m² = 0.3861 mi²
1 hectare = 10,000 m² = 2.471 acres
1 square meter = 10.7639 ft² = 1550.003 in²
1 square mile = 640 acres = 258.9988 ha²
1 square foot = 144 in² = 929.0304 cm²
 
EASY CONVERSIONS 

  Symbol If You Know Multiply By To Find Symbol
Length Mm Millimeters 0.03937 inches in
Cm Centimeters 0.3937 inches. in
M meters 3.3 feet ft
M meters 1.1 yards yd
Km kilometers 0.62 miles mi
In inches 2.54 cetimeters cm
ft feet 30.48 cetimeters cm
yd yards 0.9144 meters m
  Area cm² square centimeters 0.16 square inches in²
square meters 1.2 square yards yd²
km² square kilometers 0.4 square miles mi²
ha² hectares (10,000 m2) 2.5 acres  
in² square inches 6.5 Square centimeters cm²
ft² square feet 0.09 square meters
yd² square yards 0.8 square meters
mi² square miles 2.6 square kilomters km²
  Weight g grams 0.035 ounce oz
kg kilograms 2.2046 pounds lb
t tonnes (1,000 kq) 1.1 short tons  
oz ounces 28.3495 grams g
lb pounds 0.45 kilograms kg
      Volume ml milliliters 0.03 fluid ounces fl oz
l liters 2.1 pints pt
l liters 1.06 quarts qt
l liters 0.26 gallons gal
cubic meters 35.314 cubic feet ft³
cubic meters 1.3 cubic yards yd³
tsp teaspoons 5 milliliters ml
tbsp tablespoons 15 milliliters ml
fl oz fluid ounces 30. milliliters ml
c cups 0.24 liters l
pt pints 0.47 liters l
qt quarts 0.97 liters l
gal gallons 3.8. liters l
ft³ cubic feet 0.03 cubic meters
yd³ cubic yards 0.76. cubic meters