The S.I. and English Systems
The System International (International Standard System) use newtons (N) and meters (m) for its units of
force and length. The U.S.Customary System (English, imperial, engineering system) uses
pounds (lb) and feet (ft) or inches (in.).
The unit of time, seconds (s, sec) is the same in both systems.
Below are tables that convert units from USCS to S.I. and
back (e.g., 1 meter = 39.4 inches), as well as common conversions
within each system (e.g., 1 ft = 12 inches). In general, the Conversion
Factors are listed to 6 significant digits (even though your answers need
not be that precise).
Online resources. A useful interactive conversion tool, with many more units,
is Engineering Exchange's Unit
Conversion widget. Another tool, Convert, by Joshua Madison, may be downloaded
here (remember, pressure has the same units
as stress).
Some approximate (within 2%) conversions useful in engineering mechanics (i.e., statics, dynamics, strength of materials): 
 1 lb  4.5 N
 1 in. = 25 mm
 1 m ~ 40 in. ~ 3.25 ft
 1 ksi ~ 7 MPa (1 psi ~ 7 kPa); multiply by 7 and increase prefix by 3 order of magnitude

4a. Lengths, Areas and Volumes 
English Units to S.I. Units and Back
Underlined values represent an exact conversion factor.
To Convert From

multiply

to get / To Convert from

multiply by

to get

Length 
inches, in. 
25.4

millimeters, mm

0.0393701

inches, in. 
inches, in. 
0.0254

meters, m

39.3701

inches, in. 
feet, ft 
0.3048

meters, m

3.280840

feet, ft 
yards, yd 
0.9144

meters, m

1.09361

yards, yd 
miles, mi 
1.609344

kilometers, km

0.621371

miles, mi 
Area 
square inch, in^{2}, si 
645.16

square millimeters, mm^{2}

0.00155000

square inch, in^{2} 
square inch, in^{2}, si 
0.00064516

square meter, m^{2}

1550.00

square inch, in^{2} 
square feet, ft^{2}, sf 
0.09290304

square meter, m^{2}

10.7639

square feet, ft^{2}, sf 
square yard, yd^{2}, sy 
0.836127

square meter, m^{2}

1.19599

square yard, yd^{2} 
Volume

cubic inch, in^{3}, ci 
16387.064

cubic millimeters, mm^{3}

61.0237 x 106

cubic inch, in^{3} 
cubic inch, in^{3}, ci 
16.387064

cubic centimeters, cc, cm^{3}

0.06102374

cubic inch, in^{3} 
cubic foot, ft^{3}, cf 
0.0283168

cubic meters, m^{3}

35.3147

cubic foot, ft^{3}, cf 
cubic yard, yd^{3}, cy 
0.764555

cubic meters, m^{3}

1.30795

cubic yard, yd^{3} 
gallon, gal. 
3.78541

liters, l = 0.001 m^{3}

0.264172

gallon, gal. 
Other Geometric Properties
of a Crosssection 
Moment of Inertia, in^{4} 
416231

mm^{4} = 10^{12}
m^{4}

2.402 x 106

in^{4} 
Section Modulus, in^{3} 
16387.064

mm^{3} = 10^{9}
m^{3}

61.0237 x 106

in^{3} 
 Top  Geometric Conversions  Force and Stress Conversions 
 Same System Conversions  Pounds, Newtons & Kilograms; Density  Temperature 
4b. Force, Stress, Loads and Mass  English Units
to S.I. Units and Back
To Convert From

multiply

to get / To Convert from

multiply by

to get

Force 
pound, lb 
4.44822

newton, N

0.224809

lb

kilopound, kip = 1000 lb 
4.44822

kilonewton, kN = 1000 N

0.224809

kip = 1000 lb

Stress [Force per Unit Area] 
pounds per sq. ft, lb/ft^{2}, psf 
47.8803

pascals, Pa = N/m^{2}

0.0208854

lb/ft^{2}, psf

psf 
0.0478803

kilopascal, kPa, kN/m^{2}

20.8854

psf

kilopounds per sq. ft, ksf 
47.8803

kPa

0.0208854

ksf

pounds per sq. inch, psi 
6894.76

Pa

0.000145038

psi

psi 
6.89476

kPa

0.145038

psi

kilopounds per sq. in, ksi 
6.89476

megapascal, MPa
= MN/m^{2} = N/mm^{2}

0.145038

ksi

Bending Moment, Torque [Force x Length] 
poundfoot, lbft 
1.35582

newtonmeter, Nm

0.737561

lbft

poundinch, lbin 
0.112985

Nm

8.850732

lbin

Distributed Load
[Force per Unit Length] 
pounds per (linear) foot, plf 
14.5939

newtons per meter, N/m

0.06852178

plf

pounds per (linear) inch, pli 
175.127

N/m

0.00571015

pli

 Top  Geometric Conversions  Force and Stress Conversions 
 Same System Conversions  Pounds, Newtons & Kilograms; Density  Temperature 
4c. Same System Conversions; "Things you should
know"
From

Multiply by

to get / To Convert from

multiply by

to get

Length 
in 
0.083333

feet, ft

12

in 
ft 
0.333333

yards, yd

3

ft 
ft 
1.89393 x10^{4}

miles, mi

5280

feet, ft 
mm 
0.001

meters, m

1000

mm 
cm 
0.01

m

100

cm 
Area 
in^{2} 
0.00694444

ft^{2}

144

in^{2} 
ft^{2} 
0.111111

yd^{2}

9

ft^{2} 
Time 
second,
s (SI), sec (US) 
0.0166666

minutes, min

60

s, sec 
second 
2.77777 x10^{4}

hour, h

3600

s, sec 
second 
1.1574 x 10^{5}

day, d

86400

s, sec 
minutes, min 
6.9444 x10^{4}

day,d

1440

min 
Weight 
lb 
0.005

ton (U.S.)

2000

lb 
lb 
0.0025 
kips (U.S.) 
1000 
lb 
 Top  Geometric
Conversions  Force and Stress Conversions


Same System Conversions
 Pounds, Newtons & Kilograms; Density
 Temperature 
4d. Force, Weight and Density  English Units
to S.I. Units and Back
Caution! In the S.I. system, "weight" is often expressed in kilograms (or kilogramforce, kgf). One (1.0) kgf is the force required to accelerate 1.0 kg mass by 1.0 m/s^{2}. "Weight" given in kilograms must be converted to force: the weight of 1.0 kg mass is 9.81 N (in standard gravity)
To Convert
From

Multiply
by

to get /
To Convert from

multiply
by

to get

Pounds, Newtons and Kilograms 
pounds, lb 
4.44822 
newton, N 
0.224809 
lb 
kilopounds, kips 
4.44822

kilonewton, kN

0.224809

kips 
lb 
0.453592

kg

2.20462

lb 
N 
0.101972

kg

9.80665

N 
lb 
0.0310

slug (lbs^{2}/ft)

32.2

lb 
Weight Density
(force/volume); including kilogram force (kgf) 
lb / ft^{3} 
0.157087

kilonewtons per kN/m^{3}

6.36590

lb / ft^{3} 
lb / in^{3} 
271.447

kN/m3

0.00368

lb / in^{3} 
lb / in^{3} 
0.000578703

lb / ft^{3}

1728

lb / in^{3} 
N / m^{3} 
0.101972

kg / m^{3}

9.80665

N / m^{3} 
kN / m^{3} 
101.972

kg / m^{3}

0.0090665

kN / m^{3} 
lb / ft^{3} 
16.0185

kg / m^{3}

0.0624279

lb / ft^{3} 
Mass Density (mass/volume) 
slug / ft^{3} 
515.379

kg / m^{3}

0.0019403

slug / ft^{3} 
 Top  Geometric Conversions  Force and Stress Conversions 
 Same System Conversions  Pounds, Newtons & Kilograms; Density  Temperature 
4e. Temperature Conversions
To Convert From,

use function

to get / To Convert from

use function

to get

degrees fahrenheit, °F 
(5/9)(F – 32)

degrees centigrade, °C

1.8 C + 32

degrees fahrenheit, °F 
°F 
F + 459.67

degrees Rankine, R

R – 459.67

°F 
degrees centigrade, °C 
C + 273.15

degrees Kelvin, K

K – 273.15

°C 
change in °F (or R) 
(5/9)(D°F)

change in deg. °C (or K)

1.8(D°C)

change in °F (or R) 
 Top  Geometric Conversions  Force and Stress Conversions 
 Same System Conversions  Pounds, Newtons & Kilograms; Density  Temperature 
