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Section 1.3: FEBio Basics Up Chapter 1: Introduction Chapter 2: Running FEBio

## 1.4 Units in FEBio

FEBio does not assume a specific unit system. It is up to the user to enter numbers that are defined in consistent units. For example, when entering material parameters in SI units, the user must enter all loads, contact parameters, and other boundary conditions in SI units as well. The units of all the parameters are given when they are defined in this manual. We use a generic designation of units for all the parameters using the following symbols.
 Symbol Name SI unit L Length meter (m) M Mass kilogram (kg) t Time second (s) T Temperature Kelvin (K) n Amount of substance mole (mol) F Force Newton (kgm/s) P Pressure, stress Pascal (Pa=N/m) Q Electric charge Coulomb (C=As)
Units are given using the bracket notation. For instance, the unit for density is [M/L] and the unit for permeability is [L/Ft]. When using SI units, this corresponds to units of kg/m for density and m/N.s for permeability, respectively. Unitless parameters are designated by empty brackets ([ ]). The units for angles are either [deg] for degrees or [rad] for radians.
When adopting a consistent set of units, first choose a primary set of units, and then determine the remaining derived units. For example, in typical problems in solid mechanics, the primary set consists of three units. If you choose [M]=kg, [L]=m, and [t]=s, then [F]=N and [P]=Pa. Alternatively, if you choose [L]=mm, [F]=N and [T]=s as the primary set, then [P]=MPa (since 1 N/mm 10N/m 1 MPa) and [M]=tonne (tonne = N s/mm). The primary set of units must be independent. For instance, in the last example, you cannot choose [P] as a primary unit as it can be expressed in terms of [F] and [L] (i.e. [P]=[F/L]).
Example:
 Primary Units time s length mm force N amount of substance nmol charge C temperature K Derived Units stress N/mm, MPa permeability mm/Ns, mm/Pas diffusivity mm/s concentration nmol/mm, mM charge density nEq/mm, mEq/L voltage mV current density A/mm current A
Section 1.3: FEBio Basics Up Chapter 1: Introduction Chapter 2: Running FEBio