def _ncells_to_area(ncells, rho, ref_density=1250):
    """Return theoretical area taken up by `ncells` cells.
    
    Returns
    -------
    area  :  number or numpy array (dtype float)
        Area in units (mm^2, μm^2, etc.)
    
    Parameters
    ----------
    
    ncells  :  number or numpy array (dtype int)
        Number of cells
    
    rho  :  number or numpy array
        Cell density in dimensionless units.
    
    ref_density  :  number or numpy array
        The cell density at a dimensionless density of `rho = 1`
        in units of inverse area. Defaults to 1250 (mm^-2).
    """
    return ncells / (rho * ref_density)