Thin foils are used extensively in space plasma spectrometers, for example, for generation of a coincidence measurement via secondary electron emission in time-of-flight mass spectrometers and charge conversion of energetic neutral atoms (ENAs) in ENA imagers. An ion or neutral atom passing through the carbon foil experiences energy loss and straggling that can degrade the energy resolution or mass resolution. These effects need to be quantified in order to permit efficient instrument development and modeling. We present measurements of energy loss and energy loss straggling for H, He, C, N, O, Ne, and Ar ions between 1 and 50 keV passing through carbon foils of different thicknesses (nominally between 0.5 and 3.0 mug/cm2). We derive empirical relationships for the energy loss and energy loss straggling as a function of foil thickness, ion species, and ion energy. In particular, below ~10 keV the energy loss of hydrogen seems to vary with foil thickness and the energy loss straggling seems to vary with the square root of the thickness. Our results apply equally well to ions and neutral atoms.