Lipid membranes are highly complex systems with a hierarchy of structure and dynamics that span several decades in length and time scales which in turn have a huge influence on membrane functions. While a large body of research has been dedicated to multiscale structural investigations in such systems, dynamical characterization has lagged far behind. The experimental challenge here is the need to simultaneously access the desired length and time scales for the motions of interest. Over the past decade, neutron spin echo spectroscopy (NSE) has proven to be a unique tool for capturing the collective lipid membrane dynamics on the nanometer and nanosecond scales. For example, lipid membrane bending moduli have been estimated from the relaxation time of single membrane fluctuations in unilamellar vesicles. More recently, lipid membrane thickness fluctuations have also been measured with relaxation times on the order of 100 ns and with amplitudes of a few tenths of nanometers in fluid lipid bilayers. Here we use deformation free energy calculations to connect the experimentally measured thickness fluctuation amplitude to the bilayer elastic parameters: the compressibility modulus and wavelength of the fluctuations. The compressibility modulus is on the order of 10ˆ6 Pa irrespective of saturated tail length in zwitterionic bilayers; however, this value decreases upon mixing lipids with different tail length. Similarly, the fluctuation wavelength is also influenced by lipid composition and is on the order on 20 nm in single component bilayers and increases with temperature in mixed lipid bilayers. It is interesting to note that these fluctuation wavelengths are on the same size scale as laterally inhomogeneous bilayer structures such as the ripple phase and raft domains, which may hint at an interplay between the structure and dynamics in lipid bilayers.