Assessment of glomerular filtration rate (GFR) is necessary for dose adjustments of beta-lactam that are excreted by the kidneys, such as meropenem. The aim of this study was to compare the daily dose of 24 h-continuous infusion (CI) meropenem when GFR was calculated by means of measured creatinine clearance (mCLCR) or estimated by the CKDEPI (eGFRCKDEPI), Cockcroft–Gault (eGFRCG), and MDRD (eGFRMDRD) equations. Adult critically ill patients who underwent therapeutic drug monitoring (TDM) for the assessment of 24 h-CI meropenem steady state concentration (Css) and for whom a 24 h-urine collection was performed were retrospectively enrolled. Meropenem clearance (CLM) was regressed against mCLCR, and meropenem daily dose was calculated based on the equation infusion rate = daily dose/CLM. eGFRCKDEPI, eGFRCG, and eGFRCKDEPI were regressed against mCLCR in order to estimate CLM. Forty-six patients who provided 133 meropenem Css were included. eGFRCKDEPI overestimated mCLCR up to 90 mL/min, then mCLCR was underestimated. eGFRCG and eGFRMDRD overestimated mCLCR across the entire range of GFR. In critically ill patients, dose adjustments of 24 h-CI meropenem should be based on mCLCR. Equations for estimation of GFR may lead to gross under/overestimates of meropenem dosages. TDM may be highly beneficial, especially for critically ill patients with augmented renal clearance.