Background: The Chronic Kidney Disease in Children Study (CKiD) equation for children and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation for adults are recommended serum creatinine (SCr)-based calculations for estimating glomerular filtration rate (GFR). However, these equations, as well as their combination, have limitations, notably the problem of implausible changes in GFR during the transition from adolescence to adulthood and overestimation of GFR in young adults. The full age spectrum (FAS) equation addresses these issues but overestimates GFR when SCr levels are low. Objective: To develop and validate a modified FAS SCr-based equation combining design features of the FAS and CKD-EPI equations. Design: Cross-sectional analysis with separate pooled data sets for development and validation. Setting: Research and clinical studies (n = 13) with measured GFR available. Patients: 11 251 participants in 7 studies (development and internal validation data sets) and 8378 participants in 6 studies (external validation data set). Measurements: Clearance of an exogenous marker (reference method), SCr level, age, sex, and height were used to develop a new equation to estimate GFR. Results: The new European Kidney Function Consortium (EKFC) equation is a FAS equation with low bias (-1.2 mL/min/1.73 m2 [95% CI, -2.7 to 0.0 mL/min/1.73 m2] in children and -0.9 mL/ min/1.73 m2 [CI, -1.2 to -0.5 mL/min/1.73 m2] in adults) across the FAS (2 to 90 years) and SCr range (40 to 490 μmol/L [0.45 to 5.54 mg/dL]) and with fewer estimation errors exceeding 30% (6.5% [CI, 3.8% to 9.1%] in children and 3.1% [CI, 2.5% to 3.6%] in adults) compared with the CKiD and CKD-EPI equations. Limitation: No Black patients were included. Conclusion: The new EKFC equation shows improved accuracy and precision compared with commonly used equations for estimating GFR from SCr levels.