Purpose: To introduce a simple analytical formula for estimating T-2 from a single Double-Echo in Steady-State (DESS) scan. Methods: Extended Phase Graph (EPG) modeling was used to develop a straightforward linear approximation of the relationship between the two DESS signals, enabling accurate T-2 estimation from one DESS scan. Simulations were performed to demonstrate cancellation of different echo pathways to validate this simple model. The resulting analytic formula was compared to previous methods for T-2 estimation using DESS and fast spin-echo scans in agar phantoms and knee cartilage in three volunteers and three patients. The DESS approach allows 3D (256 x 256 x 44) T-2-mapping with fat suppression in scan times of 3-4 min. Results: The simulations demonstrated that the model approximates the true signal very well. If the T-1 is within 20% of the assumed T-1, the T-2 estimation error was shown to be less than 5% for typical scans. The inherent residual error in the model was demonstrated to be small both due to signal decay and opposing signal contributions. The estimated T-2 from the linear relationship agrees well with reference scans, both for the phantoms and in vivo. The method resulted in less underestimation of T-2 than previous single-scan approaches, with processing times 60 times faster than using a numerical fit. Conclusion: A simplified relationship between the two DESS signals allows for rapid 3D T-2 quantification with DESS that is accurate, yet also simple. The simplicity of the method allows for immediate T-2 estimation in cartilage during the MRI examination. (C) 2016 Elsevier Inc. All rights reserved.