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We derive a closed-form formula for the nonlinear interference in few-mode fibers (FMFs) in strong coupling regime. We also formulate an expression for the nonlinear FMFs capacity. This is carried out by extending the Gaussian noise model (GN-model), that has been used with single-mode fibers (SMFs), to FMFs. We inspect the derived formulas over a mode-division multiplexing (MDM) system that carries wavelength-division multiplexed (WDM) signals on each polarization of every spatial mode. The nonlinear coupling among three co-propagated modes reduces the average optical signal-to-noise ratio (OSNR) by about 11.5 dB when compared to single-mode propagation. Also, a differential mode group delay (DMGD) of 300 ps/km between the fundamental mode LP ₀₁ and the other two modes Lp _11a(b) reduces the nonlinearity penalty by about 10%.